Battery capacity test

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perossichi
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Battery capacity test

Post by perossichi »

After I read Mike Busch, I started to do annual battery capacity tests. I turn position lights, strobe, and pitot heat which gives me a known draw and record time to run batt voltage to 20. Last year, I had 86% of capacity. This year 94%. I don’t know the magnitude of the measurement error but I think this just might suggest that desulphator trickle chargers work to desulfanate. I only added the desulfator charger this past year because I moved from a tiedown to a hangar.
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Re: Battery capacity test

Post by gtmize »

What's your known draw with lights / pitot?
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Re: Battery capacity test

Post by perossichi »

sorry for the delay.

Here are the amp draws I found for equipment in my DA40. I just turn these on and leave the avionics buss off.


AI and HIS = 1
Battery Relay .5
Strobe 3.4
Position Lights .68
Pitot Heat 5.8

I found these in the Garmin 1000 STC which has to show that the Garmin 1000 doesn't overload the system so there is a load analysis of other equipment in addition to the G1000. My plane does not have a G1000 but all of this other equipment is the same.

Note: that since battery voltage is 24V not 28V, we have to scale back the draw by about 80 per cent.

I have a Concorde sealed battery rated at 15 amp hour.

I hope this helps
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Re: Battery capacity test

Post by Steve »

Just a note...

Concorde specifies that the capacity test should be performed by discharging the battery at the C1 rate (15 amperes in the case of the RG24-15M, or 11 amperes in the case of the RG24-11M). Your figures, if accurate, only total about 11.5 amps. Plus, there may be other loads (avionics fans?).

I broke down and bought an electronic load battery discharge capacity tester that connects to my laptop, is programable, and records all of the data. You set the cutoff voltage (20 volts in the case of a 24 volt battery) and discharge current, and it does the rest. The only potential issue is that my device is limited to 200W, so at 24 volts you can discharge the battery at slightly more than half the C1 rate, so I have to correct for that in my calculations (instead of multiplying 1.667 times the discharge time in minutes, it is more like 0.926 times the discharge time in minutes) to get the % capacity.

Here is the Concorde Maintenance Manual for reference:

5-0171.pdf
(277.62 KiB) Downloaded 12 times
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Re: Battery capacity test

Post by perossichi »

Steve

I think the Mike Busch approximate approach works pretty well. My evidence — when I replaced this battery some years ago, I did a load test on the new battery using this methodology and got a capacity of 99 per cent.

Virtually no one tests load capacity. So this simple method is a huge improvement. Our planes are all electric so this becomes reasonably important. If my alternator failed in IMC, I’d like to know that I really can count on at least 30 min to get down. The emergency battery pack is not adequate to do anything much but fly to VFR conditions or fly a surveillance approach.

How much did your load tester cost? When I looked into this they were very expensive, perhaps this has changed. Can you share the details?

I note your tester can’t discharge at the correct rate. So I don’t see the advantage over what I’m doing. I just work on my plane as the battery is discharging and monitor bus voltage. Am I missing something?

The load testers I saw for 24v batteries some years ago were close to $1000.
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Re: Battery capacity test

Post by Steve »

Peter:

You are correct that the load tester I have cannot discharge at the full C1 rate. But it can discharge at an exact, regulated current, not affected by battery voltage, temperature, etc. For most batteries, discharging at less than the C1 rate will give a slightly optimistic capacity value. This is the one I bought:

https://www.ebay.com/itm/Electronic-Loa ... 2749.l2649

As you can see, it is far from $1K in cost. It is typical Chinese electronics quality, but the software does work on my laptop. My concern with your method is two-fold:

1) Your values for current draw are estimated, not measured
2) The actual draw of the equipment you have powered on will vary during the test, as the battery voltage drops

Your method will give an approximation of battery life if your alternator fails, under the specified load conditions. I may be too OCD for that approach. :) As an aside, the RG battery I replaced last summer had a capacity test of 71% when I replaced it. It was 5 years old. Out of curiosity, I opened the 12 gas valves on the top and examined the interior through them. The glass mats inside seemed dry to me, so I added 7.5cc of reverse osmosis water (used for my marine aquarium) to each cell. After charging I repeated the capacity test and it improved to 93% (This was an RG24-11M battery, so my gear could discharge at about 75% of the C1 rate). I keep it in the hangar as a spare (and to see how long my "fix" will last.

You are correct - no one does them, other than seeing how fast the engine cranks during startup. But they may be cheap insurance against getting stuck on some isolated ramp somewhere, or losing your alternator in IMC and not having enough electrons to get down safely.

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Re: Battery capacity test

Post by perossichi »

Steve

How do you account for the accuracy of my method with a new battery? That convinced me it was ok.

I agree with you that a real capacity tester would be best but I’m too cheap for that. As John Deakin used to say, that makes me an honorary ex airline captain.

What most impressed me is that a year on the battery minder trickle charger actually increased capacity. This is consistent with the shift from a hot tie down (very hot here at VNY) over one year to the next with a hangar and continuous desulfating. I was very skeptical of those claims but now I believe them.
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Re: Battery capacity test

Post by Steve »

perossichi wrote:
Mon Mar 23, 2020 4:11 am
Steve

How do you account for the accuracy of my method with a new battery?
Peter:

You don't know that it was accurate - you presume it was accurate because you think that you know the load placed on it. But, as I stated earlier, your current estimates for the different loads are just that - estimates. You didn't measure them with an ammeter, and the load meter in your airplane only shows the current that the ammeter is providing to charge the battery + power the load, and only works when the alternator is running. I would actually expect that the capacity of a brand new battery might be as high as 110%.

Also, your equipment loads are not constant current loads - the draw will vary with the battery voltage. Capacity tests have to be done with a constant current load to be accurate.

How much difference does it make? I don't know. It would be interesting to use your method, charge the battery, allow it to temperature equilibrate, then retest with a constant current capacity tester like the one I have. Academic exercise? Maybe, but at least we can do it while "social distancing"...

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Re: Battery capacity test

Post by perossichi »

Steve

I already conceded that my approach is only approximate but it seems to work pretty well under the assumption that a new battery would test at 100 percent. I’ll check with the Concorde tech support opinion that. If they support my assumption then I think I’ve got something useful.

As I said, no one will know for sure without a very expensive piece of equipment which neither of us seem to be willing to buy ;).

Certainly what I am suggesting is useful for trend analysis. The equipment draw does not change from year to year.
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Re: Battery capacity test

Post by Steve »

True. And like you said, most owners and mechanics never do one. So you are definitely ahead of the game...
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