When I put them on charge, they charged for about five minutes and the charger cut off saying fully charged. I attached it to my drill and it had very little power. It turned the drill but I could stop it by hand, which I shouldn't be able to do. I then tried a different charger and the same thing happened. So now I am puzzled. Are the batteries bad? Well.....it is an 18 volt pack so I used 15 1.2 volt batteries and they are checking at 21 volts. I'm not sure why its too high but I assume that's okay. Could the chargers be bad? Could the thermistor in the pack be bad?
- First let me say that the 2800 mAH capacity is NOT the problem here. It's close enough to the original capacity to not be an issue with the charger. Even if the capacity was very much larger than the charger was expecting it wouldn't cause the issues you're getting. (Not to mention that if those 2800 mAH _batteries came from Hong Kong via ebay they're probably no more than 1700 mAH actual capacity anyway).
- Assuming that the 21 volts you're measuring is correct then that corresponds to about 1.4 volts per cell and a very healthy state of charge. If there are no faulty cells then this should power the drill just fine. Just to test this a little further, can you try charging it again and then just it sit idle for a few hours (or overnight) and measure again. Is it still about 21 volts?
- I think your most likely problem is either a faulty cell or a high resistance joint. Did you use "solder tab" cells so as to avoid putting too much heat into them while soldering? The batteries can be damaged if you put too much heat into them. If they're all soldered correctly then you may have one faulty cell that is high resistance, or one of the cells came with a solder tab that wasn't properly spot welded.
- If possible try measuring the individual voltage of each cell. They should each be close to the total voltage divide 15 (1.4 volts for example). If one cell is very much lower than the others then it may either be that it's a faulty cell or that it was initially much flatter than the other cells when the pack was assembled. Personally I measure all my cells before I assemble a pack and try have them as well balanced as possible. I find the easiest way to do this is to either full charge or to fully discharge them individually before assembly.
- Finally, are you certain that you have the thermistor connected correctly. Can you measure the thermistor resistance (typically between the negative and auxiliary terminals) and compare it with a working pack?
