Two Parallel battery strings?

[PNW_Steve]

Hey Everyone,

I understand the perils of parallel battery strings and have managed to avoid them.

The other day I read an article that addressed the topic. The info in article fit my understanding except for one item. The author insisted that 2 strings in parallel was not an issue. He said that 3 or more parallel strings is where the problems begin to show up.

Is this correct?

I have a project coming up that I was planning to run 4 - L-16 batteries in series to give me a 24v - 400a/h bank in a single string. After reading the article that I mentioned I started wondering if running 2 parallel strings of 4 GC-2's would play well or not?

I can get GC-2'S from Sam's Club for around $87 each. The L-16's are around $300 each. That yields a 30% or greater savings on the batteries.

What do you all think?

[bob-n]

I can't say what the original author was thinking, but I can tell you why some people say 2 is good and 3 is bad.

With two batteries in parallel, it is relatively easy to equalize wire length and voltage drop to each battery. With 3 or more, it is impossible.

parallel.gif

You have to use your imagination a bit, but in the above picture, the lower - wire to BAT2 is long and the upper + wire to BAT1 is long, while the other two wires are short, so BAT1 and BAT2 see the same voltage. Yes, it's an awful picture, but you get the idea.

Now try to draw a diagram where the voltage drops to each battery is the same with THREE batteries. Can't be done. The best you can do is use really fat wire.

[SunEagle]

It really is hard to say. There have been some members that have multiple parallel strings of batteries and feel they are working well. Others have seen their bank die an early death with just 2 strings.

All I can say is that any more than 1 string of batteries creates multiple charging paths across different resistances. Physics says that is not the best way to charge or discharge batteries.

But if the cost is low and the batteries don't get abused I would say you have a chance of a good system with 2 parallel strings. Although you get what you pay for and cheap batteries may not last as long as high quality type.

It comes down to what a person wants to experiment with and what they can afford.

[Mike90250]

If you have good stout cables and proper lugs crimped on them, I'd get the dual GC2 battery bank a try at least. Getting more than 3 years out of it would be awesome cost savings.

If it dies early, have a way to document the care each bank got, so something like undercharging the bank does not happen to your L-16's

[PNW_Steve]

Thanks for the replies.

I was considering GC2's from Sam's Club. I went to verify the pricing and found that they have closed all of their stores in Washington.

Back to battery shopping I go...

[Mike90250]

there is still costco , wallmart and other big boxes

[PNW_Steve]

there is still costco , wallmart and other big boxes

I have read pages and pages of positive discussion and reviews on the Sam's Club (East Penn) GC2's. I was feeling pretty good about the value.

Unfortunately, the CG2's that Costco don't elicit much as far as good reviews.

I looked for GC2's on Walmart's website and found them 'in store only'. I went to the local store and asked. They were clueless.

Back to the search

[SunEagle]

I have read pages and pages of positive discussion and reviews on the Sam's Club (East Penn) GC2's. I was feeling pretty good about the value.

Unfortunately, the CG2's that Costco don't elicit much as far as good reviews.

I looked for GC2's on Walmart's website and found them 'in store only'. I went to the local store and asked. They were clueless.

Back to the search

I have been disappointed many times with Walmart due to their website indicating something was available in the store (even their super big ones) only to find out it wasn't and had to be ordered.

[J.P.M.]

I have been disappointed many times with Walmart due to their website indicating something was available in the store (even their super big ones) only to find out it wasn't and had to be ordered.

FWIW, I gave up buying online from Wallyworld after several useless outcomes of the wrong stuff being sent. My bad for being so trusting and not smelling the coffee sooner.

[PNW_Steve]

The only thing that I have ever bought from Walmart online are mattresses. Four do far. I had good luck with those.



I may have found a reasonable buy on batteries. Solarbiz has Crown GC2's for $104 with free shipping and no core charge.

[Robbo123]

couldnt you put a supercapacitor as a firewall? both strings feeding into that with even perhaps a resistor very close to the battery bank to limit the draw. The overs and unders will be taken care of by the supercap.

[Mike90250]

Resistor = massive power loss, and will likely shut down your inverter from undervoltage

Supercap as a firewall. It may create fire in a way you do not expect

[Robbo123]

then please educate us on supercap fire risks? Cant seem to see anything on the net......

[Mike90250]

Somehow, folks have gotten the notion, that supercaps are magical sources of power. No. They store large amounts of power, which can lead to "releasing large amounts of power" uncontrolled.
Here is the tech data from one major mfg.

https://www.murata.com/~/media/webrenewal/products/capacitor/edlc/techguide/electrical/c2m1cxs-053.ashx

They have finite lifetimes, often less than 6 years
They can have high internal inductance
The cable leads have high inductance. inductance is related to resistance, and as the resistance increases, the surge power of the cap is choked off.
placing in series to increase voltage, requires balance and bleeder resistors, and the capacitance decreases, needing more stacks of caps in parallel
When they eventually fail and possibly short, a fuse is needed to prevent your battery bank from discharging into the shorted capacitor

[bcroe]

In addition to the above, no cap is a good sub for a battery, because the only way to get the
energy out is to run the voltage down to zero. Very few loads work well with such a source.
Bruce Roe