Seeking some advice regarding altering the configuration of existing 12V 306Ah batteries into a 48V arrangement

[Chris Down Under]

Hi everyone,

A few years ago I posted a discussion on repairs to the 4x 12V 306Ah LiFePO4 batteries used in my off grid solar power system. It was a bit of a disaster as the terminals were getting hot and had begun to melt into the battery case - and it was not through misuse. Hmm. It was a not something you get to experience every day. Opening up the batteries and fixing the design issue was enlightening. You can read about the dramas here: Unusual design concerns resulting in hot battery terminals with LiFePO4 batteries. Pro Power 12V 306Ah Lithium Batteries

Anyway, following on from the repairs, the batteries have been running faultlessly for the past two years. However the original battery design includes a lot of BMS's. To me it looks like the battery is a scaled up 12V 50Ah battery, but with six batteries inside the case running in parallel. Here's a photo:



With so many BMS's in each 12V battery, there's a lot to go wrong. And it would be very difficult to know if anything has actually gone wrong.

So I was thinking that longer term, the ideal solution would be to re-configure the four 12V batteries into a larger 48V battery with a higher quality BMS.

What I'd be playing with are 96x 50Ah cells.

Just throwing the idea out to the forum members: Given I run a 48V system, what arrangements for re-configuring the batteries at that system voltage would people suggest? And is it even worth it given the size of the cells? I'd especially be interested to know recommendations for BMS units?

Looking forward to hearing what everyone has to say.

Cheers

Chris

 

[DIYrich]

You could take the bms leads into a new 16s bms. That would move 4 batteries onto one bms.

If you can get to the individual cells (power connection, not the bms sensor leads), you can then put a few in parallel and cut down on the number of bms's, but then each bms will need to be higher amps. 4 cells in parallel is 200ah, which is the size of larger BMS. You could probably parallel 6 cells for 300ah, which is the size of cells today. But the bms would still be limited to passing 200a.

Roughly speaking, a 60a bms is currently $70 vs a 200a bms for $185.

 

[Chris Down Under]

Hi DIYrich,

Thanks for the ideas, and I'd not known that about the limitation with the larger BMS devices.

Yes, the batteries can be fully dismantled and the cells re-arranged. The cell terminals are a phillips head screw, which is not ideal, but is probably OK for a 50Ah cell, maybe.

I'm thinking of converting the 96 cells to 3x 48V 100Ah batteries which should require only 3x 100A BMS. That should cut down on the complexity of the existing arrangement and provide some margin for error.

The question I'm facing though is: Is all this work a case of throwing good money after bad?

Cheers

Chris

 

[DIYrich]

Compared to a 300ah cell. Each 50ah cell is worth $15 new. All it costs is bms and wires. You can use the bms on a new battery. I would do it with two 200a bms.

 

[robbob2112]

8-24s 200a Li-Ion Lifepo4 BMS + UART + RS485 + CAN

This is the best BMS that works for lithium ion and lifepo4 batteries from 8s to 24s. So 24-72v lifepo4 and 28-86v lithium ion. It is preprogrammed for lifepo4 settings but you can change that. It has all the communications to work with inverters including UART, RS485, and CAN. Bolts are...

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Li-ion/Lifepo4 3s-21s 100a BMS W/ LOW TEMP CUTOFF

--------------------Here are 100a BMS units. We carry 3s-21s and both Lithium Ion (Li-Ion) and Lithium Iron Phosphate (Lifepo4). The lithium ion version also works with Lithium Polymer (Li-Po) since they both have the same parameters. (WIRING DIAGRAM BELOW!) Some of the BMSs arrived without the...

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Lookup your cell datasheet and determine what the continuous and max charge/discharge should be before buying a bms. Also, from doing the math the other day for a post about similar, it is likely your heating issues could be solved by just replacing the wires themselves with the next higher gauge and just getting some terminals. Net of only about $40 and simple if you have crimpers

 

[robbob2112]

I talked about a few of the results here..

Post in thread 'WattCycle 12V 300Ah Mini inside look'

Jan 28, 2025

Evidence to support my theory

The positive is 4 x 8awg and the negative is 3 x 6awg

Black wire from BMS to terminal is 13" long
Black wire from BMS to B- is 11" long'
Positive red wire is 11.5" long


0.25MΩ for just the positive cable
1.10mΩ for the B- through the BMS to the P-

more resistance for the same current means more heat generated.

In theory they need to keep the negative wires as short as practical to keep the waste watts as low as possible. My 24" of wires is completely unnecessary. Looks like they increased the wire AWG size to drop the resistance a bit, but maybe they...

• robbob2112

• 

Basically it has to do with resistance in the smaller wires and the terminal materials (copper of aluminum)

 

[Chris Down Under]

Hi DIYrich,

Thanks for the clear answer. So, reconfiguring the 96x 50Ah cells into 2x 48V 150Ah batteries it is with 200A BMS each. Should be plenty of buffer for capacity in that arrangement. I think I'll go with a Daly BMS unit as they seem to be more available down here than any other brand.

Cheers, and may your solar be good!

Chris

 

[Chris Down Under]

Hi robbob2112,

The over heating issues were fixed a couple of years back - and you can see the solution in the thread.

I added a very solid bus bar onto each of the four battery cases so as to bypass the original soldered terminal. It's my belief that soldering the output from six 12V 50Ah cells and BMS's onto a single battery terminal lug is a bad idea through increasing resistance unnecessarily. I've noticed that the company I bought the batteries from appears to have ceased trading. Sometimes you have to deal with what you've got.

Cheers

Chris