Did you know that you can fairly easily replace the individual dead cells in rechargeable battery packs like the NiMH and NiCad kind found on cordless power tools? The process can be a little fussy, but if you want to save some bucks and have a few minutes to spare, you can bring a dead or dying pack back to life.
The above video, from Well Done Tips, shows you how to do it. You basically have to follow these few steps:
Test the Voltage of Your Pack
Using a multimeter set to Volts, check to see how much your fully-charged pack holds. In this example, he measures the available voltage in a healthy, fully-charged battery pack and the voltage in his failing pack and gets a difference of 2.7 volts. He guesses, given that difference, that there are two dead 1.2 volt cells inside.
Take the Pack Apart
The battery pack will usually come apart with some removed screws, peeled-away adhesive foam, and some removed spacers/insulators. Be careful taking all of this apart, save it, and be sure to remember where it all goes (take reference photos). You’ll be reassembling it the same way. BTW: Your pack may have special Torx screws that require specialty bits.
Find the Dead Cell(s)
Using a multimeter, find the dead cells. As suspected, there were two dead 1.2 volt cells in this pack. Once ID’d, he uses a knife to pry away the metal battery contact strips from the dead cells and removes them from the pack.
Replace Dead Cells
He bought his new NiMH cells in a local electronics shop for €3 EUR each (about US$3.18). The types of cells in these packs are usually designated as “Sub C.” You’ll need to know the type, size, voltage, and capacity of the cells you need. Note that there are two sizes of Sub C cells, the full-size (as seen here) and a “4/5 Sub C,” a slightly smaller version. The cells he bought came with already attached metal tabs (which most of them do). Soldering on these contacts to attach a new cell is acceptable. It’s best if you have a battery contact spot welder, but who does? If you care to, you can actually build one using little more than a 12v car battery (or similar), hook-up wire, and some thick copper wire for the contacts (and an insulating handle). There are a number of videos available for how to build one of these. They all look like a shock and a burn waiting to happen to me, but your mileage may vary.
Charge and Test the Re-Pack
With everything soldered together, it’s time to give the pack a full charge to see how the repair went. Once charged, test it with your voltmeter and it should meet or exceed the original voltage rating of the pack.
Reassemble and Enjoy!
With your battery pack reassembled and as juicy as new, it’s ready to be put to work. After you’ve been through this repair process once, and have fresh cells on-hand, you’ll be able to replace them much more quickly and efficiently. And you can save some real money. A cell is only about $3 and new battery backs cost around $50.
There are dozens of videos showing how to do this type of repair. They’re all similar to the above, but you might want to watch a few just to get some tips, especially if you can find one for the brand and type of pack that you have. There are also some Instructables on how to do these type of cell replacement.