single cell or prebuilt pack

hi
I need to build yet another battery pack and want to check which is the best way to go.
the pack will be a 72v 20ah LiPo pack it will be in the 20S 4P configuration using turnigy 20c 5ah cells. The BMS and charger are sorted.
The problem I have is what cells do I use multi cell battery packs complete with balance and power wires, thepacks will be striped of there covering then the wires will get removed and replaced with a better choice of wiring that takes up less room or go for individual cells https://hobbyking.com/en_us/turnigy-5000mah-1s-20c-lipoly-single-cell.html. Both routes will give the same result size of pack, there are pros and cons with both ways.

Time: Similar for both packs ripping the multi packs apart soon cancels out the extra time to solder the pack.
Balanced cells: with multicell battery packs the cells in each pack will be matched but not necessarily matched between packs, with the single cell route you don't know if the cells are matched. how much does it really matter with a good BMS.
Cell replacement: single cell building definitely wins there multi cells are stuck together and the contacts welded making cell replacement very hard better to just replace the pack an extra expense.
Pack shape: with single cells the shape of the pack can be more adaptable.

when building custom battery packs out of Li Ion for a large pack a huge box of cells are bought no knowing how well matched they are, these are then linked in in both parallel and series usually by welding.
Single cell LiPo's are the cheapest option but are there hidden problems I have not foreseen.
geoff

considering the same thing myself. I might go with the single cells because then you can use copper bus bars to connect everything instead of a rat's nest of wires. Let us know which way you go and document your build and I'll try to do the same.

devo1223 said:

considering the same thing myself. I might go with the single cells because then you can use copper bus bars to connect everything instead of a rat's nest of wires. Let us know which way you go and document your build and I'll try to do the same.

Click to expand...

Not sure that copper bus bars would work except for joining in parallel for the serial connection the tabs of the cells can be directly connected by soldering.

When I build the pack I will document it but that is a few months off, the pack is going to be made with a custom BMS I am designing myself. getting together 80 cells may take a bit of time as well not sure how many cells the uk warehouse keeps in stock(just checked at point of posting they had 49).
when i do build my pack I will start a thread combining building thepack and making / designing the BMS, for the BMS I have been taking advise from fechter.
geoff

have just ordered the first 40 cells, time to prototype the BMS.

kinda like this


There is enough in the US warehouse for me. Have you ever seen them on sale?

looks like the way the original "duck tape" lipo4 batteries were constructed multiple cells in parallel then add them together in series. the old duck tape batteries never used copper bus bars but that was the circuit.
geoff

about to buy this, seems like a good deal

They look ok, bus bars will not work all you will need is the silicone wires from the balance cables.
Personally I would build the pack as several 1p packs the bus bar each end once that is done link across each cell that is in the same place with balance cable wire then attach to a BMS I already have one 20s battery waiting for a bike.
for my next battery pack I'm taking things a few steps further each 4 cells in series will be linked to a slave BMS board with a connection to the main controlling board.
geoff

Watch out for zippy. The C rate is overrated by a factor of 2 usually and they almost always fall short on capacity.

So really about the same actual c rating as multi stars? Are the turnigys better? I'm looking for something that can handle more amps than the multistars

Considering same thing. I bought one zippy compact 5Ah and 7s with 25c/35c raiting.
Will test it with my icharger duo in sync mode with 70A and see how it handles.

second choice was Turnigy Heavy Duty 5000mAh 7S 60C Lipo Pack w/XT90 but with these my pack would be 5kg heavier and also 12055cm3 rather than 9156cm3 with zippy compacts.

But after reading i'm not sure these compacts will suit me, but lets see how my test goes.

Did the test with Zippy but first impression is that I need "Turnigy Heavy Duty" instead.
Will try to do more Amp test if I can get my iCharger DUO to get it to draw more amps.

how do you read that graph? You discharged at 29 amps? what's that green line way at the bottom? I just bought some 12c multistars. I couldn't resist the discount sale

Green is amps, you can see green y axis in left.
Yellow is discharged mAh
Blue is temp
Red is pack voltage
Others are cell voltages.

Each has its own y axis in left or right
And x axis is elapsed time