Battery Life

[Marauder]

If they attached the "ground supply" directly to the battery your ammeter would show a "discharge" but the voltage should have been well above 12v (e.g. around 13.8) assuming the Aspen's voltage readout was accurate.

The amperage displayed on the Aspen when running the avionics from the battery (with or without a supply or charger attached to the battery) should give you an idea of the current required of an external supply for "ground operation" of your avionics although you should add a couple amps for the master contactor.

If you have a "basic" stack (a GPS, navcom, transponder, and audio panel) plus the Aspen I'd expect your nominal current to be in the 8-12 amp range. The least expensive option (other than borrowing a supply) would be something like this:
tinyurl.com/12vSupplyOnEbay

You'd need to add a power cord and appropriately sized wires to the battery (preferably with and inline 25A fuse in the positive lead). Most supplies like that one allow you to raise the output voltage to a bit over 13 which would be adequate. You might even find a 14v or 15v supply, either of which could be adjusted down to 13.8v which is ideal.

You can use a simple (manual) battery charger but there are issues with this because they are not regulated or even filtered. They can damage a fully charged battery if left on too long when the load is light and are likely to let the battery discharge when the load is more than half the charger's rating. An automatic charger may be worse depending on it's complexity. Many modern automatic chargers involve a "computer" scheduled charge cycle and a load connected to the battery being "charged" can confuse the "computer".

If you do go the charger route, make sure the battery is always in the circuit (i.e. don't connect the charger to an external power connection on the airplane without the master turned on) and pay attention to the battery voltage during the operation. If the voltage is less than 12.6 the battery is discharging and you will need to operate the charger without the avionics for some time to replenish the battery's charge.

Thanks for the detailed explanation and the link (is it correct?)! I am pretty sure the voltage was below 12VDC while on the ground supply because my airplane's low voltage indicator was on.

I don't have an external power connector, so the connection is solely on the battery.

 

[Bill Watson]

If you do go the charger route, make sure the battery is always in the circuit (i.e. don't connect the charger to an external power connection on the airplane without the master turned on) and pay attention to the battery voltage during the operation. If the voltage is less than 12.6 the battery is discharging and you will need to operate the charger without the avionics for some time to replenish the battery's charge.

... very important and totally agree.

In my case, I anticipated 1) doing a lot of avionics operation in the hangar 2) entering flight plans and clearances before engine start. So I designed my RV10 electrical system to facilitate both things by having 2 main batteries. I simply use 1 or both of my batteries to run the panel as needed. Then I charge them.

Certified avionics are spec'd to operate at 10 or 11 volts minimum. So, assuming the collective load of say a G430W, SL30, audio panel and 3 experimental EFIS screens can run off a battery for the required time without dipping below 10 or 11 volts, that has worked for me. To facilitate that I did 2 things.

1. I have an easily accessible charging port for the hangar work.
2. I have 2 batteries with one driving all the avionics required for flight path and clearance entry. The other is used for engine start. The two batteries can also be linked together when needed.

I understand that all this may not be feasible for certified AC but I would make the point that running your panel for extended periods on the battery is doable. But if you do it, be sure to use something other than a Battery Tender type maintainer to recharge the battery and don't leave the battery lying around in a discharged state. I learned this the hard way and it cost me 2+ batteries during my panel build. I later found that using the Schumacher charger I described earlier would fully recharge the batteries and do it relatively quickly (generally less than 1 hour).

I never left the charger on while operating anything, so I have no experience with that option.

 

[jesse]

Certified avionics are spec'd to operate at 10 or 11 volts minimum. So, assuming the collective load of say a G430W, SL30, audio panel and 3 experimental EFIS screens can run off a battery for the required time without dipping below 10 or 11 volts, that has worked for me.

Pulling the battery down to 10 or 11 volts is quite damaging.

 

[Bill Watson]

Pulling the battery down to 10 or 11 volts is quite damaging.

