Normally, you should replace the set. If one battery is bad, chances are the other one is not far behind, especially if they’re several years old. Placing a new battery in series with one that’s half used up will result in much reduced service life of the new battery. The only exception is if a battery goes out within the first 6 months of service and you have been charging them regularly, the one bad battery can typically be changed without causing a problem to the set.
Posted in Battery.