Designing a flat, structurally sound battery pack

[Sepial]

Hi,

I'm currently putting together a 14s4p battery pack with the following requirements:
* Pack split in 2x 7s4p serially connected for having 2 equal weights, required for transportation (don't ask...) also, no BMS
* the 7s4p's need to be flat, arranging them as 2x14 matrix is my preferred form factor which gives me dimensions of minimum 260x65x33 mm per block

My problem is, 260mm by 2 cell wide is starting to look bad for flex... Therefore I am splitting each 7s4p in 2 parts. No connectors here, I'm just connecting those 2 parts with 12cm non-removable wires. It has the additional (not required) benefit of being foldable now.
I could have split them into uneven 4s4p and 3s4p but I chose to split the 4th parallel block into 2, then connect them in parallel with 12awg wire (battery should deliver 30A tops). It has the additional advantage of being even when folded but this is not a requirement either.

This is what it looks like, I would make total 4 blocks like this:



With my (poor) level of electronic engineering, I can't think of anything wrong doing that... However if somebody thinks of a the tiniest thing, I would rather play it conservative and do the 4s4p/3s4p design. Can I neglect the wire resistance as oppose to the cells resistance and will the current flow nicely and evenly in this split parallel group? is there some created inductance I should know about? (I am not too good with inductance...)

Thanks

 

[Punx0r]

A short (few inches) piece of 12AWG wire should be OK to do what you propose.

However, if you did want a single, long "stick" pack I would just use two strips of something like fibreglass PCB material attached tightly with filament tape to sandwich the pack. This would make it pretty damn stiff for only a marginal increase in pack volume and weight.

Oh, and while it's a bit late now, many people say it's not safe to rely on the cell shrink wrap to isolate the cells and some additional insulation (heat shrink) should be added if the pack is not built with cell spacers.

 

[Sepial]

I didn't think of PCB boards, I will have to see if I can find some 30mm long and how much they flex. To be honest the glued cells are actually much stiffer than I expected so your suggestion should be very much stiff enough.

Thanks for the heads up about heat shrink. I tore down some batteries after gluing and I can see why... that heat shrink is easily torn. It's not exactly too late because I did only half of the blocks. I wanted to use spacers but I don't like those available ones than goes over the top of the cell. Unfortunately I don't have suitable heat shrink, I have some nice 25mm but it's a few mm to small...

 

[Punx0r]

Yes, unfortunately the cell wrapper can be brittle when glued. Also sadly cell spacers considerably increase the size of the pack, so it's a case of picking the least-worst option...