replaced the batteries in a string trimmer. Recently I replaced the batteries in a low cost cordless drill. You can buy a replacement battery pack or replace the cells yourself. The drill I have had a battery pack with ten NiCd 1300 mAh sub-C cells (as fat as a disposable C size battery, but shorter). I found that AA size "pre charged" (or "Low Self Discharge") rechargeable nickel metal hydride (NiMH) cells are now available with 2300 mAh capacity at a reasonable price. So I could replace the batteries with smaller ones with more capacity and which will hold the charge longer.
Nickel-cadmium (NiCd) batteries can be replaced with NiMH, but you may need a different charger. The Lithium batteries used in laptops and newer
power tools require very specialized charging and can explode or catch
fire if not charged correctly. Only a Lithium battery specifically designed for the tool should be used. Do not substitute lithium batteries for other types.
The first step should be to ensure the battery
to be replaced is discharged (check with a multimeter). Then remove the screws holding the pack together. Remove the sub-C cells (ten x 1.2 Volts for a 12 volt pack). Two cell in my pack were holding the battery's connectors in
place and I decided to retain these (while electrically insulating them
from the new cells with tape).
To make up the new pack I wrapped a strip of silicon self amalgamating tape around one AA cell, then added one next to it and wrapped the tape around both, then a third and fourth. Then I soldered the batteries in series, using a soldering iron
and insulated wire. This made a flat pack of four cells held slightly apart by the tape to provide some ventilation and cushioning. I then built another pack of four cells and pushed the two together, held by the stickiness of the tape. I then make a pack of the two remaining batteries and stuck them to the eight, wrapping tape around all. I used some closed cell foam to hold the cells in the battery case. If you are not confident with soldering, you can use a battery holder.
I tested the battery with a multimeter,
to make sure the voltage and polarity were correct (and matched that of the charger). Apart from the drill not working correctly, connecting
the batteries the wrong way (negative terminal to positive), may damage
the drill, the charger or cause the battery to explode.