Several types of rechargeable batteries are available, such as nickel cadmium (NiCad), nickel metal hydride (NiMH) and lithium polymer (LiPo). NiCad and NiMH batteries produce 1.2 volts; LiPo batteries produce 3.7 volts. An 18-volt rechargeable battery is made from 15 cells, each having an output of 1.2 volts. Cells in a battery pack are wired in series to combine the voltage from each cell. If an 18-volt rechargeable battery pack is discharging quickly, rebuild it.
Charge the 18-volt rechargeable battery pack until it's fully charged. Let the pack rest for several hours without using. This process allows bad cells to discharge while the good cells retain their charge.
Remove the cover from the 18-volt rechargeable battery pack. Unclip the cover or use a small electrical screwdriver to undo the screw it. The 15 batteries are wired in series. Label each battery numerically. Remove them from the battery pack. Write one through 15 on a piece of paper.
Set the voltmeter in the range 0 to 1.5 volts. Each cell, when fully charged, has an output of 1.35 volts, which quickly reduces to 1.2 after the cell is in use. Place the red positive connector from the voltmeter on the positive terminal of battery 1. Place the black negative connector on the negative terminal of battery 1.
Write down the voltage displayed on the meter next to number 1 or the piece of paper. Repeat the process for the other batteries. When finished, you should have the voltage written down for each battery in the same numerical order as the labels on the batteries.
Separate the 15 batteries into two sections. Place all the batteries that have voltmeter readings between 1.2 to 1.35 volts in one area. These batteries are usable. Put all the batteries with a voltmeter reading less than 1.2 volts in another area. Dispose of these batteries.
Replace the batteries you have discarded with new ones. Do not label them. This procedure helps you identify the new batteries if you need to rebuild the battery.
Insert the 15 batteries in the battery pack. Replace the cover. Put the batteries on charge for a short while to ensure they have equal voltage.
If you have a sealed battery pack, don't break it open. Purchase a new pack, ensuring it has a removable cover. Always dispose of rechargeable batteries responsibly.