voltage slow to come up

[Dan Thomas]

The regulator varies the field voltage to control the output voltage. As loads increase or RPM decreases, the field voltage must rise to keep output voltage constant.

I made a device to measure all these things while the engine is running. Makes troubleshooting much easier.

 

[xander75]

And Dan, not dimishging your expertise or anything, but probably a picture says more then 1000 words...
I always talk from experience and personal knowledge, that is what I share here.

 

[Dan Thomas]

And Dan, not dimishging your expertise or anything, but probably a picture says more then 1000 words...
I always talk from experience and personal knowledge, that is what I share here.

View attachment 121178

That is output voltage, not field voltage. Surely you know the difference?

 

[3393RP]

Maybe I should enhance my write up that you'd want to see the 13.9 when the engine is running and the alternator is working.
I agree with you that the batt only voltage needs to be 10+ in order to start the engine, but from my experience if your battery is on the way out it shows a drop on the initial voltage when you flip on master and of course dropping definitely during start.

Point I was making is merely to look at battery and alternator separately...
x

Battery voltage of "10+" isn't going to start an engine.

When battery voltage drops below 11.7 or so it's at a zero state of charge.

Just my "experience and personal knowledge."

 

[Dan Thomas]

https://www.cessnaflyer.org/magazin...item/1306-alternators-electrical-systems.html

Too many folks, even some mechanics, do not understand the charging system. When mechanics don't understand it, the repairs can get costly as numerous expensive components get replaced in the hope that the problem will go away.

 

[GeorgeC]

Then pull all circuit breakers or remove all fuses. Turn on master and reset each breaker one at a time till you see a voltage drop, or better yet get a clamp on ammeter and check each circuit. The ammeter will show the current flow to each circuit. Keep a log of your readings.

No joy. About half the voltage drop was the alternator field, but there's still 0.2V or so unaccounted for. Maybe I'm barking up the wrong tree and/or at the point of diminishing returns.

 

[Bell206]

Maybe I'm barking up the wrong tree

Have you determined yet if this is on the indicating side vs the actual charging side? Did I miss that?

 

[xander75]

No joy. About half the voltage drop was the alternator field, but there's still 0.2V or so unaccounted for. Maybe I'm barking up the wrong tree and/or at the point of diminishing returns.

Nah, I am very similar, I need to have an explanation for everything, you're very detailed, as one should be as a plane owner obviously, from all the detail you shared it starts to feel you have a battery that is getting tired.
Note btw that OC (open circuit) voltage is always higher then when on load. To test a battery (of whatever kind really) you want to measure the voltage when it experience some (reasonable) load.
A(ny) battery that is getting old(er) would exhibit increased internal resistance, while a volt meter by nature has a very high resistance, you don't really see that effect. As you don't see the voltage drop over the internal resistance.

How old is your battery? And are you at times in a cold(ish) climitate? Is it a wet or dry battery?
For comparison, I had to change my Concorde XC after 5 years, this apparently is "normal" (ugh, another $500), but yeah had the same thing with starter cranking I dropped to 10.5V at the time, in the winter.

If you're FBO has a "starter pack" maybe you can do that test with that on your battery, if you see something readings you'd expect, it is likely your battery is suggesting time for replacement.
xander

 

[Bell206]

I had to change my Concorde XC after 5 years, this apparently is "normal"

Not necessarily normal. Battery life is usually directly related to the aircraft electrical system condition and how the battery maintained. The averages I've seen with a good system and no abuse are over 7 years with some to 11 years on the private GA side with AGMs.
And as a side note performing an OEM battery capacity check will give the most accurate condition of a battery which is sometimes required for certain requlatory requirements.

 

[Dan Thomas]

To test a battery (of whatever kind really) you want to measure the voltage when it experience some (reasonable) load.

A good shop has this, or similar:



You set the battery's rating (such as 25 or 35 amp-hour) and the tester puts a drain on the battery until the voltage drops to a calibrated level. The tester then displays the battery's capacity in percent of new. Gill says anything under 80% needs to be replaced.

I had to change my Concorde XC after 5 years, this apparently is "normal" (ugh, another $500), but yeah had the same thing with starter cranking I dropped to 10.5V at the time, in the winter.

Might not have been the battery. An awful lot of batteries get replaced because the master and/or starter contactors are bad, with eroded or oxidized or burned contacts. Everything goes fine for awhile until the new battery gets a little bit weaker, and then the voltage at the bus gets like your 10.5. In your case, it would be the master contactor.

Ohm's Law matters. The voltage drop across that contactor is relative to the contactor's contact resistance. Big current draws, like the typical 250 amps for the starter (and yes, it passes through the master contactor) will cause a big voltage drop even at tiny contact resistances. E=IxR. 1/50th of an ohm will cause a five-volt drop; .02 ohms times 250 amps equals five volts.

A good mechanic knows how to take voltage measurements across the contacts when the starter is cranking. He can sometimes replace a $40 contactor instead of a $500 battery and get more life out of that battery.

 

[GeorgeC]

No joy. About half the voltage drop was the alternator field, but there's still 0.2V or so unaccounted for. Maybe I'm barking up the wrong tree and/or at the point of diminishing returns.

Speaking of Ohm's Law... duh. I forgot that the solenoid itself pulls some current (book number, 0.8A), and the battery has an internal resistance (book number, 11mΩ). My Odyssey is a couple years old, so I reckon a drop of a tenth of a volt or two is in family.

 

[GeorgeC]

An awful lot of batteries get replaced because the master and/or starter contactors are bad, with eroded or oxidized or burned contacts.

That was my hypothesis going in, and I certainly wanted to believe it when I saw that my master contactor was original.

I couldn't have been more wrong. I got the old part opened up and it looked brand new inside, not a spot of carbon or corrosion.

The only battle scar was some of the plastic housing was melted, like someone shorted it a long long time ago.

 

[Dan Thomas]

That was my hypothesis going in, and I certainly wanted to believe it when I saw that my master contactor was original.

I couldn't have been more wrong. I got the old part opened up and it looked brand new inside, not a spot of carbon or corrosion.

The only battle scar was some of the plastic housing was melted, like someone shorted it a long long time ago.
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You sure that's the master contactor? Located right next to the battery, with two thick cables on it? That one wire looks to be maybe 12 gauge, not nearly what we'd find on a master.

Other than that, it does look very good.

 

[GeorgeC]

Yes, there's a pic of it installed earlier in the thread. The small wire you see jumpers the positive side of the battery to the coil. The master switch grounds the other side of the coil to complete the circuit, energize the coil, and close the contacts.