Eighteen-volt batteries are commonly used in such professional power tools as drills, which require more electrical energy than power tools designed for the DIY enthusiast. However, an 18-volt battery is not a single battery cell, it's made from multiple battery cells that are wired in series, a method that combines the voltage from each cell. Batteries used in power tools and many other devices are rechargeable. A typical 18-volt battery is made from 15 1.2 volt nickel cadmium (NiCAD) or nickel metal hydride (NiMH) cells. Battery cells don't go dead at the same time so although you are having a problem with your battery retaining its charge, it may only be a few battery cells causing the problem.
Read the label on the 18-volt battery pack to determine the type of battery cells the pack contains. If it says NiCad or NiMH, continue to follow the steps to rebuild your battery. If the label says lithium ion (Li-on), do not proceed. Rebuilding lithium ion batteries needs specialist knowledge as the battery packs contain power surge breakers. An 18-volt battery does not contain lithium polymer (LiPo) cells as each cells produces 3.7 volts and is not a multiple of 18 volts.
Put your 18-volt battery on charge until the charger indicates the battery is full before attempting to rebuild it. Don't use your fully charged battery. Instead, leave it overnight. This ensures that any bad battery cells lose their charge. Good cells retain their charge.
Remove the cover from your 18-volt battery. Some battery packs have small Phillips-head screws, others have clips. Use a suitably sized Phillips-head screwdriver if it has screws or a small flat-head screwdriver if the cover clips in place. Insert your flat-head screwdriver under the side of a clip and twist it to release the clip. Once the cover is removed you will see 15 battery cells.
Remove the 15 battery cells from your 18-volt battery. Most cells can be pried out using a flat-head screwdriver. If they are soldered into place you will need to heat a soldering iron then touch each battery cell terminal. Let the solder melt then quickly pries the cell out before the solder solidifies. Repeat this for all the 15 cells.
Label each battery cell numerically and write a list of corresponding numbers. Your 18-volt battery has 15 cells so number the batteries one to 15 and write one to 15 on the sheet of paper.
Use a voltmeter to test the output voltage from each battery cell. Set the voltmeter to read volts in the range of 0.5 to 1.5 volts. Touch the clip attached to the end of the red wire from the voltmeter to the positive terminal of battery No. 1 and the clip attached to the black wire from the voltmeter to the negative terminal of battery No. 1. Take the voltmeter reading and write it down next to the place you wrote "No. 1."
Repeat the process for the remaining 14 battery cells. Write the readings from each battery next to the corresponding number on your sheet of paper.
Separate the batteries into two places. Batteries that have a reading of 1 to 1.2 volts are good batteries and need to be kept. Batteries that have a reading of less than 1 volt are losing their charge and should be replaced, even if the reading is as high as 0.9. Dispose of the bad batteries.
Work out how many new battery cells you need to buy. You'll need 15 cells so if you disposed of five you need to buy five new ones. NiCad and NiMH cells are widely available from electrical outlets. Get the same type as the rest of your good battery cells. Never mix Nicad cells with NiMH cells.
Replace the 15 cells in the battery pack. Put the positive and negative terminals the correct way around. The cell compartments are labeled "+" and "-." Either push them in place or use your soldering iron to melt the solder on each terminal. You may need to add extra solder to get a secure fit. Only touch the terminal using your soldering iron for a couple of seconds. You don't want to damage the cells or melt the plastic battery pack.
Replace the battery pack cover using a screwdriver or press the clip in place. Put your rebuilt battery pack on charge for a few hours to ensure that all the cells have the same charge. Your 18-volt battery is ready to power your device.