PA28-140 Starting Issues

[Brent Perry]

So I’ve done some research and haven’t quite seen anything like what I’m seeing, so here it goes.
1964 Piper Cherokee 140
Battery: Concord (2021)
Starter: Either original or OEM
Plane is kept outside in a shade hangar

When it’s cold, the prop barely turns over, it turns, just VERY slowly. Engine starts as it should and away I go. I do turn the prop over by hand a few times before I start it.

I noticed the other day when it was pretty cold and I couldn’t get it started, after trying to start it I went back out side and went to turn the prop over a few more times by hand there was a noticeable grinding noise, I assume this is because the starter was still connected as I could hear the gears turning. (I assume this is a thing with these old starters. I went back inside the plane and got it started. (This is the worse I’ve ever seen it

If it’s warm outside and I go for the first start of the day the prop spins freely and plane starts fine.

Now, temperature not withstanding, when I fly somewhere after several hours of flying and I shut down and get fuel and then go to start it, the plane does the exact same thing it does when it’s cold outside, like the battery and or connectors are hot or something. If I wait 20 mins and go try it the prop spins freely like it should and of course the plane starts without issue.

The plane has always done this (since I’ve owned it in 2021). I replaced the battery shortly after I got it as the old Gill battery was 5yrs old.
My mechanic seems to think that putting a skytec starter in will fix my issue. I agree it will help but it seems to me that could also be a wiring issue. Something is not making a great connection when it’s cold (32-45 degrees) or when it’s hot after flying.

Thoughts?

 

[Bell206]

Thoughts

Without some voltage readings can't offer much. Perhaps search on here for some threads on starter troubleshooting and start there to check voltages.

 

[Bob Noel]

clean all connections - while that might not solve anything, it can't hurt.... unless you fail to reconnect all the wires correctly.

does the plane have aluminum cables or copper cables? (copper has less resistance and therefore provide more power to the starter)

how cold is cold?

do you preheat? (yes, I saw the part about the "hot" start)

are you using 100LL or autofuel?

 

[cowman]

Define cold, both planes I've owned struggled to crank in below freezing temps without pre-heating. If you're using 20W50 engine oil that stuff is as thick as maple syrup on a below freezing day, makes the engine harder to turn. A simple oil pan heater plugged in for as little as an hour will cure that pretty effectively.

 

[baboss]

Aircraft Starter Performance Issues | Sky-Tec

Basic aircraft starter testing is the place to start when your parts aren’t performing as well as they should.

skytec.aero

 

[bonefishpete]

Could be the starter also could be the 61 year old battery cables. I believe there’s battery cable kits that are stc’d for that problem.

 

[ArrowFlyer86]

I have a Pa28r with a new concorde battery + starter as of last yr. It's been great at cranking the prop, whereas the previous 32y old one was weak. Especially painful during hot-starts where it's been running for a while. Sounds like your experience is similar.
However, on the 2 occasions where I've had it sitting in the cold this year (<35 F), it took a couple tries for it to catch, waiting 1 minute inbetween attempts. Hot-starts can still be a PITA, especially on hot days, but MUCH better now than with the old starter.

Separately, I don't attempt to spin the prop by hand during preflight. And FWIW I replaced my aluminum cables with copper. I didn't notice a difference but like nearly everything in aviation: YMMV.

 

[3393RP]

Unless an investigation of the system from the battery to the starter for excessive voltage drop is performed, you're just throwing darts.

If the mechanic replaces the starter and there's no improvement, he'll say "Oh, it's the starter contactor," and replace that. No improvement? "It's the master contactor." Not yet? "Your wiring is bad."

Do the work first before changing parts.

 

[AV8R_87]

So, this might be a slightly unpopular opinion, but even the "better" battery choice for your plane (Concorde RG-35AXC) is underdimensioned for the cranking requirements of those engines, especially with the high compression pistons.

Realized that a few years ago when my battery started failing, and I ended up having to use a lithium jumpstarter box to get the engine going. That little 2lb box spun the engine better than any new battery.

 

[RyanB]

I noticed the other day when it was pretty cold and I couldn’t get it started, after trying to start it I went back out side and went to turn the prop over a few more times by hand there was a noticeable grinding noise, I assume this is because the starter was still connected as I could hear the gears turning.

If you engage the starter but the engine doesn’t start, the bendix will typically remain engaged with the fly wheel.

The issue is that it could be a multitude of things. Test the battery and put a load on it. If it’s fine, then move on to the starter and check for proper voltage. It very well could be a wiring issue.

 

[Dan Thomas]

Unless an investigation of the system from the battery to the starter for excessive voltage drop is performed, you're just throwing darts.

If the mechanic replaces the starter and there's no improvement, he'll say "Oh, it's the starter contactor," and replace that. No improvement? "It's the master contactor." Not yet? "Your wiring is bad."

Do the work first before changing parts.

Exactly. This thread here: https://www.pilotsofamerica.com/com...-turn-over-when-starting.149721/#post-3584303

The key is in knowing how to use a multimeter and how to take voltage DROP tests, not voltage tests. Big difference. Voltage drop tests find the tiny resistances that make big problems in high-amperage circuits like the starter's. Ohm's Law. With a half-hour's work, max, the health of the whole system can be determined, and the faulty stuff repaired or replaced after that.

What would we think of a doctor that kept removing or transplanting organs in an effort to cure some illness? A doctor that didn't use MRIs or X-rays or blood tests and so on. That's what some mechanics are like. Throwing darts, throwing money.

 

[Dan Thomas]

Define cold, both planes I've owned struggled to crank in below freezing temps without pre-heating. If you're using 20W50 engine oil that stuff is as thick as maple syrup on a below freezing day, makes the engine harder to turn. A simple oil pan heater plugged in for as little as an hour will cure that pretty effectively.

If you're seeing 20W50 as thick as maple syrup just below freezing, you're not seeing 20W50, or you're a long way below freezing. The whole idea of multigrades is that they will act like the lower number when cold (SAE 15 or 20) and the higher number when hot (SAE 50). We ran 15W50 all year round on the Canadian prairies in the flight school airplanes. We flew down to -25°C, same as -13°F. The airplanes were hangared overnight but sat outside during the day.

