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pokerfjes asked

Lithium battery bank composition

I know this question is bordering not relevant for Victron, but I may not be alone with this issue - and there are so many skilled techies here so I hope you can forgive me.
If I get this wrong I'm in for a very expensive detour on my camper build.

I have a Quattro 12/3000/120 which is going to be the heart of my offroad camper electrical.
I purchased it because I want to use VW e-Golf Lithium Ion packs for my battery bank, and was recommended to purchase the REC 4S ABMS as it is compatible with the Quattro, supporting the required voltage.
I want to build a bank around 10kWh capacity, and each VW module inside the VW battery is 4S3P, based on Samsung 37aH cells. I need 6 modules to reach the wanted capacity.

So far so good, but I fear I have an issue with battery bank composition and my expected result.
I purchased a couple of test modules the other day, and the guy says - sure - just connect the two modules in paralell, and connect the BMS to cells in one of the modules. He says the cells at the same parallell level will balance automatically across all 4S3P modules, and I will only need one BMS even if I add more of them. I'm in doubt though... since the 4S3P modules have no connection at cell level, how can they balance to the same level?

Attached are an image of one module from the VW e-Golf battery pack, and my 3rd grade level drawing of how I am supposed to connect this (according to the seller of the modules).

The individual 37aH cells are welded together and I suspect messing with that is going to cause more pain and cost more than I would like.

ANY advice is welcome at this stage.

battery
vw-4s3p-modul.jpg (391.6 KiB)
batterybank.jpg (86.6 KiB)
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2 Answers
AvB avatar image
AvB answered ·

Thanks for asking, I use same style batteries a lot on Victron. BMS is not optional, but absolutely required. Cells are easily permanently damaged by overcharging.,though this version does not easily explode. Yes I tried it a few times.

I highly recommend Rec BMS ,works very well with Victron. Also recomend safety relay as second safety layer.

picture below 1 battery with original bms and one without. I lack information to make original bms working.

indeed in parallel batteries you need to add the 3 wires to balance each pack internally.


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pokerfjes avatar image pokerfjes commented ·

Thanks so much for responding. I am very interested in seeing some schematics on how you've connected these. You don't use a separate REC ABMS per module? The BMS would be 3x as expensive as a module.

The reason I chose the VW modules is because they are at least 30% less than the Tesla 7s packs when looking at cost per Wh. The Samsung 37Ah cells also have an amazing watt to weight and volume ratio.

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AvB avatar image AvB ♦♦ pokerfjes commented ·

Indeed 1 Rec BMS for entire battery bank. In a well running system all the batteries only need once in a while a few mAmps of balancing. Rec BMS has quite big resistors build in. The BMS thus is able to balance many parallel batteries.

Sorry don't have time to draw you a schematic. In fact I just took a look at your schematic and it is almost correct. The middle three wires have to also be attached to the other batteries. All cells in parallel must also be connected to the Rec BMS. Schematics in Rec BMS manual should tell you enough I guess and the support from REC bms is good.

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pokerfjes avatar image pokerfjes AvB ♦♦ commented ·

The Rec BMS manual was not very clear on this case and their support actually said straight out that it was not possible. Must have been some misunderstanding then.

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kai avatar image
kai answered ·

I believe your suspicions are correct. For me the story started going sideways when the 4S3Ps are put in parallel before considering BMS. If each 4S3P pack on its own has its own BMS, and the BMSes talk to each other, sure I would say parallel them up.

The "balance" that would be achieved in the proposed scheme is only at the pack level. The cells inside the non-BMS'd packs will do their own thing according to chemistry. If the cells are well matched, then maybe the risk isn't too bad.

I can only offer some thoughts about possible approaches:

1) Reconsider whether you can live without the BMS and put in other controls (may be procedural, or maybe you sample the voltage of all cells and review them regularly).

2) BMS for each 4S3P... But this would involve engineering an interface between multiple BMS and the Quattro.

3) Is there a way of adding reliable connections onto the cells? If you can stack the packs vertically and connect up/down you turn the 4S3P into 4S(3*8)P. You wouldn't have to disassemble the packs but will need to physically support the packs externally with some structure. The cross connections need to be substantial to minimise cell voltage differences in each 24P group. Suggest you want to incrementally build and test this - this is not a typical way of approaching the problem and will cost if it goes wrong.

* I'm assuming you meant 8 packs to get to 10kWh.

All of the above will carry some level of risk that you probably weren't expecting to carry. Sorry to say that you may need to have a think about your resources to engineer a solution vs repurposing/ebaying the equipment you've already bought and chalking it up to a lesson.


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mka avatar image mka commented ·

I agree with you a part from;

1) Reconsider whether you can live without the BMS and put in other controls (may be procedural, or maybe you sample the voltage of all cells and review them regularly).

with lithium batteries having a bms to manage the cells is more efficient and secure.

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pokerfjes avatar image pokerfjes commented ·

Thanks so much for your input. You've confirmed my suspicion.

The cells are most likely very well balanced. They are one of the latest generations Samsung SDI prismatic cells and I expect them to be of high grade. However, I do not wish to attempt to run them without a BMS.

I am looking into splitting the 4S3P module and create a 1S12P. This allows me to use the factory casing which is very rugged. I have a good welder at hand, but after a few hours of google, I want to avoid any kind of hedat applied to the cells.
I am currently investigating using conductive silver epoxy to bind the 4 3P packs in each module. As you can see from the image, there is plenty of area to make contact to a new connector and with some threaded mounts I can also apply some physical pressure to the connections.

I actually meant 6 modules to get 10kWh. 4S lithium-ion at 14.8v and 37x3 Ah = 1642Wh per module, but if I want to create 1S12P modules in the factory enclosure, I will have to create 1S24P and then serial them for a 4S24P setup which should result in a 12.7kWh pack.

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