Charging in series
 

Is charging multiple batteries, like 2 or 3 packs in series damage them?

I was told the amount of damage you are doing to the cells completely outweighs any time or financial advantage you get from one charger that'll do 12 or more cells.

Yet I do this all the time, problem free, with both Nimhs and Lipos.

Heat kills
As long as you're not exceeding the current and heat capacity of the cells, you'll be OK.
1c should be fine with any amount of cells. Your bottleneck will be the charger and power supply capacity.

It's not a problem IF all the cells are balanced... nimhs included. You will be doing damage if the cells don't all peak together.

The more cells you have in series, the higher the odds that there is a cell getting over/undercharged.

But we know there are plenty of battery packs out there with a lot of cells... like all those 18v-36v nickel cordless drill packs, etc. Some may have a balancing circuit built in, but I'm sure most don't.

Yeah, they really need to be balanced! and they also need to be of the same quality.. and the same capacity, and the same amount of stored energy when charging is started..

Not really adviced.

And the same number of cycles...