Thanks for the comments and concerns.

I measured the voltages of the cells and they all measure the same. No issue there. We don't quite understand the issue of why one gets so hot and the other only warm.

I am looking at batteries, and:

Is 30a maximum discharge enough? The maths say 30a isn't enough, but our current batteries, although they get hot, seem to to the job, and I doubt they're designed to give 30a discharge as drill cells. Would paying double and going for 60a be worth it?

I can't seem to find a fast enough charging charger that doesn't cost more than the batteries themselves. Any help with this? Is there a golden charger that everyone uses?

What do you guys recommend we do? We could get 3x 6v batteries, put 'em in series and use our current charger, or buy two 9.6v packs and buy a new charger?

Decisions! All of them costly!

Thanks for the kind words guys, really!