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Old 09-05-2006
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Well, Dave, I guess I'm the Devil's Advocate here.

GE, in their old industrial battery book, says right out that parallel batteries can work perfectly well. But then goes on to say they will work best when each set of CELLS is paralleled, and then the parallel banks of cells are connected in series. (i.e., three cells in parallel, six banks of them, to make 18 cells combined in the final battery. Not, two six-cell batteries put in parallel.)

I would guess that they suggest this because it "plays the odds" better, averaging the capacity of each group of cells rather than playing one group of six against another, where the variations in SIX cells accumulate in each battery.

But, as you note, there are real life considerations to be taken view of and I suspect it is the real life variations versus the pure ideal where the problems can arise. Or, be eliminated by simply not paralleling.

So, I'd like to suggest some investigation along these lines:

Consider your case of two 100AH batteries running in parallel from one charger, which for systems of that capacity usually means one 60-90Ah automotive alternator with integral automotive regulator. I'm not sure where to begin quantifying anything that specific. We know that the regulator will throttle back the charging amperage when it sees about a 90% charge, and we know that there will only be one charge sense lead, which is reading the average of the two batteries. So we can begin by noting that when the AVERAGE state of the two batteries has reached 90% capacity (80, 90, whatever, that's another example of the differences in specific equipment) capacity, the system will go into trickle charge mode. Which is a problem regardless of battery configuration, but even more so with parallel batteries I'd think one of them will be cooking (boiling out electrolyte and overcharging) while the other is undercharging more than it would alone.

Or don't you think this "charge abuse" will be greater for two batteries in parallel than it would be for separate banks?

Have you actually measured or metered parallel batteries to see the charge states they reach, versus the same batteries each separately charged?

And, given the variation in batteries these days, have you compared mutiple batteries "off the shelf" to see how closely they do or don't compare in voltage and capacity, even when they are twins off the same line? (Assuming twins stay together all the way into the boat?)

"...but with two in parallel, the amount of current may be limited by the “good” battery, depending on the charger." Current limited by the good battery? Not when there's just the one charge sense lead, reading the average of them. Of course, as soon as that average (pulled up by the good battery) reaches a set point, the *voltage* produced by the alternator will be dropped and the current reduced. But AFAIK in the common automotive alternators, they may be limiting voltage or current, there are a number of different schemes and float ranges.

And then, at what point, at what size of battery banks, do we assume boat owners have switched to real marine outboard regulators and real marine high power alternators?? Because at that point, charging can or will change, won't it?

"Because of the limiting effect of the good battery, both batteries will reach full charge state at about the same time. (Assuming full discharge was the initial state of both batteries. ..."

Ouch! Assuming full discharge?! That's outright abuse. Who ever recommends taking batteries below the 50% discharge point? Makers may claim they get the most "lifetime power" out of a battery by cycling it to 40% or 70%, but they all seem to center on 50% and they all say a full discharge can kill a deep cycle battery at least 10x faster than any lesser charging state.
And reaching a state of full charge? Well, again, unless you've been motoring all day, or using a marine regulator, the more common automotive type won't get you above a 90% charge for hours after that point. If you're just running the engine as much as you need for recharging, and shutting it down after that...odds are you'll never see more than a 90+% charge.

"During discharge, ... Both batteries will be at the same state of charge during the discharge." Is that from theory, or observation? At what discharge rate?

And I think it really should be pointed out, the case of cruisers who are regularly cycling their batteries with daily cycling, is going to be very different from the typical weekend sailor--where the batteries may be sitting (one hopes OFF) for five days, then sitting in parallel but feeding only a small load, like the instruments, for six or eight hours during the day.

"If the batteries are left off the charger they will drift down to their open circuit voltage after several hours." That's too generous. I've seen battery makers who suggest that it will take 10-24 hours for the electrolyte to equalize out the charge in the battery, not "several" hours. With an older 12v battery, I've seen that initial charge burn off overnight but then continue to settle, visibly, for longer.

"If ...the open cell voltage of both batteries should be the same for given state of charge. (close enough anyway). Since there will not be a potential difference between the batteries there should be minimum to no current flow between batteries. " But that's a gross assumption. Unless the batteries come from the same maker and the same production batch, the alloys used in them can differ. Different makers and lines intentionally use different alloys making that the greatest problem, but battery making is not watchmaking, mechanical differences in assembly are normal.

"Any difference will be equalized once the batteries stabilize any specific gravity differences. " I'd been told that's not the case, and that is the problem. They WILL equalize, yes. And then as each battery is different, and has a different rate of self-discharge (among other things caused by the physical changes and sulphation happening differently in each battery), as soon as they have equalized they begin to drift apart and the current loop between them starts to flow again. This process does not stop, it continues and it drags them down.

I suppose the simplest way to test this would be to take two "good" batteries, put them on a very smart charger to make sure they are properly charged, and then record their discharge rates over the next month. Then repeat the process, alternating between batteries that were in series and in parallel. Perhaps using two sets to get better data. (What, a month if too long? OK, two weeks? One week? How about run the month, and let's see where the data cross?)

I'd volunteer to do that, but don't have the resources to do it.

"However, any charge taken from the “good” battery will be stored in the “bad” battery minus resistive loss again. Again the battery system capacity should remain constant." So you're familiar with the endless looping...but isn't that the catch? Just resistive loss? Can we put a number, a percent, on that? Again, measured, and with varying installations?

"Does this mean that you should not replace all of your batteries if you find one bad? Maybe, or maybe not." I'll agree with that!

But given all the variables, I still think that paralleling batteries gives the system opportunies for failure that simply do not exist when using batteries in series, or separate banks. Given optimization of the charging system (capacities and rates matched, etc.) I don't see anything to be GAINED by paralleling, except the chance to buy cheap 6V or 12V batteries from convenient sources.

Maybe we can get Practical Sailor to give us a stipend to do the lab testing, because without that...all we can say is "if, maybe, it depends, it might". Personally, I'd rather use a system that doesn't rely on those terms.

Last edited by hellosailor; 09-05-2006 at 04:00 PM.
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