Here's an excerpt from the Phillips 20W50 datasheet:



It has a pour point of -33°C, or -27°F. It will pour until it gets below that. Compare that with the specs for a straight-grade like W100, which is an SAE 50 oil:



Pour point of -20°C, or -4°F. That is thick stuff. When I taught Aircraft Systems in college, one of the classes was on lubricants and a few other things. I had put quarts of 15W50, W80 and W100 in the freezer overnight, which was about -10°C. Just before class I put three other quarts, same grades, in a pan of water on the stove and brought the water to a boil for a half-hour or so to get the oil up to a typical operating temp. With the students I showed them the difference in pour between the cold and hot 15W50; they could see that it ran fine at -10 but did thin out some at 212°F. The W80 and W100 showed much larger viscosity differences at the temp extremes, much worse than the 15W50. The W100 at -10°C was indeed like heavy syrup, compared with the free-running 15W50. The danger is that the oil pump cannot suck that thick single-grade oil up the pickup tube from the sump to push into the engine, so the bearings and everything else run dry and the engine is ruined.

One more thing: An oil pan heater does little in an hour. The problem with thick oil is that it won't suck, as I said, but that oil is also between the crankshaft and its bearings, the camshaft and its bearings, and the pistons and the cylinders, and it is stiff and sticky and now the starter can't move that engine much. You have to heat the whole thing for a long time, since the crank especially is heavy and slow to heat up. That's where a multigrade can save the day.

So, this might be a slightly unpopular opinion, but even the "better" battery choice for your plane (Concorde RG-35AXC) is underdimensioned for the cranking requirements of those engines, especially with the high compression pistons.

Baloney. From the PA-28-140 TCDS, which is what the OP's model is:



The Lycoming O-320-E2A is a low-compression version of the O-320. Compression ratio is 7.00:1, as opposed to the 8.5:1 or 9.0:1 in some other O-320s. Furthermore, that Concorde RG-35AXC is a 35-amp-hour battery, far bigger than the 25-amp-hour batteries used in tens of thousands of Cessna 172s with the O-320, where it works just fine.

So yes, that opinion is unpopular because it is wrong. The OP has a problem, more likely several problems, with anything from the battery to the starter to the ground path between them. There are the master and starter contactors, all the cables and their connections, a whole raft of things that need checking for voltage drops.

Realized that a few years ago when my battery started failing, and I ended up having to use a lithium jumpstarter box to get the engine going. That little 2lb box spun the engine better than any new battery.

Sure, that little lithium battery has a lot of jam, especially when connected in parallel with the existing battery when jumpstarting. If you were using a lithium battery alone to start, you'd need one that weighed more than a couple of pounds, and it would cost a whole lot of money, especially for aviation where it cannot ever be allowed to catch fire. I went fishing this summer with a friend that finally got fed up with the monstrously heavy lead-acid battery he had to lug around for his electric trolling motor. He bought an automotive lithium battery of the same physical size that weighed about 7 pounds and had more capacity. Nice. But he also paid C$1000 for it (US $700 or so).

 

[Brent Perry]

clean all connections - while that might not solve anything, it can't hurt.... unless you fail to reconnect all the wires correctly.

does the plane have aluminum cables or copper cables? (copper has less resistance and therefore provide more power to the starter)

how cold is cold?

do you preheat? (yes, I saw the part about the "hot" start)

are you using 100LL or autofuel?

Cold here is 32 degrees (I'm in NW Florida)
Plane has copper cables
I do not preheat

Will look into cleaning the connections
Thanks

 

[Brent Perry]

Define cold, both planes I've owned struggled to crank in below freezing temps without pre-heating. If you're using 20W50 engine oil that stuff is as thick as maple syrup on a below freezing day, makes the engine harder to turn. A simple oil pan heater plugged in for as little as an hour will cure that pretty effectively.

I Use 20W50 oil

 

[Brent Perry]

I have a Pa28r with a new concorde battery + starter as of last yr. It's been great at cranking the prop, whereas the previous 32y old one was weak. Especially painful during hot-starts where it's been running for a while. Sounds like your experience is similar.
However, on the 2 occasions where I've had it sitting in the cold this year (<35 F), it took a couple tries for it to catch, waiting 1 minute inbetween attempts. Hot-starts can still be a PITA, especially on hot days, but MUCH better now than with the old starter.

Separately, I don't attempt to spin the prop by hand during preflight. And FWIW I replaced my aluminum cables with copper. I didn't notice a difference but like nearly everything in aviation: YMMV.

I think it have it figured out on how to get it to actually start (I use the primer method - per Tom-D) and that has worked great. The issue is getting the prop to actually spin vs just barely a revolution every few seconds.

 

[Brent Perry]

I have a Pa28r with a new concorde battery + starter as of last yr. It's been great at cranking the prop, whereas the previous 32y old one was weak. Especially painful during hot-starts where it's been running for a while. Sounds like your experience is similar.
However, on the 2 occasions where I've had it sitting in the cold this year (<35 F), it took a couple tries for it to catch, waiting 1 minute inbetween attempts. Hot-starts can still be a PITA, especially on hot days, but MUCH better now than with the old starter.

Separately, I don't attempt to spin the prop by hand during preflight. And FWIW I replaced my aluminum cables with copper. I didn't notice a difference but like nearly everything in aviation: YMMV.

I turn the prop by hand per the manual, not sure if that really does much to help me though.

 

[AV8R_87]

Baloney

We'll agree to disagree on this one.

where it works just fine.

The fact that even a tiny increase of resistance in a contactor or connection causes a poor start actually shows how marginal they are. We're just used to it and take it as normal.
The lithium jumpstarter pack is able to maintain voltage under cranking a lot better than even a new and properly charged/conditioned RG-35. Proven by actual cranking RPM being significantly higher.

So yes, that opinion is unpopular because it is wrong

Is it, though? Here's a way to prove me wrong. Put a voltmeter and an amp clamp on a battery and record what the battery is doing during cranking. Then place the lithium jumpstarter in parallel with the main battery, with its own amp clamp. See which way the current is split.
Bonus points, disconnect the main battery and attempt to crank solely on the lithium pack. Most of them will crank the engine just fine.

