Battery Renovation

I have a cordless drill that is no longer holding a charge. It is a cheap one and its not worth buying a new battery. My brother has a similar battery that also was not holding a charge so he gave it to me. I wanted to see if there were some bad cells in the battery that were draining the good cells. If I can identify them and swap the bad ones for good ones, maybe the battery would work better.

Here is a picture of the inside of the battery:

There are 15 cells at 1.4 Volts each. The cells are in series. First, I charged the batteries. Then I used a multimeter to measure the voltage across each cell. The chart below shows the voltage measured across each cell:

The "Meas total" is the voltage reading on the battery output that goes into the drill(in other words the voltage across all the cells). The "Calc total" is the sum of the measured voltages across each cell. I calculated this to make sure that I didn't have a bad connection when measuring one of the cells. I figure if the "Calc total" and the "Meas total" are almost equal then that means I measured everything correctly. You can see from the chart that most of the cells have the same voltage, but a few of them are not contributing anything.

My idea is to cut out the cell that is reading 0 volts on my battery and replace it with a cell that is reading 1.4 volts on Zeke's battery. Below is a picture of a cell that I cut out of Zeke's battery. The cells are connected by a thin metal tab. I had to find a cell with the tabs going in the same direction as my bad cell. I cut the tabs as long as possible so I would have some overlap.

I used rubber bands to hold the new cell in place. Then I soldered the tabs together. I do not recommend you try this. It could be dangerous to overheat one of the battery cells.

The result is a battery pack that is much better, but not back to new condition. It will hold a charge for longer and it takes longer to self-discharge while sitting on the shelf. But it still won't hold a charge for longer than about 1 day on the shelf.