My (nearly) 20 year old cordless drill not longer holds charge and replacement battery packs are no longer available. The posts shows the steps I took to get it working again.

On the outside of the battery pack there is a warning saying "do not open". Defiantly, I open the battery pack. There are only 4 screws holding it together. Then carefully lift out the cells.

Luckily, the wrap is labelled. We need 8x 1.2v 4/5 Sub C Ni-Cad cells.

Further inspection shows the state of the battery pack inside - covered in corrosion and rust. I think these are past the point of reconditioning.

It was difficult to source these type of batteries. I considered using Ni-MH or Lithium, but this would have meant modifying the charger. Eventually I found some from AllBatteries.co.uk: https://www.allbatteries.co.uk/rechargeable-nicd-battery/4-5sc/standard/ft.html. Shipping was from France and it took over 3 weeks. The alternatives on Amazon had bad reviews and were 3x the price.

To do this job, I got myself a cheap battery spot welder from AliExpress for £20, shipped in 5 days from China. I used 0.15mm nickle-plated steel strips and not pure nickle because of increased price and the low amperes of the drill.

Ampacity charts that I found on various battery and EV forums show amp ratings of different materials. I yield to their expertise since they are building big Lithium packs - a little more dangerous to handle than Ni-Cd, Nickel-Cadmium.

I cut the strips to size and spot weled the batteries together in pairs. Each pair are 2 cells connected in series.

I then used clear heatshrink wrap to bind each pair together to stop them flopping around. The original battery pack was bound together in pairs with a paper wrap.

Once my pairs were done, I pre-positioned them so that I would weld them correctly the first time. The complete battery pack is 8 cells connected in series.

1.2 x 8 = 9.6

I cleaned up the terminals on the outer casing of the battery pack with abrasive paper and files, removing the corrosion. Then soldered two new wires and some new red and black heatshrink tubing. Finally I reassembled the pack.

The total cost of repair (excluding the spot welder) was £24.14. This has saved another piece of perfectly working electronics from landfill and saved me a ton of money (new drills are crazy price because of Lithium battery packs).

Remember to dispose of battery cells safely and in accordance with local guidelines.