Sure, that little lithium battery has a lot of jam, especially when connected in parallel with the existing battery when jumpstarting.

When the existing battery develops a high resistance cell and can't provide more than 50 amps or so during cranking, virtually all the cranking current comes from the lithium jumpstarter, because it will maintain a higher voltage under load.

If you were using a lithium battery alone to start, you'd need one that weighed more than a couple of pounds, and it would cost a whole lot of money, especially for aviation where it cannot ever be allowed to catch fire.

Nobody brought up the cost, certification requirements and saftey implications. Just how marginal the existing batteries are, by comparison.
Regarding lithium battery certification: the catch fire is actually the easy part, compared to dealing with the qualification of the BMS hardware and software. You dont't want a BMS software fault disconnecting your battery while in IMC after a generator failure.

 

[ArrowFlyer86]

I turn the prop by hand per the manual, not sure if that really does much to help me though.

Huh. Interesting. That was never in my start-up/preflight procedures or checklists. I'll have to take a look at that.

 

[Bell206]

The issue is getting the prop to actually spin vs just barely a revolution every few seconds.

Which could be caused by a voltage drop at the starter or some upstream location. The cold weather can simply make it more evident by changing clearances, etc. One of the links above has a quick easy way to check for proper power at the starter

 

[Bell206]

I turn the prop by hand per the manual, not sure if that really does much to help me though.

FYI: don't know which manual you refer to, but Lycoming does not recommend this if the engine hasn't been run in a weeks time.

 

[Brent Perry]

Huh. Interesting. That was never in my start-up/preflight procedures or checklists. I'll have to take a look at that.

 

[Bob Noel]

fwiw - my cheroke 140 was always a little sluggish to start when the weather got down near freezing or below. IIRC, even with the new starter, copper cables. Once I got a preheater (and used it), the engine would turn over very nicely - using preheat made starting much better.

Of course, that doesn't explain the OP's problem with "hot starts"

 

[Dan Thomas]

We'll agree to disagree on this one.

On what basis do you disagree? What about the fact that many thousands of 172s and other airplanes using the low-compression E-series Lycomings use 25-amp-hour batteries that work just fine, while you claim that the 35-amp-hour battery is "marginal" for the application?? Go figure.

The OP said these things:

If it’s warm outside and I go for the first start of the day the prop spins freely and plane starts fine.

So how is his 35-amp battery still marginal?

The plane has always done this (since I’ve owned it in 2021). I replaced the battery shortly after I got it as the old Gill battery was 5yrs old.

He's had a problem since he bought it in 2021. He installed a new battery, a 35-amp-hour Concorde (actually rated at 33 amp-hours), and it still doesn't start properly at times. That's not a marginal battery; that's an electrical system problem. That RG35AXC is certified for use in the Cherokee 140, by the way, meaning that Concorde ran successful tests with it.

The fact that even a tiny increase of resistance in a contactor or connection causes a poor start actually shows how marginal they are. We're just used to it and take it as normal.

It does not show how marginal the batteries are. Not at all. Like I said, Ohm's Law is supreme here, and when we don't understand it we start throwing parts at the problem. The situation is so ludicrously bad that Skytec had to publish that troubleshooting diagram to show mechanics how to measure voltage drops. Skytec no doubt got fed up with the constant warranty claims of "defective" starters, when the real problem was an old airplane with old cables and old contactors and dirty/loose/oily/corroded connections. It's not the starter (probably, though in this case it might just be) and it's not usually the battery. Those are the components that owners and mechanics see in the airplane and assume that one or both must be bad, so they start spending money uselessly. They look at the cables and connections and say, "Looks good." But they can't see tiny resistances, nor can they see the shot contacts inside the battery and starter contactors.

Ohm's Law says that E = I x R. Voltage is equal to current (in amps) times resistance (in Ohms).

So we need to know the amperage for the starter to make any sense of this. The typical Lycoming has an Prestolite MZ-series starter, if it doesn't have a newer permanent-magnet Skytec or Lamar starter. From Prestolite's starter manual we see this info for a 12-volt direct-drive (ungeared) starter:



Look at those amperages. At no load, not driving the engine, the current is only 75 amps, max, at 5500 RPM. Under load, the current goes WAY up, as inductive reactance decreases with RPM decrease, and we see as much as 560 amps when the thing is totally stalled. More realistically, when cranking a cold engine, it will be around 250 amps, and that's what we'll work with.

So if that starter tries to draw 250 amps though an old contactor, say, that has burned and eroded contacts, it only takes 0.02 Ohms (one-fiftieth of an ohm!) to cause a five-volt drop across that contactor. SO the battery voltage that has already dropped from about 13 down to 11 volts due to the high draw, now gets cut another 5 volts and the starter gets only 6 volts, assuming that there is nothing else offering such tiny resistances to the flow. Ohm's Law matters.

From that same Prestolite manual we see this:



Look at that. Even at a relatively low flow of 100 amps, they want no more than a 0.3-volt drop between the battery's positive post to the starter's terminal. That is a system total of 0.3 volts, or 0.3/100 = 0.003 ohms. Serious stuff, that. It doesn't take much oxidation or arcing or dirty/oily corroded stuff to make a tiny resistance like that.

Ignoring this stuff just leads to ignorantly replacing expensive parts. Doing the voltage drop tests is the smart way. Proposing that batteries are "marginal anyway" is no way to do the science, is it?

The Prestolite manual: https://www.csobeech.com/files/PrestoliteStarterAlternatorManual.pdf

Cessna uses 35-amp-hour batteries in their big-engined airplanes that use the Continental O/IO-470 and 520 or Lyc O/IO-540. The 25-amp-hour batteries work just fine with the smaller engines. When they put the O-320-H2AD engine in the 172N, which has a 9.0:1 compression ratio instead of the E-series 7.0:1 as in the Cherokee 140 and other 172s, they went to a 24-volt battery of 11 amp-hours. If we do the wattage figures, we see that that battery actually produces less wattage (over time) than the 25-amp-hour 12 volt battery, which is actually 24-amp-hours in the RG25AXC.

