Many makers consider batteries to be a consumable object. Even rechargeable models have a finite lifespan, and when they're done they're done. Some, though, dive deeper: we've looked at Micah Toll's customisable Maker Batteries in the past, and the impressive ground-up iron-air open hardware battery project, but Well Done Tips' latest Instructable sits somewhere between the two in terms of complexity and accessibility: refurbishing old multi-cell nickel metal-hydride (NiMH) batteries.
The project began as a means of eking a little more life from power tool NiMH battery packs. Inside each pack is a series of cells, soldered together using spot-welded tabs; when the battery goes 'bad,' it's often the case that only one or two of the individual cells is actually faulty. Rather than throw out the entire pack, Well Done Tips demonstrates how to find the faulty cells, cut them out of the pack, and replace them. While admittedly a lot more involved than simply buying a new battery pack, it's a considerable money-saver: where a brand-new pack for a power drill can cost upwards of £30 for even the lowest capacity in an off-brand compatible, the cells used in Well Done Tips' guide cost just €3 each with the pack requiring two replacements.
There are a few caveats, of course. The project does require handling batteries that can hold quite some considerable charge, which should always be done with care. Under sustained use the connectors can be subjected to considerable currents, too, meaning your soldering skills should be reasonable before attempting the repair. You're limited, too, to buying cells which already have solder tabs applied, increasing the cost - unless you've something like Jack Davies' homebrew resistance spot welder somewhere in your workshop. Still, it's a great way to get a little more life out of your hardware, and a good overview from Well Done Tips.
I have two Bosch NiMH batteries from cordless power tools. One is performing very well, but other not so good. Most of the case there are two reasons of that: first - natural battery wear down or only few cells just die inside battery pack.