I have a Bosch (blue) GSR12-2 cordless drill that I bought with a
mains SDS drill on sale from Screwfix in 2009. (IIRC, someone here
pointed the sale out to me. Thanks, whoever!)

The drills are still in good working order, but one of the two
batteries that came with the cordless one is dead. Specifically, if I
put it in the charger, the light blinks for a while (charging) then
indicates "charged", but when I put it in the drill, absolutely
nothing happens. The other battery is still working, but it doesn't
last as long as it used to, & I have to be careful not to run low in
the middle of a job.

I googled the battery number, 2607335709, & it appears to be extinct,
but you can buy "compatible" ones for £25 & up. I'm interested in
opinions on whether replacing one or both batteries is worthwhile.

Thanks.