The RG25AXC has a cold cranking amps rating of 350 amps. The RG35AXC has a cold cranking amps rating of 440 amps. Even the 350 amps is plenty for any version of the O-320.

When the existing battery develops a high resistance cell and can't provide more than 50 amps or so during cranking, virtually all the cranking current comes from the lithium jumpstarter, because it will maintain a higher voltage under load.

That means that the airplane needs a new battery, not a lithium jumpstarter. Fix the thing properly.

 

[pfarber]

this sounds like a typical ground or connection issue. Clean everything in the starting circuit. Might also be a starter solenoid. Put a voltmeter on the solenoid relay and see if its low. Use a good conducting paste that prevents the copper from oxidizing.

 

[Barrett50]

Measure the resistance from the ground connection on the engine, that goes directly to the firewall ground. That cable never gets touched, and if it's the type which is braided steel, they can get an internal resistance at the ends. Would be a good idea to clean it well and maybe even just replace it first. It's a quick and easy place to start.

Post 22 mentioned copper cables. It reminded me that some PA-28 left the factory with alum cables from the batt to the solenoid, and solenoid to starter lug. Those should be replaced no matter what with copper cables. I would question the solenoid also. It's hard to get a good test on the solenoid. Best way is to take the cable off the starter, now check the voltage reading at the + of the battery, and then the hot side of the solenoid with the key in the start position. Next, check the voltage readings at the cold(starter) side of the solenoid, and finally get the voltage reading at the lug that connects to the starter. This is all done with no load on the battery except the coil of the solenoid. Drops in voltage along the string may be indicative of cable problems.

 

[Dan Thomas]

this sounds like a typical ground or connection issue. Clean everything in the starting circuit. Might also be a starter solenoid. Put a voltmeter on the solenoid relay and see if its low. Use a good conducting paste that prevents the copper from oxidizing.

As if there are only ground connections in the system? Hardly. We once had a Champ that began to get hard to start. O-200 engine. All connections were clean and tight. I finally found the problem: it was nearly-invisible corrosion between the cable from the battery master contactor and it's big crimp terminal at the starter contactor's input lug. Similar to this:


It was a 40-year-old airplane at the time. Looking at the visible copper exposed inside the lug, there was a green color to the copper, proving corrosion. We didn't have the 6-gauge cable nor those terminals at the time, so I warmed that terminal, wicked a bit of soldering flux into it to strip out that corrosion, and wicked solder into it. It is still working 30 years later. Places like this are often overlooked.

Conducting pastes are used for heat-sinking transistors. They're not used on terminals on solenoids (contactors). If anything, we use a dielectric silicone paste (dielectric means it's NON-conducting) to keep the oxygen off it. And you can't get anything on the contacts inside the solenoid. Most of them have been a permanently-crimped-shut style for the last 60 years.


The fact remains that voltage-drop tests need to be done to find the problem, and that includes between the starter and the airframe, which is not shown here. The voltmeter would connect between the starter's case and the firewall.

 

[Mikey52]

That battery can show 12.6 volts and still have a weak cell in it, so when you hit the starter the voltage will drop significantly. I had one do that on a O200. Took the battery out to an auto parts store to test and sure enough the cranking amps was less than half of what it should have been. Just out of the warranty period of course.

 

[Dan Thomas]

That battery can show 12.6 volts and still have a weak cell in it, so when you hit the starter the voltage will drop significantly. I had one do that on a O200. Took the battery out to an auto parts store to test and sure enough the cranking amps was less than half of what it should have been. Just out of the warranty period of course.

And that would have shown up if you had just put a voltmeter across the battery terminals and cranked the engine. The voltage would have dropped way down to maybe four or five volts, maybe even less. And by taking the measurement on the battery terminals themselves, not on the cables or their terminals, you are leaving out all other connections and contactors in the system and you KNOW that the battery is bad. A low voltage means a low amperage, of course. It takes volts (pressure) to push current (amps).

A loss of half the voltage means that the amperage could also be half, and if we multiply the two to get wattage, the measure of power, we see that the wattage is a quarter of what it should be. For example, if we multiply 12 volts times 200 amps, we get 2400 watts. If we multiply 6 volts by the resultant 100 amps, we get 600 watts, and the starter doesn't do anything useful.

 

[AV8R_87]

as inductive reactance decreases with RPM decrease

It's the Back EMF that varies with RPM, not inductive reactance. I won't go on a 2-page electrical machinery theory tangent, though.

At 0 RPM the stall current is 560 amps (that's at 4V, not at the 7V or so that the battery might deliver, but we'll gloss over that for now). That's when you need to deliver the current to get the starter turning fast enough for the Back-EMF to build up and start limiting the current. The brand new battery, fully charged and conditioned, is rated for 440 cold cranking amps. A battery that sat for a month will lose 5-10% of its stored charge. A battery that's not new will have a slightly higher internal resistance. That will decrease its cranking capacity. But somehow you don't think of that set-up as marginal.

On what basis do you disagree? What about the fact that many thousands of 172s and other airplanes using the low-compression E-series Lycomings use 25-amp-hour batteries that work just fine, while you claim that the 35-amp-hour battery is "marginal" for the application?? Go figure.

You sound like the management at Morton Thiokol arguing that they've always had blow-by on the SRB field joints, why should they all of the sudden be concerned before launching Challenger?
Normalization of deviance is not the way, no matter how accustomed we are to that deviance. You should read the Rogers Commission report when you have some spare time: https://www.nasa.gov/history/rogersrep/genindex.htm.

Also, we're not talking about C172s here, with the battery mounted on the firewall and a couple feet of starter wire. We're talking about PA28s with the battery mounted under the rear seat. Different set-up with different wiring resistances.

