Hi all. I got a bunch (18) of Sanyo lithium batteries. They are the flat prismatic type maybe 1.4” x 1.8” x .4” and have a built in protection board. I think they’re 2200 mah. They came out of some 6 cell (3s2p) tablet PC battery packs. According to Sanyo, they say not to discharge them at loads of 2A or more. Here’s my question…id like to build a 6S2P 12 cell battery to power a light project in the near future. Since they take up less space than comparable 18650’s and are pretty much free. I’d be powering a 36v led that draws 3.6A connected to a boost converter driver. Would these batteries handle that? If so, that would be great because the alternative is 8 18650GAs that cost over $40.
Thanks!
The power required for your 36V 3.6A led is 36*3.6 = 129.6W
Because the boost converter doesn't have 100% efficiency you have to add maybe 10% so now that's 129.6*1.1 = 142W
When fully charged the 6S2P will output 4.2*6 = 25.2V and the converter will try to suck 142/25.2 = 5.6A but while with 2P the current in each cell will be higher than 2A (5.6/2 = 2.8A) the voltage will be much lower and the current should be even higher so 2P is not ok even with 4.2V/cell
Now we can see how many parallel cells you need for staying within the specification.
If you want to be able to run the led at max power without going higher than 2A/cell with 6S and low cell voltage of 3.0V you need :
6 cells, 3.0V each 3*6 = 18V
143W required with 18V pack mean the current is 143/18 = 8A
2A/cell maximum means 8/2 = 4 parallel cells minimum
So that 6S4P minimum (more cells than you have) and you will be using the cells at their maximum current
You'd better lower the power in the led or use some better cells
Great thanks for the detailed response! I kind of figured as much, that’s why I had planned on using 18650s in the beginning but sticker shock made me wonder if I could go the other route and have higher capacity then using 8S 18650s. Plus, my balance charger won’t do higher than 6S, otherwise I’d use more series cells 8S2P for 16 cells.