The other day I found another very nice pack. This time a 5Ah DeWalt pack, and it looked brand new.
Look at the specs on this thing, 100Wh!!!
It has a state of charge meter on it but it didn’t light up, maybe the reason it was tossed. So I took it apart to see what was up. All cells were sitting at 2.67V, maybe just low enough that the charger wouldn’t recognize it. This pack contains 10 Samsung 25R’s.
So I charged this pack up manually, making sure all cells charged equally. I let it rest a day and then discharged it through a 3 ohm power resistor to measure the capacity.
Here are my data points, voltage measured across the 3 ohm resistor versus time in minutes.
This DeWalt pack has no Low Voltage Protection in the pack, it must be that DeWalt includes that in their tools. Because of that I had to monitor each cell as it was being discharged to make sure they all tracked the same so as not to over discharge any one. I shut the pack down at 14V as at that point each cell was at just about 2.80V while under load.
My capacity estimation for this pack is: (17.8V/3 ohms) * 50 minutes * (1 Hr/60 minutes)= 4.944Ah
This post explains why I use those numbers in my calculation.
I had to estimate the capacity of an 18V drill pack, here is how I did it.