We constantly see threads about poor starts, shows that you don't have that much of a margin. There's one here every few months or so, and this is a relatively small community. The fact that we have to chase every single bit of voltage drop across all the wires and contactors shows that there's almost no spare capability. Take a look at what automakers install in vehicles. A typical engine of half the displacement of the O-320 usually comes with a battery that has almost twice the capacity and the cranking amps of the RG-35AXC. Do you think those penny pinching CFOs, the same people that skimped on a 3 cent spring in the ignition system, causing multiple deaths and massive recalls in the process, did that from the kindness of their hearts? No, it was done that way because they wanted to have a margin so that their vehicles didn't struggle to start when their start contactor (built into the starter and exposed to the elements a lot more than what we have in our planes) had an extra half a milliohm of resistance and the battery was not brand new anymore.
I have a 30+ year vehicle, 250k+ miles on it, original starter and cables going to the battery in the trunk. Needs a new battery every 7 years or so, usually when one cell develops high resistance. I never have to chase voltage drop across wires and contactors. But again, that is a 50Ah+ battery rated at 750+ cold cranking amps, for a 2.8L engine.

Why do automakers use double the battery for half the displacement? Because in a car an extra 20-30lbs doesn't really matter, when reliable starts (and consequently owner satisfaction) are one of the most important things. In an aircraft, we're always looking at ways to shave every single pound. Some OEMs used to offer 10 thousand dollar bonuses for each pound saved during design, once they realized they were significantly over their target weight. Why do you think Piper installed aluminum cables in the Cherokees? If I were presented with the choice of hauling around another 25lbs of battery to ensure more consistent starts in cold conditions when the battery and the electrical system weren't at peak performance, I'd probably pass as well. I'll take the extra 4 gallons of gas. Or that extra bag of stuff I think I need to bring with me to OSH.

But that doesn't make it better just because we accept the compromise. It's still marginal.

He's had a problem since he bought it in 2021. He installed a new battery, a 35-amp-hour Concorde (actually rated at 33 amp-hours), and it still doesn't start properly at times.

My statement wasn't intended to imply that it's strictly the battery, but, as discussed above, that the battery magnifies any small degradation in the electrical system, that would never be noticed if a battery capable of a higher cranking current was installed. OP probably has a couple connections that are not in prime condition anymore, but he'd never notice them if he had 50% more cranking amps out of the battery.

That's not a marginal battery; that's an electrical system problem. That RG35AXC is certified for use in the Cherokee 140, by the way, meaning that Concorde ran successful tests with it.

Have you seen their certification package? Is it anything more than "meets or exceeds the performance of the original battery specification for the airframe, and it is a form/fit/function replacement"?
A long time ago, an OEM was in the process of certifying a new engine on an existing airframe. More power, bigger starters, but not much weight margin. Also wanted to get rid of the existing Ni-Cd battery and replace it with an AGM one. They quickly realized they might not pass the cold weather start test, so they cherry picked the best battery from a whole pallet (something like 8% over nameplate capacity), kept it on the charger as long as possible, burnished every electrical connection and torqued things by the German method (gut' n tight) to ensure the best chance of success. Even like that, they barely passed the test. Customers had to deal with that for years after, some of them resorting to replacing batteries every six months or so just to get reliable starts. But the OEM ran "successful" tests.

That means that the airplane needs a new battery, not a lithium jumpstarter. Fix the thing properly.

Quoting Richard Nixon during the presidential debate with Jimmy Carter, "There you go again!" Please pay attention. This is only in the context of "a lithium pack had a lower voltage drop under load and provided most of the cranking power for the engine, while the battery that couldn't crank the engine anymore provide none". That was counteracting your statement that the lithium pack only did a good job because it was in parallel with the dead battery. And as I stated in my previous reply, hook up measurement equipment and prove me wrong.

It does not show how marginal the batteries are. Not at all. Like I said, Ohm's Law is supreme here, and when we don't understand it we start throwing parts at the problem.

When you can only afford to lose half a volt (if that) in the wiring and contactors because of a marginal battery, then it all becomes critical. Now, if that battery had more oomph and only dropped to 9V during cold cranking instead of 7V, there would be some extra margin and the end results would be better starts in cold conditions. Even if the start contactor had a resistance of 1 milliohm, it wouldn't really matter. But it does, because a brand new, fully charged RG-35AXC will drop to 7.2V or so during cold cranking. Add a bit of wear on the battery, a couple contactors with a couple tenths of a milliohm of extra resistance, and the starter sees 6V. As you stated earlier, all of a sudden you're only sending a quarter of the power through the starter and then nothing happens. Have a battery that can deliver 9V under cranking load and now you get 100% more power to the starter.

 

[ArrowFlyer86]

It's the Back EMF that varies with RPM, not inductive reactance. I won't go on a 2-page electrical machinery theory tangent, though.

At 0 RPM the stall current is 560 amps (that's at 4V, not at the 7V or so that the battery might deliver, but we'll gloss over that for now). That's when you need to deliver the current to get the starter turning fast enough for the Back-EMF to build up and start limiting the current. The brand new battery, fully charged and conditioned, is rated for 440 cold cranking amps. A battery that sat for a month will lose 5-10% of its stored charge. A battery that's not new will have a slightly higher internal resistance. That will decrease its cranking capacity. But somehow you don't think of that set-up as marginal.


You sound like the management at Morton Thiokol arguing that they've always had blow-by on the SRB field joints, why should they all of the sudden be concerned before launching Challenger?
Normalization of deviance is not the way, no matter how accustomed we are to that deviance. You should read the Rogers Commission report when you have some spare time: https://www.nasa.gov/history/rogersrep/genindex.htm.

Also, we're not talking about C172s here, with the battery mounted on the firewall and a couple feet of starter wire. We're talking about PA28s with the battery mounted under the rear seat. Different set-up with different wiring resistances.

We constantly see threads about poor starts, shows that you don't have that much of a margin. There's one here every few months or so, and this is a relatively small community. The fact that we have to chase every single bit of voltage drop across all the wires and contactors shows that there's almost no spare capability. Take a look at what automakers install in vehicles. A typical engine of half the displacement of the O-320 usually comes with a battery that has almost twice the capacity and the cranking amps of the RG-35AXC. Do you think those penny pinching CFOs, the same people that skimped on a 3 cent spring in the ignition system, causing multiple deaths and massive recalls in the process, did that from the kindness of their hearts? No, it was done that way because they wanted to have a margin so that their vehicles didn't struggle to start when their start contactor (built into the starter and exposed to the elements a lot more than what we have in our planes) had an extra half a milliohm of resistance and the battery was not brand new anymore.
I have a 30+ year vehicle, 250k+ miles on it, original starter and cables going to the battery in the trunk. Needs a new battery every 7 years or so, usually when one cell develops high resistance. I never have to chase voltage drop across wires and contactors. But again, that is a 50Ah+ battery rated at 750+ cold cranking amps, for a 2.8L engine.

