I recently built a 4S-9P battery, using 2nd life 26650 LFP cells. I tested and grouped them prior to building the pack, choosing only the best cells so that the capacity and resistance were very similar, for each of the parallel strings. I was very confident in the pack when I sealed it all up, but ran into trouble when I tried to charge the pack, after fully discharging to test the total capacity. I used a 30 amp Daly BMS with bluetooth dongle and the Smart BMS app to watch the cell (string) voltages, and they were going wildly out of balance. The 2nd string was showing 3.400+ while string 3 was only 3.180, so 220mv of deviation. Each time the BMS attempted to balance the strings, the 2nd string would show 5.00 volts and the whole thing would shut down!
I disconnected the balance leads from the BMS and checked each of them with my multimeter, which is when I discovered that the 2nd string was not showing the appropriate ~6.6 volts. In fact, it was only showing around 50mv, yet the whole pack (on the output side) showed the expected ~13.5 volts. I started inspecting the spot where the BMS leads were attached to each string and (no surprise) the one for the 2nd string was attached, but not as securely as I thought. In fact, by just pressing down on that spot, so that the lead was firmly in contact with the top of the cell, the Smart BMS app showed all of the strings had less than 10mv of deviation! If I stopped pressing down on the balance lead, it quickly started showing more of a voltage spread. I can only surmise that this "loose" connection was creating a much greater resistance?
Anyway, I cut the end of the lead, stripped off a new section, and carefully soldered it onto the nickel strip tab I had left at the end of the string, so as not to overheat the individual cells. I plugged the connector back into the BMS and it showed 006 volts (6mv) of deviation across all four strings. Charging is now going exactly as it should and I learned a valuable lesson about BMS balance leads!
I disconnected the balance leads from the BMS and checked each of them with my multimeter, which is when I discovered that the 2nd string was not showing the appropriate ~6.6 volts. In fact, it was only showing around 50mv, yet the whole pack (on the output side) showed the expected ~13.5 volts. I started inspecting the spot where the BMS leads were attached to each string and (no surprise) the one for the 2nd string was attached, but not as securely as I thought. In fact, by just pressing down on that spot, so that the lead was firmly in contact with the top of the cell, the Smart BMS app showed all of the strings had less than 10mv of deviation! If I stopped pressing down on the balance lead, it quickly started showing more of a voltage spread. I can only surmise that this "loose" connection was creating a much greater resistance?
Anyway, I cut the end of the lead, stripped off a new section, and carefully soldered it onto the nickel strip tab I had left at the end of the string, so as not to overheat the individual cells. I plugged the connector back into the BMS and it showed 006 volts (6mv) of deviation across all four strings. Charging is now going exactly as it should and I learned a valuable lesson about BMS balance leads!
