7p10s Lawn Mower Battery Rebuild

In this video, I review the issues I had with the new 18650 battery I built for my Black and Decker lawn mower. I’m having charging issues with 2 of the cell banks, one of the banks is undercharging and the other is overcharging. I’m hoping the overall issue is the BMS but it might be one of the cells has gone bad. I will start by replacing the BMS and if that does not help I will look into each of the cells.

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6 thoughts on “7p10s Lawn Mower Battery Rebuild

  1. best way and i have this problem with all 4 of my 4S bms's capacity check the one bank and compare it to the others with out the bms connected and see what the difference is. If the diffrence is minimal then yes its the bms causing the problem. You can try another one or add a parallel cell to that one bank to get it more balanced with the others through the bms its been my experience that only one bank of the S series out of the 4 with mine that will do this ya just gotta find it and add a parallel cell to offset the difference. I went nuts finding this problem when i seen it come up much like yours one cell row runs down faster while the others still have 3.2 to 3.5 volts and the pack shuts down because of that one row. You put alot of effort in the pack so verify cell bank capacity to confirm the bms is acting odd. Hope this helps.

  2. we have a very similar situation with the mowers! As for the BMS i have heard many situations where people have a BMS that cuts power but does not do true balance. The one you have now may actually balance however it only balances on the top if the charge, from what i hear. So, that means you have to charge it, let it get to full then let it sit plugged in and it balances the cell banks at a very low amperage. So the balance feature may take quite some time. Get a hold of the BMS seller asking about the balance feature. If his name is micah (look at shipping label) then try heading to ebikeschool.com or the youtube channel. Lots of BMS info there. good luck with it!

  3. If mowing your whole yard completely discharges your pack then the pack is not big enough. Is there room for a second pack connected in parallel on top of that one. Adding a second pack should half the imbalance as well as reduce draw and double your run time. For whatever reason your pack is discharging unevenly. Probably different types of batteries with different internal resistances. If you are going to charge with a balancing charger then why do you need a BMS? Your pack will balance every time you charge it which should be every time you mow. My experience with off the shelf BMS boards is that they don't work on large packs because they have a limited discharge ability and the rate of charge on a large pack exceeds their ability discharge fast enough. They do nothing when discharging! they only discharge cells that reach 4.2 volts to keep them from overcharging. A BMS that was able to balance while discharging would have to have both battery leads and leads for your load separate from each other on the BMS. Something like this would probably need to be custom made or maybe something like the opensource BMS project I saw would be configurable enough.

  4. The charge input is not all that special, all cheaper BMS boards use top end single cell discharge balancing, the cell gets to 4.2 and it switches a resistor across the cell to discharge it. It probably also turns off the charge input ( or at least slow it down with PWM) you should test it with a clamp meter on the balance leads, I haven’t seen many videos of BMS behavior in action

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