Why do automakers use double the battery for half the displacement? Because in a car an extra 20-30lbs doesn't really matter, when reliable starts (and consequently owner satisfaction) are one of the most important things. In an aircraft, we're always looking at ways to shave every single pound. Some OEMs used to offer 10 thousand dollar bonuses for each pound saved during design, once they realized they were significantly over their target weight. Why do you think Piper installed aluminum cables in the Cherokees? If I were presented with the choice of hauling around another 25lbs of battery to ensure more consistent starts in cold conditions when the battery and the electrical system weren't at peak performance, I'd probably pass as well. I'll take the extra 4 gallons of gas. Or that extra bag of stuff I think I need to bring with me to OSH.

But that doesn't make it better just because we accept the compromise. It's still marginal.


My statement wasn't intended to imply that it's strictly the battery, but, as discussed above, that the battery magnifies any small degradation in the electrical system, that would never be noticed if a battery capable of a higher cranking current was installed. OP probably has a couple connections that are not in prime condition anymore, but he'd never notice them if he had 50% more cranking amps out of the battery.


Have you seen their certification package? Is it anything more than "meets or exceeds the performance of the original battery specification for the airframe, and it is a form/fit/function replacement"?
A long time ago, an OEM was in the process of certifying a new engine on an existing airframe. More power, bigger starters, but not much weight margin. Also wanted to get rid of the existing Ni-Cd battery and replace it with an AGM one. They quickly realized they might not pass the cold weather start test, so they cherry picked the best battery from a whole pallet (something like 8% over nameplate capacity), kept it on the charger as long as possible, burnished every electrical connection and torqued things by the German method (gut' n tight) to ensure the best chance of success. Even like that, they barely passed the test. Customers had to deal with that for years after, some of them resorting to replacing batteries every six months or so just to get reliable starts. But the OEM ran "successful" tests.


Quoting Richard Nixon during the presidential debate with Jimmy Carter, "There you go again!" Please pay attention. This is only in the context of "a lithium pack had a lower voltage drop under load and provided most of the cranking power for the engine, while the battery that couldn't crank the engine anymore provide none". That was counteracting your statement that the lithium pack only did a good job because it was in parallel with the dead battery. And as I stated in my previous reply, hook up measurement equipment and prove me wrong.


When you can only afford to lose half a volt (if that) in the wiring and contactors because of a marginal battery, then it all becomes critical. Now, if that battery had more oomph and only dropped to 9V during cold cranking instead of 7V, there would be some extra margin and the end results would be better starts in cold conditions. Even if the start contactor had a resistance of 1 milliohm, it wouldn't really matter. But it does, because a brand new, fully charged RG-35AXC will drop to 7.2V or so during cold cranking. Add a bit of wear on the battery, a couple contactors with a couple tenths of a milliohm of extra resistance, and the starter sees 6V. As you stated earlier, all of a sudden you're only sending a quarter of the power through the starter and then nothing happens. Have a battery that can deliver 9V under cranking load and now you get 100% more power to the starter.

Personally I'll have to disagree with that take. I see where you're coming from, but I feel like that mentality takes you down the wrong path.

Increasing the capacity of the battery to mask underlying problems of unexpected voltage drops across corroded/poor connections is a band-aid solution. The starter can/does work with the batteries provided. Jacking up the size of the battery to compensate for other unknown shortcomings in the electrical system is kind of like a programmer looking at a sluggish, slow-running computer program and saying "we can just throw more compute power at it". Yeah, sometimes you can, but most of the time it's worthwhile to open it up and see what's draining your resources before you do that. Sometimes from a practical, engineering POV increasing the resources does end up being the better all around solution... But it shouldn't be your immediate go-to if you have ample reason to believe current resources should suffice, and I think that's particularly true in this analogous case with aircraft batteries.

Because if you have a poor electrical contact resulting in increased resistance it won't get better on its own, and compensating by using a bigger battery is only going to postpone the inevitable while masking your root cause (while toting around a bigger batt). And if you don't address that base cause what'll happen? You'd just continue to neglect the wiring/contacts (cuz why wouldn't you? everything is working 'well') until they degrade so much that even the supersized battery doesn't cut it. Cuz that will eventually happen.

Even though it can be a bit of a headache on a 60+ year old plane, I think there is a lot of wisdom in opening up the hood and getting at the root problem so that the aircraft functions as it was designed (I say this as someone who sunk quite a bit of time this past June/July doing exactly that). It can be a PITA to hook up the test leads and get your measurements, but in the end if it means you can make the right diagnosis and get the aircraft back in the condition it was engineered to be used -- I think that's a good trade.

Just my $0.02. Not an A&P or an EE (just an electrical tinkerer and aircraft owner that likes things to work as designed)

 

[Capt. Geoffrey Thorpe]

I recommend randomly cleaning connections and/or replacing parts one at a time.

A few dozen hours in labor and a few thousand in parts can easily save you two hours worth of diagnostics. Not to mention the cost of the 49.99 multimeter.

 

[Bob Noel]

I recommend randomly cleaning connections and/or replacing parts one at a time.

A few dozen hours in labor and a few thousand in parts can easily save you two hours worth of diagnostics. Not to mention the cost of the 49.99 multimeter.

don't forget the cost of learning to use the multimeter.

 

[Bell206]

We constantly see threads about poor starts, shows that you don't have that much of a margin. There's one here every few months or so, and this is a relatively small community. The fact that we have to chase every single bit of voltage drop across all the wires and contactors shows that there's almost no spare capability.

