EI Battery capacity test ?

Larry DeCamp · #1 10-30-2018, 06:41 PM

I have read the process for loading the battery with a ? known wattage? and then time the decay to a specified level such as when your EI CPU will quit. I have my RV4 set up with two totally independant power source charged by the alternator (with diode) for the second ignition. Recall R Knuckles says the main battery is the most reliable source of power.

My question is, instead of guessing ? on the work bench ? what load constitutes a resonable demand rate, why not turn off the alternator field inflight and watch the main and essential buss voltage ( second ignition ) decay . You then have two real measurments on endurance before it gets ?quiet? for both systems ?

rv6ejguy · #2 10-30-2018, 07:16 PM

You could do this but be aware that over time, all batteries will lose some of their current delivering capacity so time to battery depletion will reduce over time.

I load test my main backup battery once a year running a blower motor which draws 6 amps and 2 car headlights which draw around 2.5 amps each for about 11 amps total which is my actual current draw to keep my EFI running. I'll run that making sure battery voltage doesn't drop below 10 volts. If I don't get 45 minutes out of it, I change the battery.

For SDS CPIs running on Lycoming engines, figure 1.5 amps each for 4 cylinder ones and 2 amps for 6 cylinder versions each as a worst scenario. Old car headlights make a good load source using the low and high beams to tailor your current draw. 9V would be the minimum threshold for our EIs to test to although they will run lower than that.

I'd base your endurance on a conservative time from this test. If the battery lasts 1.5 hours, call it 1 hour in actual flight time to be safe.

Larry DeCamp · #3 10-31-2018, 05:54 AM

My goal here was to avoid any set up on the bench requiring an ammeter etc. You have done an excellent job of posting your current demands for various configs. I am proposing to periodically ( while flying) just watch the voltages go to 10V and thus have an easy update on battery endurance under real flight conditions. Just inviting the braintrust to point out something negative I may have overlooked. 😊

Bicyclops · #4 10-31-2018, 04:08 PM

I use a battery analyzer to check actual capacity using the same load my plane draws. Plot it on a graph that overlays the graph from last year, and I can easily see the trend. I terminate the test at 10.5v and come back later to recharge the battery. You don't want to leave it discharged too long. The whole test and recharge can be done in the plane using the battery charge cables that are already there.

Ed Holyoke
http://www.westmountainradio.com/cba.php

Carl Froehlich · #5 11-02-2018, 07:32 AM

As has been posted, there are various ways to verify you have the electrical capacity (amp-hours) that you have set for your design objective. Early on with the first plane I turned off the alternator and flew for 2.5 hours watching battery voltage, but I did shift the loads to just one battery so the protected battery was there if needed.

While such battery test discharges tell a story, I find that adhearing to a maintenance routine brings me more piece of mind for IFR flight. Here:
- One battery is replaced every three years. This means neither battery is more than six years old (PC-625 batteries). The pulled batteries go on to a second life in law tractors, test batteries for other builds, etc.
- If either battery is abused, it is replaced before next flight. This means if you leave a master on or such, pull the battery. Doing the heroics of bringing it back to life does not repair the damage you did to it. Its reserve capacity is now simply unknown.
- Never jump start an engine. If the battery is abused, replace it. If drained because of extended ground use or such, recharge it before flight. Taking off with after a jump start means you are launching into the ether with a significantly degraded reserve capacity on the hopes that the battery will recharge before you need that reserve.

Carl

BillL · #6 11-02-2018, 06:26 PM

Quote:

Originally Posted by Larry DeCamp

My goal here was to avoid any set up on the bench requiring an ammeter etc. You have done an excellent job of posting your current demands for various configs. I am proposing to periodically ( while flying) just watch the voltages go to 10V and thus have an easy update on battery endurance under real flight conditions. Just inviting the braintrust to point out something negative I may have overlooked. 😊

PbA batteries don't like to be cycled, especially a wide range for DoD, depth of discharge. The Odyssey AGM and it's more pure lead material is more tolerant of that, but not like the batteries designed and developed for it like Li and NiMh. A test using the main battery should be limited and start when it is fully charged.

VW, back in 1974, had a diagnostic plug put into the beetles. It ran a 4 second starting battery load test with low amps and measured the voltage drop in the battery to project reserve capacity. I talked to the field engineer and he said it had to measure milliamps and voltage to three decimal places. It was pretty good at predicting starting issues.

The point is . . . you can and should use a shorter test period with a low DoD. Since my EFIS voltage records voltage only to 0.1 volts, I would have to be careful to catch the time just as the voltage dropped the next tenth to reduce the error when plotting data. I am considering a 0.50 volt drop as a test, then plot against the published battery discharge data.

I think a ground test would do just fine if performed at similar temps, and ignoring the first 30 seconds of data.

Larry DeCamp · #7 11-03-2018, 04:13 AM

BillL makes a good point regarding depth of discharge affect on battery health/longevity. Since discharging to a specified voltage over time is frequently referenced as THE reserve capacity measurement, I will consult Earthex and Odessy for min voltage for recurrent testing and report back.

BillL · #8 11-03-2018, 07:07 AM

Quote:

Originally Posted by Larry DeCamp

BillL makes a good point regarding depth of discharge affect on battery health/longevity. Since discharging to a specified voltage over time is frequently referenced as THE reserve capacity measurement, I will consult Earthex and Odessy for min voltage for recurrent testing and report back.

Larry, as a start, Odyssey has a pretty good technical manual on their batteries. It addresses a discharge test and the ideal charging amps etc. It is a bit fluffy, but the technical information is in there. I got other good documents from their website. Brantel posted a good procedure for the rejuvenation of the starting battery. It is presented in a better way than the manual. IIRC it is about 6 amps. I can something in that range with selection of panel loads.

Larry DeCamp · #9 11-08-2018, 05:31 AM

The question is ? how to check battery condition without an external load and amperage monitor? ?
My proposal was to monitor voltage under normal use loads and confirm the time to cause a specified voltage drop that meets your endurance criteria.

Odessey says 11.2v is the lowest level to avoid battery degredation and 10v will be doing permant damage to the battery.

Earthx suggests a low limit voltage of 12.5v for routine testing of time interval. They also advise that when the time degrades 20 to 30 percent of new condition, replace the battery. Just a CYA note in the Earthx response ? this should be done on the ground and rechargjed with a charger not the aircraft?. This is understandable but not useful for the independant endurance buss for an EFII system.

I hope this helps someone else .