Question on battery bank configuations

elesaver

If a battery string has 4 ea 6V batteries wired in series to create a 24V bank, what is the issue with having 3 strings for a total of 12 batteries to increase the amp hours available?

Cariboocoot

Re: Question on battery bank configuations

elesaver wrote: »

If a battery string has 4 ea 6V batteries wired in series to create a 24V bank, what is the issue with having 3 strings for a total of 12 batteries to increase the amp hours available?

Parallel connections create different resistances so the current flow is not necessarily divided equally between the three battery strings. One string may handle most of the current while the other two have different amounts. This creates an imbalance in the battery usage which will increase over time.

So even though the battery bank should be 3X Amp hours one string may be discharged 40% another 20% and the third 35% or any such combination meaning you don't have full battery capacity available. The reverse is true on charging: one may be brought up fully while others never quite make it. The effect won't show up immediately, so it is not always apparent that there is something wrong (if there is something wrong; parallel strings can work they just create a potential problem) until it's too late.

waynefromnscanada

Re: Question on battery bank configuations

I had a 3 string parallel bank for over 10 years. Worked OK, but definitely required a good bit of monitoring and equal length battery cables etc. Their death was sudden. One string died, the other two covered for the dead one, then the second failed, followed quickly by the third. Like cops gone bad, they covered for each other till the lights went out. Present system is single string of 2 volt batteries. The difference for me in many ways is awesome and I'd never want to go back to a multi-battery bank.

elesaver

Re: Question on battery bank configuations

OK. Thanks for completing my understanding. I'd love to do the 2V batteries but I cannot imagine working with this kind of weight in a battery. I do well to move the 65 "pounders."

Now, since it is not desirable to have multiple pairs of strings, how in the world do you get to a larger amp battery bank? I'll bet I know after all...have them hooked to a different controller and daisy-chain the controllers as midnite solar show their "follow me" hook up. Is that the way it is done for those who can't work with such heavy batteries?

Cariboocoot

Re: Question on battery bank configuations

Your first level of defense against parallel batteries is larger size. But weight per unit does then become a problem.

A 500 Amp hour 2 Volt cell weighs about 75 lbs. Not much worse than a GC2 at half the capacity. That's the trick: you're only dealing with one cell at a time.

Batteries are heavy when they're made of lead. LiFePo's are lighter if weight is a problem. But you need to plan differently to use them.

Your second level of defense against parallel batteries is higher system Voltage. Again you run into limits and it's not convenient to have to change the inverter (and a few other things perhaps) because you started at 24 and now need 48 for the extra capacity it offers.

Parallel battery strings can work, you just have to realize the possible trouble you may have and keep an eye on them. If you want to go whole-hog you can put separate shunts on each string with its own monitor (a Pentametric will actually monitor three shunts on its own) and watch for imbalances, correcting them before they become an issue.

The arrangement of charge controllers and arrays is irrelevant to the battery bank configuration. If you go above 800 Amp hours you probably need multiple controllers & arrays so yes the Classics in Follow Me is the best way to go there, but it's not strictly necessary; many systems have multiple controllers that aren't even the same type much less model. Not ideal, but can be made to work.

And then there is the multiple system option, where loads are separated and you have fully-independent power systems for each load group. (This usually comes about as a side effect of expansion where people put in a small system "just for lights" and then discover they need something much larger for power the other loads they suddenly can't live without.)

elesaver

Re: Question on battery bank configuations

So much to know...so little time.:D I've always said that "education is a process." And, now I see that's an understatement! Thanks again for the time to help educate me.