Some Battery Recharge and Capacity Data for the Curious

[BFL13]

People might be interested in this for various aspects of it depending on what applies, if you need to figure out what kind of shape your own batteries are in.

My battery bank is four 6s, consisting of a pair of 4 1/2 year old Interstates (232AH) and a pair of 4 year old Exides (226AH) total 458AH as rated new.

Recent load test using the 20 hr rate for each pair done separately seemed to show they are now somewhere between 90 and 95% of rating (hard to be exact due to picking when at 50% by SG and then voltage bounce back, and ambient temp effect on capacity)

So cross-check is on a recharge using the Trimetric AH counter starting with full batteries (by SG) on leaving home and then normal camping draw downs to find assumed 50% before the recharge begins.
In this case, down 192AH and showing 12.1 volts, which looks like 50% to me. (no SG taken)

Did the recharge to assumed 90% marker (5 amps per batt at 14.5v, so in this case somewhat higher amps per batt at 14.8v--I used 7 amps here as a WAG.)

The recharge started at 156 amps then tapered as usual and at the 2 hour point (provincial park 9-11 gen time allowance) which is when I hope to complete a 50-90 with my set-up, I was back to -33AH and ambient temp was about 52F average (started at 48F at 9) and amps were down to 27 amps with batt voltage at 14.6. So I did the 50-90 in the two hours.

So now the big cross-check to measure bank capacity.

First I am declaring that was a real 50-90 and 192-33 = 159AH =40% restored and am assuming the Trimetric allowance for heat is correct.

So 100/40 x 159 = 398 for 100% full and 398/458 =87%. But how much of that is due to temperature and how much to battery wear and tear?

According to a graph I have, temperature at 52F would put them down somewhere around 7%. That would be on their actual capacity at four years old.

Playing with some numbers, if they are down 92% to 458 x 92 = 421 and temp effect is 7% of that, 93 x 421 = 392 which is pretty close to the 398 I got above.

So it looks like the 20 hr rate draw down load test and the recharge agree on about 92% capacity remaining in these 6s after four years. I have no idea how many cycles they have done, but it is a mix of 50-90s and fast recharges like above, and shallow daily cycles with solar.

I am very pleased with these batteries as my second set since starting RVing this way. My "learner set" pair of Interstate U-2200s were pretty much shot at the four year point and I had to toss them. With a lot of help from the Forum, learning what to do better, (Thanks all! 🙂 ) it looks like these batts will do way better.

[pianotuna]

Hi BFL13,

Dancing on the needles again?

Thanks for the report.

[BFL13]

LY, you are the scientist! My brain hurts too much trying to follow the voltage drop vs the angle of the dangle or whatever like you do. You do know where you are at all times, which is what counts.

The main thing is for people to keep track somehow! Total accuracy is not required for my situation, but it might be more important for others, especially if they have AGMs so they can't use an hydrometer.

Mex, I do worry, based on what you have posted, that I am living in a fool's paradise. I would not be surprised if my batts look good now, but have hardly any positive plate left and also have antimony poisoning and battery syph or whatever, so they have only two more cycles to go. 😞 I can't tell. All I can do is carry on carrying on.

The worst that can happen is I might have to buy new batteries, each of which costs the same as a tank of gas for the truck.

[MEXICOWANDERER]

Thanks BFL, as I sit here with the fan playing on me dressed in shorts and tee shirt.

That last 7% is not the easiest thing to deal with. With time limited charging my 14.8 volts @ 20c allows the specific gravity to keep raising after cessation of charging. The time it takes to raise another .05 on the hydrometer or if it even raises a full .05 is an excellent abstract (logarithm) to augment your tinkering. But it does require battery isolation. Just thought I'd mention this...

[landyacht318]

Thanks for the report.

My own data is less scientific. I pretty much watch my amp hour counter and voltmeter like a hawk, and my overnight loads are pretty consistent.

I'm not always awake to check morning voltage but my monitor does record minimum voltage and I usually reset that daily.

I've close to 200 deep cycles on mY group27 Northstar AGM rated at 90AH.

When I am at the 45AH from full mark about 6 hours into the discharge, voltage is in the 12.1v range under a 3 to 4.5 amp load and bounces back practically instantly when I shut down tv and laptop, upto 12.2x under a 0.7 amp load.

I think this battery's capacity is underrated.

What is noticeable is whether my solar was able to hold absorption voltage long enough to taper to 0.4 amps, or whether it was only 1.2 amps when the sun got too low in the sky to hold ABSV/Vabs.

Voltage under load is lower when it cannot, or after 4 solar only low and slow recharges from 50% even when it achieves that taper to 0.4a at 14.4v.

But give this battery a 40 amp meanwell charge or 75+ amps from my alternator and the voltage under load impressiveness is back.

A battery which requires the occasional high Amp recharge is both a curse and a blessing. Great as there is no fear of packing in the amps at as high a rate as possible, curse because solar only recharges to 50% is not enough to keep the battery happy, even when amps taper to the 0.5% of capacity at Vabs/ABSV.

I am going to continue to work this battery hard and heavy.