Not necessarily. While the theory and math may not work out to your satisfaction, in practice the original aircraft electrical system is more than adequate to handle the requirements... provided that system is properly maintained.

Unfortunately, the most neglected/abused system on private recreational aircraft is the electrical system by a factor of 10 which is the reason we see repetitive posts on the same problems. And not because the battery has lack of “spare capability.” If that were the case, then once those electrical system discrepancies/abuses are corrected one should not be able to get 7+ years out of that same battery... or in some cases getting 11 years before the battery fails a capacity test.

Same with the wire run lengths. They’re not an issue provided the system is to spec. There are plenty of examples of battery relocations that would support that to include installs on turbine powered aircraft. Bottomline, most owners, and some mechanics do not want to spend the time and money to fix those underlying electrical problem that affects many private aircraft. And when you read those old posts, you’ll find the same mitigative steps are always mentioned but few follow them. I probably made more money fixing electrical system problems than any other type of disc and the only times I found the electrical system to be lacking any "spare capacity" was when the actual aircraft electrical load exceeded the recommended 80% level due to 3rd party alterations.

 

[pburger]

Quoting Richard Nixon during the presidential debate with Jimmy Carter, "There you go again!" Please pay attention. This is only in the context of "a lithium pack had a lower voltage drop under load and provided most of the cranking power for the engine, while the battery that couldn't crank the engine anymore provide none". That was counteracting your statement that the lithium pack only did a good job because it was in parallel with the dead battery. And as I stated in my previous reply, hook up measurement equipment and prove me wrong.

COMMUNITY NOTE:
It was Ronald Reagan that debated Jimmy Carter in 1980, and said "There you go again..."

 

[AV8R_87]

COMMUNITY NOTE:
It was Ronald Reagan that debated Jimmy Carter in 1980, and said "There you go again..."

D'OH! That's what I get for discussing Watergate while typing this.

 

[Dan Thomas]

It's the Back EMF that varies with RPM, not inductive reactance. I won't go on a 2-page electrical machinery theory tangent, though.

Well, now. Are you an EE? I find this:

Whenever an alternating electric current is traveling through the loops of wire of an inductor, that changing current will create a changing magnetic within the loops. This in turn creates a changing magnetic flux and by Faraday's Law, we know this will induce an emf. By Lenz's Law, this induced emf (also known as the back emf) will tend to oppose the change in magnetic flux. The induced current produced by the back emf will tend to oppose the original current as long as the original current is increasing. This idea of the back emf providing resistance to the current flow is known as inductive reactance.
https://aklectures.com/lecture/lr-circuits-lc-circuits-and-lcr-circuits/inductive-reactance

At 0 RPM the stall current is 560 amps (that's at 4V, not at the 7V or so that the battery might deliver, but we'll gloss over that for now).

A smaller battery could easily fall to 4 volts with a draw of 560 amps. We're talking about a nearly dead-short here. A starter that draws 560 amps at 4 volts has a resistance of .007 Ohms. No battery can maintain its nominal voltage under conditions like that. Its internal resistance prevents it.

Also, we're not talking about C172s here, with the battery mounted on the firewall and a couple feet of starter wire. We're talking about PA28s with the battery mounted under the rear seat. Different set-up with different wiring resistances.

The 172 used a 25-amp-hour battery. The PA-28-140 used a 35 amp-hour battery, and both systems worked just fine. There are many thousands of both models flying around.

We constantly see threads about poor starts, shows that you don't have that much of a margin. There's one here every few months or so, and this is a relatively small community. The fact that we have to chase every single bit of voltage drop across all the wires and contactors shows that there's almost no spare capability. Take a look at what automakers install in vehicles. A typical engine of half the displacement of the O-320 usually comes with a battery that has almost twice the capacity and the cranking amps of the RG-35AXC.

Two things: A car with a bigger battery has to run more than the starter. It often has headlights that stay on for some time after you park it so you can see your way home. There are many electricity-gobbling accessories like heated windshields and mirrors and seats, all which have to kept happy at idle. Compression ratios are considerably higher. A car is not an airplane. Never was, never will be.

I have a 30+ year vehicle, 250k+ miles on it, original starter and cables going to the battery in the trunk.

I worked on airplanes from the 1950s all the way up to the restart Cessnas from '96-on. Guess where the electrical problems were? In the old airplanes, where age takes its toll. Things rot whether they are being flown or not. Connections and contacts oxidize simply because there is oxygen everywhere. I was constantly fixing such stuff in those 40-50-60-year-old airplanes. The restart models? Nope. Only once did I fix any electrical supply problem in a restart, and that was a burned-out $2 diode in the external power circuit. The airplane didn't need it to operate, only if it needed boosting. That airplane had an 11-amp-hour 24-volt battery to start the IO-360 Lyc, and it did it handily.

Why do automakers use double the battery for half the displacement? Because in a car an extra 20-30lbs doesn't really matter, when reliable starts (and consequently owner satisfaction) are one of the most important things. In an aircraft, we're always looking at ways to shave every single pound.

There. You just showed us why airplanes use smaller batteries. An extra 20-30 pounds is a big deal, especially when the system works just fine when it is maintained and rotten stuff is replaced or fixed.

My statement wasn't intended to imply that it's strictly the battery, but, as discussed above, that the battery magnifies any small degradation in the electrical system, that would never be noticed if a battery capable of a higher cranking current was installed.

No matter how much battery you install, small resistances in the high-amp circuits will cause big voltage drops. Getting the battery to fall only to 9 volts instead of 7 doesn't change the fact that a fiftieth of an ohm will cause another 5-volt drop, and in fact, the higher battery voltage will push more current which will cause a BIGGER voltage drop across that resistance. It just runs circles around any mechanic that doesn't do the diagnostic work. Mechanics cost money, and mechanics that don't know how to isolate the fault just cost even more. Upgrading to a larger battery is about the dumbest way to fix such problems.

The math there is similar to installing twice the horsepower in the airplane and expecting it to go twice as fast. It doesn't.

OP probably has a couple connections that are not in prime condition anymore, but he'd never notice them if he had 50% more cranking amps out of the battery.