I think you mean that discharging a battery so it only produces 10 or 11 volts with no load is damaging (?). I'm under the impression that engine starts routinely pull batteries down that low if measured during the start, but that the voltage should bounce back up to over 12 when the load is removed.

I never put a volt meter on it so I don't know how far I pull them down when using the avionics. Turns out that while my certified avionics will continue to operate, my experimental EFIS units go dark before any of the certified units go dark so they kind of act as an inadvertent discharge limiter.

Not sure where I picked this up at but I've been using this test for several years on my batteries at inspection time. (Does anyone know if this is similar to what Concorde specifies for battery testing?) Note that the battery can be pulled down below 10 volts while loaded and still be considered 'good'. Generally, my good batteries test around 10.5 volts at the end of the load test with the load still applied. They test 12.6+ volts with no load after the load test. I have not seen any damage from this yet though I am careful to fully recharge the battery immediately after the test.

TEST PROCEDURE:
Step 1: Battery terminal voltage must be 12.60V or greater to test. If the battery voltage is below 12.60V, charge battery before testing.
Step 2: Connect the Load Tester cables and the test leads of a separate Digital Voltmeter (±0.25% DC accuracy, if the Tester has no direct DC Voltage Meter) to the battery terminals.
Step 3: Adjust the Tester Current Load to achieve ½ the Cold Cranking Amperage (CCA) rating of the battery and apply it for 15 seconds.
Step 4: At the end of 15 seconds, note the battery voltage on the Tester Voltmeter or the Digital Voltmeter and discontinue the test.
TEST EVALUATION:
This test is a pass or fail. Compare the recorded test voltage to the chart below. If the battery test voltage is above the appropriate voltage at the determined battery temperature, the battery tests OK. If the test voltage is below, then the battery failed and needs to be replaced.

BATTERY TEMPERATURE vs. END OF TEST VOLTAGE

70° + - 9.60V
60° - 9.50V
50° - 9.40V
40° - 9.30V
30° - 9.10V
20° - 8.90V
10° - 8.70V
0° - 8.50V

 

[bbchien]

Lynn, I use this one, and my Concordes make in excess of 7 years.....http://www.northerntool.com/shop/tools/product_200589669_200589669

 

[hiflier]

"While I can see this being an important requirement in the summer it seems that in the winter when it is cold it couldn't be that big of a problem."

Good point. There seems to be a certain amount of shadow boxing going on between the charger and aviation battery companies. The Battery Minder literature shows an optimum charge voltage at <40* of 14.9 and float at 13.9. At >120* 13.4 and 12.45. Big difference. I think most of the cheap little smart chargers from Battery Minder and Battery Tender companies charge the battery to about 14.4 briefly, then drop to about 13.3. Should be fine to use below 65* or so. If you have a volt meter, you can easily check your charger's profile.

 

[gismo]

Pulling the battery down to 10 or 11 volts is quite damaging.

Depends on the battery and the load (assuming a lead acid battery). Discharging a 12v LA battery to 10v over a relatively short period of time won't significantly affect it's longevity as long as it's returned to a normal state of charge shortly after and neither the discharge nor the recharge heats the battery up enough to warp the plates.

 

[jesse]

Depends on the battery and the load (assuming a lead acid battery). Discharging a 12v LA battery to 10v over a relatively short period of time won't significantly affect it's longevity as long as it's returned to a normal state of charge shortly after and neither the discharge nor the recharge heats the battery up enough to warp the plates.

I was replying to the notion that you can discharge a battery to 10 volts while ground running your avionics which would be a very bad idea.

Just because your avionics would work at 10 volts doesn't mean your battery will be around much longer.

I'd be limiting myself to 12.5 or so while running the avionics.

 

[gismo]

I was replying to the notion that you can discharge a battery to 10 volts while ground running your avionics which would be a very bad idea.

Just because your avionics would work at 10 volts doesn't mean your battery will be around much longer.

