Wiring 42 12 volt 35 AHr Batteries in a 12 volt configuration

hermit · April 2015

These batteries were gifted, what to do with them? Make my shed a source of power. I have a 3000w 12v inverter with 2 sets of inputs (2+ AND 2-) I have bus barred 3 sets of 14 parallel batteries. Question which is the best way to hook up these 3 sets of batteries to each other for both charging and for the inverter. The reason why there are 14 X 3 is that what fits on the shelf. I used aluminum bus bar. The inverter needs to be hooked up with 2 positives and 2 negatives. Do I tie all strings at both ends together both on the positive and negative sides, or do I tie 1 end of string to the opposite side of string then again to the opposite side of the string to the other string (kind of like series but not positive to negative) or tie the 3 strings together with just 1 positive on 1 side and 1 negative on the other side. The answer will determine which way I hook up my charging. Then which would be the best way to wire up the inverter which needs 2 positives and 2 negatives? I know this parallel configuration is bad for charging and troubleshooting but this is what I have. Thanks for you input.

inetdog · April 2015

hermit wrote: »

These batteries were gifted, what to do with them? Make my shed a source of power. I have a 3000w 12v inverter with 2 sets of inputs (2+ AND 2-) I have bus barred 3 sets of 14 parallel batteries. Question which is the best way to hook up these 3 sets of batteries to each other for both charging and for the inverter. The reason why there are 14 X 3 is that what fits on the shelf. I used aluminum bus bar. The inverter needs to be hooked up with 2 positives and 2 negatives. Do I tie all strings at both ends together both on the positive and negative sides, or do I tie 1 end of string to the opposite side of string then again to the opposite side of the string to the other string (kind of like series but not positive to negative) or tie the 3 strings together with just 1 positive on 1 side and 1 negative on the other side. The answer will determine which way I hook up my charging. Then which would be the best way to wire up the inverter which needs 2 positives and 2 negatives? I know this parallel configuration is bad for charging and troubleshooting but this is what I have. Thanks for you input.

First of all, my advice is to forget this configuration. The operational problems of 42 lead acid batteries in parallel are just too great to contemplate.
If you insist on doing it anyway, read the information at smartgauge.co.uk, The page How to correctly interconnect multiple batteries to form one larger bank will give you some idea of the problems involved.

The "ideal" four battery solution shown there can be extended to 8, 16, or 32 batteries without much trouble (but a spaghetti pile of long wires and a maintenance nightmare?!), for 42 you will have to compromise some.

Is there any way that you can use a 48 volt system instead? That would reduce the complexity of the battery wiring but you could only use 10 strings of four batteries each and keep the other two as spares. I would not really recommend that either, but it would be better than 42 in parallel!

And before you start, do you know enough about the history of the batteries to be confident that they are good? If they sat for months without being charged they could have lost much of their capacity.

CALLD · April 2015

If they were given to you for free then for sure - give it a try why not? It's a pity you have to go in an all-parallel 12v configuration but then I suppose if you're stuck with a 12v inverter you have no choice. If possible use heavy copper bars to link all the + terminals together and all the minus terminals in a long "train rail" configuration. As for attaching them to the batteries you want to have as little electrical resistance as possible at the terminals and since the batteries were free I would just solder them securely to the battery terminals. Now with all 42 batteries connected like sleepers on a railway line with the copper tracks above them connect the loads and charging points at the diagonaly opposite ends of the setup;)

Dave Angelini · April 2015

Sell them to a recycler or buy alot of fuses and a sprinkler system with a dialer for the fire department.

tampasolar · April 2015

Ditto on the go 48 volt arrangement, 12v will potentially leave cells overheating and evaporating regularly.

hermit · April 2015

The batteries are 3 years old and taken from one of our decommissioned facilities. The batteries are Discovery AGM VRLA`s. If it was easier to get a higher voltage inverter I would but usual order time is 2 to 6 months. Maybe I should be a little less aggresive on the number of batteries in this installation.

westbranch · April 2015

FYI if you went 48V @ 35Ahr you would get ~ 800Watt hr of usable power per 48V string, not very much power, about enough to run a single 40w light for 20 hours and be able to reuse the battery continuously. If those are UPS batteries they may not be designed for many discharges before starting to fail...
hth

waynefromnscanada · April 2015

Generally, the max parallel batteries you can successfully get away with are 3. You are looking at 14 times what can be expected to reliably work. Sorry, it's a no go.
Even wired for 48 volts, you're over 3 times what can be expected to work. Try to sell them now as used but working batteries - - - or keep them and prepare to sell them for scrap.

MarkC · April 2015

I've found a nearby battery wholesaler that accepts old batteries for recycle in trade for new ones. It worked out to be about 20-30 cents per pound. With almost thousand pounds of batteries, you might get a couple of 6 volt deep cycle batteries in exchange! Might even get a couple of 12 volts. Working with many small AGMs in UPSs, I cannot imagine the pain (and maybe danger) of keeping so many old ones working together.

inetdog · April 2015

MarkC wrote: »

I cannot imagine the pain (and maybe danger) of keeping so many old ones working together.

Among other things, consider the need for 42 separate fuses. Otherwise a shorted single battery could draw enormous current from the other batteries that are in parallel with it.
At least fuse groups of three or four batteries maximum.

hermit · April 2015

Free isnt so cheap. I think I will listen to the words of wisdom and save grief. Thanks for the replys.

Wiring 42 12 volt 35 AHr Batteries in a 12 volt configuration

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