Study of dissimilar battery strings in parallel

techntrek · February 2016

For daily solar discharge from shallow to the usual 50% DOD, the old rule still applies - keep them the same size and age. But the rule seems to differ for other uses like occasional discharges in an RV or UPS, or for regular deep discharges. This is the summary:

Such “asymmetrical” batteries can be operated quite successfully and yield nearly their complete cycle life potential when deep discharges to 100% d.o.d C3 are carried out. If power outages cause only shallow discharges then the “low” impedance string furnishes an overproportional share of the Ah discharged and may thus suffer in terms of achievable cycle life

http://neuralfibre.com/paul/wp-content/uploads/2007/05/can-we-now-sin.pdf

MarkC · February 2016

I can sleep well tonight knowing my spare set of (new) Trojan deep cycle batteries (in a battery case) will likely perform well with my (old) Interstate deep cycles (in my golf cart) in emergency conditions as I will likely take them both to high DOD's every night - but hopefully only for a couple of weeks at the most. The "test" emergencies have so far indicated both strings deplete (in SG anyway) nearly the same.

However, would FLA's give differing results? Also, in my setup, there is probably not so much "asymmetry" as the normal aging with the attendant increase in impedance might be quite minor? Other factors of impending failure of individual cells (especially in the older batteries) might be the overriding factor. If I'm understanding correctly, the lower impedance (in my case the newer?) string gets the early high current loads, which taken to an extreme, might actual damage the new batteries? Maybe they will do a similar study on strings of identical batteries of one aged and one new.

Anyway, thanks for referencing an interesting study!

vtmaps · February 2016

From the paper:

The discharge and charge cycle routine, carried out at room temperature, consisted a) in a discharge with the combined C3 rate current of 27.8A (6CP100) + 152A (2CP550) = 180A to 1.80Vpc or 21.60V and b) in a recharge under constant current-constant voltage conditions with 100A starting current and a charge voltage of 2.25Vpc (float charge voltage). The charge time was limited to 21h and one full discharge/charge cycle was carried out per day.

OK... so they discharged the batteries at C/3 rate, and flattened the batteries. It took about 3 hours.

Then they recharge the batteries and limit the charge time to 21 hours.

Maybe this research is applicable to folks living above the arctic circle on the summer solstice... 3 hours of night and 21 hours of daylight.

--vtMaps

Study of dissimilar battery strings in parallel

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