How to Rebuild a NiMh Battery Pack
Nickel metal hydride (NiMH) battery packs are becoming more popular than nickel cadmium (NiCad) battery packs as they are not as susceptible to the commonly referred to term "memory effect." If a NiCad battery pack is not regularly fully discharged before recharging, crystals in the cells grow, reducing the surface area, meaning the battery pack can't hold as much energy. While NiMH battery packs don't get this problem, they can suffer from voltage reduction, which means that your device may not operate at full power. If this occurs, try fully discharging the battery, then recharging it. If there's no improvement, then it's time to rebuild your NiMH battery pack.
- Charge your NiMH battery pack in the usual way until fully charged. Remove from the charger and leave overnight. You need to allow time for bad cells to lose energy while good cells will retain their energy. There's little point in replacing good cells.
- Place the NiMH battery pack on a workbench. Remove the cover from the battery pack. Small Phillips screws usually hold the cover in place, so use a Phillips screwdriver to remove the screws, and then take the cover off. The NiMH cells are visible. The number of cells depends upon the output voltage of the battery pack. For example, a 9.6-volt battery pack contains eight 1.2-volt cells.
- Use a voltmeter to test the output voltage from each NiMH cell. Starting at the top of the battery pack, place the sensor on the end of the red wire that's attached to the voltmeter onto the positive terminal of the first battery cell. Put the sensor on the end of the black wire that extends from the voltmeter onto the negative terminal of the first battery. The terminals are marked on the cell "+" and "-."
- Read the voltmeter display. If it reads between 1.1 and 1.2 volts, then the cell is good and doesn't need replacing. If the reading is less than 1.1 volt, the cell needs replacing. Use a marker pen to make a mark on the cell if it's bad so you know which ones to replace. Repeat the process for all the remaining cells using the voltmeter. Ensure you mark bad cells.
- Get replacement NiMH cells from your electrical outlet. They are widely available. Get the same number of cells as you've marked as bad using your pen.
- Remove the bad cells from the battery pack. Some cells simply push in place, others are soldered. If they push in, use a small flat-head screwdriver and carefully pry the bad cells from the battery compartment. If they are soldered in place, heat a soldering iron, then put the end of the soldering iron on a cell terminal. Let the solder melt then remove the soldering iron and very quickly pry the cell out of the battery compartment using a small flat-head screwdriver before the solder goes hard. Repeat the process until you have removed all the bad cells. Turn off the soldering iron.
- Get a clear plastic bag and put the bad cells in it. You need to dispose of the cells in a battery recycling unit; they mustn't be put in your regular trash.
- Put the replacement cells into the battery compartment. Push them into place if that's how they fit, or heat a soldering iron. Put the end of the soldering iron onto a cell terminal. Introduce the solder and let a small amount melt. Remove the soldering iron and let the solder harden. Repeat the process until you have solder all the cells in place. Turn off the soldering iron.
- Let the battery cool completely. Replace the battery cover using the Phillips screws and your screwdriver.
- Put your rebuilt NiMH battery pack on charge for about 1/2 hour. This ensures all the cells are fully and equally charged. Remove from the charger and your battery pack is ready for use.
Things You Will Need
- NiMH battery cells
- Small Phillips screwdriver
- Small flat-head screwdriver
- Soldering iron
- Some battery pack covers have clips rather than screws. To remove the cover, pry open the clips with your fingers, or slide a small screwdriver under the clip and lever the clip open.
- If you find your battery pack is a sealed unit, don't tamper with it. You will break the battery pack and it could be dangerous. Get a replacement battery pack ensuring it has a removable cover.