Chris Holmes wrote:
They are Nicads. And I think the charger is dumb.

I wonder if I could use NiMH instead?

You can, although NiMH have higher internal resistance so don't have as much
'grunt' in power tools. It's worth getting 'low self discharge' cells
otherwise they self-discharge over a month or two - if you put the tool away
they'll be flat next time you use it.

If you go for lithium ion it'll give the tool a new lease of life but it's
more work - you can probably get 3x 18650s to fit inside the pack, although
you'll need a BMS board (search ebay for '3S BMS' for whatever current
rating) and a new 'dumb' 12.6v CC/CV charger, eg:
https://www.ebay.co.uk/itm/DC12-6V-1...r/254777843396

Although if you're doing that you'll probably get better performance by
buying a new drill...

Theo