Today was my first real day of pack building after gluing stuff together, and unfortunately it ended in disappointment. About 1 hr into the pack (only one and a half P groups bc I’m still learning and had to cut nickel, etc etc.) my battery for my spot welder just ceased to work. No clue what happened, it looks perfect, no damage or anything crazy. I measured the voltages and it appears one cell has gone MIA and the pack total voltage is 7.2mV - really??
I just submitted a hobbyking rma, we shall see what happens. Obviously if they refuse to replace it I will have to repair it but I really wouldn’t want to have to open this pretty lipo pack.
Anyway this thread is for the battery building part, here’s what I’ve done so far.
Personally I think it is coming along nicely, I just wish that my battery hadn’t shat itself. I’ve got tweleve of these to do so being 1.5 in with a broken battery is very annoying to say the least
Yeah I was kind of thinking the same thing. I was welding at 49J on my kWeld, the nickel is 0.2mm and that’s about what they recommended in the kWeld guide for that thickness (50J) should I just turn it down until there’s no bluing on the extremeties of the weld circles?
Does anyone know how the mathematics works on this? When 2 cells of differing voltages are combined in parallel what direction and how much current flows from the higher voltage cell to the lower voltage cell?
The voltage difference between cells will try to equalize with the maximum current available, which is dictated by total circuit resistance (and the voltage delta). That comes from cell internal resistance, resistance of your welds and the resistance of nickel. Looking at only cell resistance is a good simplification. For example, for 30Q it is 0.0221 ohms. Two of those connected together are 0.0442 ohms. If you have a 0.05 volt difference in the cell potential, you will have 0.05/0.0442 = 1.1A equalization current for a short while. That’s totally ok.
