Testing Cells

[BrianG]

Originally Posted by coolhandcountry

I have a pack that is showing resistance on my charger. Is this a sign of a bad cell.

All cells will show some resistance. The lower the better. The higher the resistance on a cell means less voltage will be available to the motor under high currents since more will be dropped across the cell itself (and will heat up). You generally want resistances at, or lower than, .005 ohms per individual cell. If all your cells show about the same resistance, then you're fine. If one cell has a substantially higher value than the others, then you might have a bad cell. It depends on how much higher the resistance is. .001 ohm difference it OK. .01 ohm difference is not. Since cells in a pack generally are series connected, you'll just see the summed resistance from all the cells of the pack and could be affected by one bad cell.

With the simple method I stated in my previous post, you can calculate how much resistance there is on a cell:

CellResistance=(Vunloaded-Vloaded)\loadedCurrent

To get a better (more accurate) calculation, you really don't want to use a truly unloaded measurement. You'd want to load it with about 1-2 amps to get some type of drop. Then:

CellResistance=(vLightLoad-vHeavyLoad)\HeavyLoadCurrent

To get the "HeavyLoadCurrent", you'd simply take the measured voltage at the heavy load and divide by the test load resistance.

Unfortunately, the loading curve of a cell is not truly linear. You'd have to measure and calculate values at several increments of load - like at 5A, 10A, 20A, etc.