I’ve been following this guide on assembling a DIY 18650 battery pack and I’ve run into something I can’t quite figure out.
I built a small pack using spot-welded nickel strips and tested each cell before assembly. All the cells were balanced around 4.1V when I started, but after wiring everything up, one parallel group is showing about 0.2V lower than the others. I double-checked my welds and connections, and they seem solid.
Is this normal cell drift after assembly, or does it point to a weak connection or bad cell in that group? I’m hesitant to throw a BMS on it before I understand what’s causing the mismatch.
Hi @igor_X, thanks for jumping in, it’s really appreciated!
Here’s what I’ve checked so far since your tips:
Weld Quality: I went back and re-inspected the battery welds with a magnifier, visually, they seem clean and consistent.
Cell Testing: I swapped the cells between the low-voltage parallel group and a properly balanced group. The voltage difference moved with the cells, so it seems like the cell itself might be the weak link, not the wiring or weld.
Resistance Check: I measured the voltage drop across each group while applying a small load. The “bad” group shows slightly higher voltage drop, suggesting internal resistance is indeed higher in that cell.
So it seems likely that one of the cells has developed elevated internal resistance rather than any wiring issue. That said, before replacing it, I want to make sure of the diagnosis.
Next steps I’m considering:
Get a proper cell analyzer and measure the internal resistance (IR) directly to confirm.
Leave the suspect cell partially discharged on its own and test again later to see if it continues to drift down.
Replace the cell and re-balance the pack, then monitor behavior over time to confirm the issue is resolved.
Does that sound like the right course of action? I’m hoping to diagnose it correctly before throwing on the BMS.
Thanks again. your insight helped me pin down where to focus!
