Balancing current/current sharing between parallel cells during discharge - Page 1
Forum section: electronics / projects

author: h3po
author_rank: Newbie
timestamp: April 04, 2017, 06:13:54 pm
content: Hi to all my earthling viewers,I'm about to add a hub motor to my kick scooter (Oxelo Town 9) and am thinking about where to put the batteries. Because that scooter, when folded together, can be dragged on its front wheel like a trolley I would prefer to have as much of the weight in the front as possible. I'll only be able to decide on the number of series cells once the motor arrives; currently I'm planning with 10S2P (NCR18650GA), which might be reduced to 8S2P. The steering column is a good place to start putting cells, I have 3D printed spacers that allow me to put 6-7 cells in the hollow column.However I am worried about balancing current between the parallel cells with the long wires that this approach to battery placement will necessitate. I estimate the available space will at most allow for 19AWG (~0.65mm²) between each two cells (output wires will be 12AWG for 15A motor current).Do you think this will lead to problems during normal operation due to slightly different internal resitances of the cells? I am aware the thin wires might go up in smoke if a cell fails (at 15A/2 max current), but that can be tolerated or mitigated wit PTC.

author: Codebird
author_rank: Regular Contributor
timestamp: April 05, 2017, 09:47:40 am
content: Handy trick: Connect one power wire to the row-of-cells at one end, and the other to the opposite end, like so:http://m3.i.pbase.com/g4/84/622984/3/140504413.Kxrddugl.jpgThat effectively eliminates differences in wiring resistance, as they all have the same length in total. It won't perfectly balance the loading due to differing capacities and internal resistances, but those are mostly self-correcting (a cell discharging faster than the others will quickly provide a little less current until they reach equilibrium), so it does get you very close.

author: h3po
author_rank: Newbie
timestamp: April 05, 2017, 06:04:56 pm
content: Thanks, that is indeed a neat trick. But I'm less worried about the resistance of the (oversized) main wires, more about the wires that connect the cells in parallel.What I'm thinking about doing would look like this:

author: edpalmer42
author_rank: Super Contributor
timestamp: April 05, 2017, 06:51:22 pm
content: Maybe rearrange the pack so that the bottom 6 cells are in a 3S2P configuration - same for the six vertical cells. This will minimize the wire length used between the cells in each pair. You also won't have to try and stuff 6 wires into the vertical column. You'll have one large wire, (maybe two smaller wires in parallel would fit better), plus short jumpers or straps to connect the cells that are paired together. Careful arrangement of plus-minus versus minus-plus orientation of the cells might reduce the strapping even more.The biggest issue with this arrangement is making sure that everything is properly insulated so that the pack won't develop shorts after months of temperature-cycling and vibration. Tape or heat-shrink by itself probably won't be adequate.Ed

author: h3po
author_rank: Newbie
timestamp: April 05, 2017, 07:39:56 pm
content: Hmm, that might well be the solution, it would balance out the lengths of wire of the two separate packs and also reduce the length of the paralleling wires. Thanks!I'm not worried about insulation, the cells will be held in place and spaced apart where needed by 3d printed parts. To deal with expansion, I'm thinking about adding nickel plated springs at one end of each string.