Cell voltages and advice on wiring 15s BMS

[Mitch3514]

Hey guys i'm soon to attempt to wire up my first BMS for a 15s LiPo battery pack based on 3 of these: https://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=22586

So far i have opened one of these packs and separated the cells into 2 groups of 5s as to wire up each half in series with one of the other packs to create 2X 15s packs ( each pack is made up of 2 5s packs wired in series with separate balancing cables for each group of 5 so easy done ) these remade packs will be wired in parallel to provide 15s at 9Ah

I have measured each cell voltage:


*Total v refers to the voltage difference between the Negative wire coming from the battery pack and the most positive wire on the balance lead (red).

There is one cell at 3.51V which is a concern at this point. note that cells (1,2) (5,6) are the premade packs, and (3,4) has been separated into 2X5s

I now plan on wiring these cells like this:


Packs 1 and 2 will be wired in parallel and balance leads combined to wire into the BMS.

Am i on the right track? my concern is that when combining the cell at 3.51v with the one at 3.82v it will damage the cell.

 

[Ykick]

Paralleling 3.51 to 3.8V probably won't harm the cell (although the wiring might get hot) but why not bring the low 3.5V cell up to around 3.8V before connecting?

Around here we commonly refer to these Hobby King packs as "bricks" and assembled bike battery as packs.

 

[dnmun]

why not post up a picture of your BMS?

if you connect a cell that is totally discharged @ 3.5V to a cell that is about 80% charged at 3.82V then there will be such a large current surge through the sense wire it will melt it and maybe catch the plastic on fire.

how is it that the pack got so far outa balance?

 

[Mitch3514]

I have no means of balancing a single cell unfortunately. Maybe ill invest in a hobby lipo charger and do this to be safe.

These are the cell voltages as they came from hobbyking AU so not sure on why its lower than the rest.

The bms is one of these: http://bestechpower.com/555v15spcmbmspc ... -D166.html

 

[dogman dan]

Get yourself a super cheap 50w RC charger. It will come in handy as a single cell charger, or discharger at some time along the way.

YOUR STARTING POINT,, is to charge them all with this small balancing charger, sort out any weak cells after a few mild DOD cycles, THEN start disassembling things to rewire them.

Awkward now of course, I'd suggest that you rewire as you planned, but again after charging all to full charge. Unless I'm confused, you will have 5s sections when done? This will work to cycle it a few times on a tiny cheap 6s charger.

Then when you have it sorted, all weak cells removed, all cells that tend to self discharge more than the others, ones that puff, stink with leaks, or have obviously higher internal resistance gone, then assemble your pack and begin using the bms and bulk charger.

That one cell that is at 3.5v may turn out fine, but only a few full charges, a few cycles, and a resistance test will tell. It may puff in 5-10 cycles or so. So when you build this pack, put it on the outside edge easy to get at.