Replace the cells in the battery pack
When your power tool battery pack bites the dust, probably your first inclination is to buy a new one. But if the battery is out of production or the price of a new one is sky high, consider rebuilding it yourself instead of paying a professional rebuilder. That'll save you about $25. You’ll need new cells, a soldering iron and rosin-core solder, a glue gun, and some wire and tape. The whole process takes about an hour.
Most power tool battery packs use 1.7-in. x .9-in. “Sub C” size batteries. Remove the screws that hold the pack together and yank the top off. Then remove the cluster of batteries and measure one of the cells. Then get the milliamp hour (mAh) rating off the old batteries (or search online for the cell specs). If you're willing to spend more, upgrade to cells with a higher mAh rating—they hold more juice. Just make sure they have a discharge rating of 5C or higher. And always buy cells with the tabs already welded in place (all-battery.com is one online battery seller). Then get to work.
Using the old battery pack as a template, arrange the first row of new cells in the same configuration (Photo 1). Then solder the tabs (Photo 2). Duplicate the process with the additional rows of cells. Transfer the temperature sensor and insulators from the old battery and reinstall in the plastic case. Charge it up, test it out and start your next home improvement project.