I'd be limiting myself to 12.5 or so while running the avionics.

A fully charged aircraft battery won't provide 12.5 v for more than a minute or two with a 5A load (typical for one or two radios). But you're right that draining one to 10v with the same 5A load (should require several hours) isn't a good idea.

 

[Tom-D]

I was replying to the notion that you can discharge a battery to 10 volts while ground running your avionics which would be a very bad idea.

Just because your avionics would work at 10 volts doesn't mean your battery will be around much longer.

I'd be limiting myself to 12.5 or so while running the avionics.

read the data tag of the av equipment, it will give the min voltage, and it usually a lot lower than that.

I run the coach batteries down to 9.5 volts before I start the gen set and recharge to 14.8. they are over 5 years old.

 

[jesse]

read the data tag of the av equipment, it will give the min voltage, and it usually a lot lower than that.

I run the coach batteries down to 9.5 volts before I start the gen set and recharge to 14.8. they are over 5 years old.

Just because the avionics are certified to run at 10 volts does not mean that draining a 12 volt lead acid battery down to that 10 volts is a good idea. That's very hard on a lead acid battery. The voltage drop isn't linear, it'll fall fast towards the end.

As to your RV I don't know what batteries it has. But even a deep cycle marine battery shouldn't be brought below 12.

I have no alternator or generator in the Flybaby but do have strobes, nav lights, and a com radio. Runs off a 12 volt motorcycle battery. I don't let it get below 12.3 volts.

 

[bbchien]

read the data tag of the av equipment, it will give the min voltage, and it usually a lot lower than that.

I run the coach batteries down to 9.5 volts before I start the gen set and recharge to 14.8. they are over 5 years old.

I'd be willing to bet they are "deep cycle" batteries with huge egg grates for the plates....

 

[Bill Watson]

The key distinction in measured voltages is whether the reading is being taken while the battery is under load or whether the reading is taken with no load.

The batteries we use are all designed to start engines. This puts a substantial load on the battery for a short period. When loaded during the start, the battery's voltage will be well below 12 volts but this is not damaging to the battery. But once the load is removed, the voltage of a healthy battery will immediately return to a 12.5+ reading (and it will slowly rise up a few more tenths of a volt over the next 10 or 20 minutes).

As far as running avionics on battery-only, seeing a voltage of 11 volts with all the avionics operating is one thing. Running the panel for 2 hours on battery, then shutting it down and measuring the no load voltage at 11 volts is another thing. In the first case, I don't think any damage is done as long as the battery voltage returns to 12.6+ volts once the load is removed. In the second case, if the voltage reads under 11 volts with no load, the battery has been trashed.

I'm running 2 Odyssey 680 batteries in my RV10. There is a PMA'd version of the battery too but the non-PMA version is of course, cheaper. Great price performers they are.

 

[Jim_R]

I run the coach batteries down to 9.5 volts before I start the gen set and recharge to 14.8. they are over 5 years old.

Maybe, but like Dr. Bruce says, that's likely a different battery. From Concorde's RG-series battery owner's manual:

State of Charge vs. Open-Circuit Voltage (12V battery)
100%: 12.9V or above
75%: 12.6V
50%: 12.3V
25%: 12.0V
0%: 11.7V

Note that there is this also, though, which may play into those numbers:

WARNING: LOW CAPACITY HAZARD. Aircraft batteries are certified to have a certain minimum capacity for emergency operations in the event of an electrical generator systems failure. Never use a battery that has less than 80% of rated capacity.

In general, if you deep-cycle a battery not designed to be deep-cycled, you'll kill it faster than if you use it the way it was intended (very short, high-current load followed by immediate recharge).

 

[denverpilot]

read the data tag of the av equipment, it will give the min voltage, and it usually a lot lower than that.

I run the coach batteries down to 9.5 volts before I start the gen set and recharge to 14.8. they are over 5 years old.

Those are typically deep cycle batteries in RVs.