Like I said, he has an old airplane. Various versions of the 140 were produced between 1964 and 1977, so the youngest of them is 48 years old this year. How many 48-year old cars are on the road that have not had new battery cables installed? Or starter solenoids? The oldest 140s are 61 years old now. Very few cars like that on the road, and NONE of them will have the original stuff in them. Even in ten- or 15-year-old cars one often needs new cables.

Now, if that battery had more oomph and only dropped to 9V during cold cranking instead of 7V, there would be some extra margin and the end results would be better starts in cold conditions.

A small extra margin, instead of fixing the faulty system. Sounds like false economy to me.

Have a battery that can deliver 9V under cranking load and now you get 100% more power to the starter.

100% more of next-to-nothing is still next to nothing.

Edit: Additionally, connections or contacts with resistance create heat when large currents run through them. The heat accelerates the oxidation process of the metals and also results in expansion-contraction cycles that loosen the connection. Heat cokes up a connection contaminated with oil or grease, adding further resistance. Installing more battery capacity does absolutely nothing to correct the heating problem.

What is you training and experience as a mechanic? When I wrote the tests to get my AME license, I had to know the regulations and "M" license stuff as per this document: https://tc.canada.ca/en/aviation/pu...ance-engineer-technical-examinations-tp-14038 and that was just the basics. I also had to prove experience in repairing the stuff. Getting the ticket in Canada is far more involved than in the US. In the end, I learned far more from experience than I did from any textbook.

 

[AV8R_87]

This idea of the back emf providing resistance to the current flow is known as inductive reactance.

That statement confuses two different phenomena. And completely misses the fact that reactance requires the presence of an alternating current.
One of them is caused by inductance, and appears due to a change in current which changes the magnetic field inside the coil. That's what you might see in a start solenoid.
The other one, which is what you see in a motor, is not caused by inductance (or inductive reactance), but by the motor acting as a generator and producing an opposing voltage. Which will happen even when the current through the motor is constant. This can become a problem, so on large starters you might see field weakening being employed to reduce the back-EMF in order to maintain a large enough current to continue accelerating the engine. If it was inductive reactance at play, this wouldn't be possible.

There. You just showed us why airplanes use smaller batteries. An extra 20-30 pounds is a big deal, especially when the system works just fine when it is maintained and rotten stuff is replaced or fixed.

Yes, but that still doesn't mean the system isn't marginal. It is so by design.
Just because it's intentional doesn't make it good. On the topic of maintenance, see below.

A small extra margin, instead of fixing the faulty system

Would you say the same about an engine that refused to start or make any useful amount if power if compressions dropped to 77/80? That's the same 3% decrease as is with the 0.4V loss in a 12V system.
Having to do significant engine maintenance every time you could't get 77 on a compression test would make you feel like that engine is a bit marginal, right? Especially if it all could be fixed by a less restrictive air filter, that doesn't drop your manifold pressure to half of the ambient.

What is you training and experience as a mechanic?

A lot less than yours, and not ashamed to admit it. That doesn't mean I don't have a clue about aircraft electrical systems requirements and design, though.

 

[Dan Thomas]

That statement confuses two different phenomena. And completely misses the fact that reactance requires the presence of an alternating current.

You don't seem to understand the series-wound electric motor as used in a starter. From https://studiousguy.com/dc-motor-working-principle/comment-page-1/



The armature gets its DC power via carbon brushes running on the commutator. In this animation, there are only two segments in the commutator, and the "coil" is a single loop. In the real thing, there are many coils, many segments, four brushes and four field poles.

And the current in those coils is being reversed twice in every revolution. See the reversing direction of the yellow dots. That is alternating current. And we get inductive reactance in the process, lots of it, which is why the amperage runs from 75 amps at full no-load RPM, to 560 amps when it's stalled. Faster RPM means a higher frequency of the AC in the armature, and much more reactance and resistance to the flow.

All this applies whether it's a series-wound motor, where the armature brushes are wired in series with the field electromagnets, as in the Prestolite, or with permanent-magnet field magnets as with the Skytec and Lamar and others, and with most auto starters now. That armature is still running on AC created by the brushes and commutator, while the field gets DC or is a pair of, or four, permanent magnets.



And as far as 77/80 compression being the same as a small resistance in a circuit? That's a false equivalency. We are testing static compression when we do the differential compression test, and it tells us where the leakage is and how bad it might be. There are places where it is acceptable (rings) and places where it is not (the exhaust valve). And that test does not tell us what the actual leakage is while the engine is running. There is VERY little time for much air to escape at running RPMs, which is a dynamic situation, not static, and the proof there is whether the engine meets static max RPMs when it is run up as per the TCDS. It actually takes a really leaky engine to fail that test. Brakes locked, throttle firewalled, RPM taken. For instance, for the OP's Cherokee 140, we get this from its TCDS:



Not over 2425 RPM and not under 2150 RPM, with a Sensenich M74DM or 74DM6 prop. There are slightly different numbers for a later airplane serial number series using a different prop. This is the normal means of measuring overall engine health for fixed-pitch prop airplanes. In a constant-speed prop engine, we have to rely more on manifold pressure.

Now, static leakage measured in the shop is on an engine far below operating temperatures, even if it's been warmed up. A hotter engine has smaller clearances at the rings and pistons, limiting the leakage. While running, oil is constantly being thrown into the cylinder, and one of the oil's functions is sealing.

Continental wants the differential compression tester calibrated before every use, to make allowances for the ambient conditions at the time. Atmospheric humidity, air pressure and temperature all affect the reading somewhat. They issued an SB on it:
https://www.aircraftspruce.com/catalog/pdf/servicebulletin.pdf

I regularly saw 43/80 when I did that calibration, meaning that Continental was fine with compressions as bad as that. I wasn't but they were, which is a stark indication that static pressure testing cannot tell the whole story. I'd want to see those cylinder bores if the rings were leaking that badly. I sure wouldn't pass an engine with an exhaust valve doing that.

 

[AvNavCom]

I can't speak for anyone else, but this thread has turned into an education for me!

 

[Dan Thomas]

I can't speak for anyone else, but this thread has turned into an education for me!

I just added some stuff regarding engine compressions.