# Series/Parallel grid cell design w BMS



## dcb (Dec 5, 2009)

paralleling everything at the cell level seems to be the norm. In fact life would be MUCH simpler if you made 1s24p modules, then put 32 of those in series with a bms tap between each.

just make sure they are all the same voltage before you parallel them.


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## rubbaducky3 (Oct 17, 2015)

Thanks for the quick response! 

This would be the easiest! The thing is the steel trays to hold them and the sub assemblies are assembled to hold 64 cells and 24 wont evenly divide into that. so it would mean some of the parallel connections would need to be made across different modules which is a real hassle.

It wasn't a complete ground up build for us otherwise this would have been the design from the beginning. The club in the past I believe was going for a higher voltage motor and we had to modify the design for the lower voltage DC.


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## dcb (Dec 5, 2009)

maybe 21P per module then? pack 3 per tray in series, 21p36S?

how did you arrive at 24p anyway?


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## rubbaducky3 (Oct 17, 2015)

So the batteries were originally donated and put together with fiberglass boards with cut outs for the tabs. There are 8 together and are held together by a copper bus bar that is screwed into standoffs in the fiberglass. There are then another 8 the opposite polarity and a copper bar connects them. 4 of these cubes are in a module. This was just how we got them so rearranging means remaking all the boards which is possible but work and money. So the quick fix was to stack the module's if possible.


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## rubbaducky3 (Oct 17, 2015)

So is the consensus then not to assemble the modules like in the stack picture?

We could try to do the 21p36s the only thing is then is we have to remake the boards, we'd have to check with the converter/charger (which we might have to change) that we could operate at the higher voltage, and that the BMS connections could handle being made across the bus that would be linking two modules.


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## dcb (Dec 5, 2009)

Part of the problem is who knows if those batteries are any good, or if they are suitable for a warp 9. What sort of tests/spot checks have you performed on them?


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## rubbaducky3 (Oct 17, 2015)

So the cells actually came donated from the company, one of the members parents actually works there now. They have also never been used the've been kept at nominal voltage. We've checked all of the packs to make sure they still hover around 3.7 and have done some charge/discharge tests on a number of the packs with a balance charger.

They are 8ah 2c cells originally manufactured by Dow Kokam but we would also have no way of replacing them.


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## dcb (Dec 5, 2009)

sorry, I can't really make sense out of what you are saying. Something isn't adding up.

you have 768 cells? and they are 8ah each? and they are in blocks of 8? or blocks of 8x8?

do you have pics from all sides of one of those blocks? Are there connectors on the bottom?


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## rubbaducky3 (Oct 17, 2015)

Sorry I always have a hard time describing this. Here is a physical picture of a pack.








Here is a drawing of the assembly which uses a number of these packs, there would be two of these in series for final design.








Basically just trying to parallel 3 packs as they go up and do 4 in series. As it stands each step has 8 individual cells together so 3 of them in parallel gives 24.
Also I know these are the same two images as before now, I went back and changed them so pics would actually show up.


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## dcb (Dec 5, 2009)

hmm, the balance wires don't have to be super huge, you can probably screw bare wire looped around a screw for that.

The main current channels are a little concerning, i.e. the bar at top should probably be at least 3x the cross sectional area of the other bars, and the bars at the bottom as well, sort of a "manifold". 

how many ah is this picture at 59.2v?


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## rubbaducky3 (Oct 17, 2015)

so they are 8ah cells 8 in a cube and 3 parallel so 8*8*3=192aH. That bar is a 1/8"x1" copper bar so cross sectional area 81mm2.
edit: should also probably note in total there would be two of what you see in picture so voltage is 120 and they are 2c cells.


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