How to Repair a Battery Pack

Batteries for power tools and other industrial devices can often be repaired by replacing one or all cells. Finding a NiCd and NiMH cell is relatively easy; locating the correct Li-ion cell can be more difficult. Naked Li-ion cells are not readily available off the shelf and a reputable battery manufacturer may only sell to certified pack assemblers. Incorrect use or lack of an protection circuit could cause stress and disintegration of the replaced cell. When repairing a Li-ion pack make certain that each cell is properly connected to a protection circuit.

If a relatively new pack has only one defective cell, you may replace only the affected cell. On an aged battery, it’s best to replace all cells. Adding a new cell with full capacity in between neighboring cells that have faded would cause a cell mismatch. Matching the replacement cell with one of a lower rating may work but this fix is often of short duration. Always replace with the same chemistry cell.

A well-matched battery pack means that all cells have similar capacities. An anomaly can be drawn with a chain in which the weakest link determines the performance of the battery. Read more about When replacing all cells, the rating is less important as long as the differences are not too large for the charger to handle. Cells with higher Ah will simply take a bit longer to charge.  The state-of-charge of all cells being charged for the first time should have a similar charge level, and the open-circuit voltages should be within 10 percent of each other
Many visitors of BatteryUniversity.com ask if NiCd can be replaced with NiMH? Theoretically, this should be possible but charging may be an issue. NiMH uses a more defined charge algorithm than NiCd. A modern NiMH charger can charge both NiMH and NiCd; the old NiCd charger could overcharge NiMH by not properly detecting full charge state and applying a trickle charge that is too high.
Welding the cells is the only reliable way to get dependable connection. Limit the heat transfer to the cells during welding to prevent excess heat buildup.
Simple Guidelines when Repairing Battery Packs

• Only connect cells that are matched and have the identical state-of-charge. Do not connect cells of different chemistry, age or capacity.
   
• Never charge or discharge Li-ion batteries without a working protection circuit unattended. Each cell must be monitored individually.
   
• Include a temperature sensor that disrupts the current on high heat.
   
• Apply a slow charge only if the cells have different state-of-charge.
   
• Pay special attention when using an unknown brand of cells. Some may not contain a high level of intrinsic safety.
   
• Li-ion is sensitive to reverse polarization. Observe correct polarity.
   
• Do not charge a Li-ion battery that exhibits physical damage or has dwelled at a voltage of less than 1.5V/cell.
   
• When repairing Li-ion, assure that each cell is connected to a protection circuit