Does this mean the rest goes to heat? So I'd be better of with say a 9V supply than a 12V supply because the 12V supply will just make the regulator get hot?
But I don't understand what you mean when you say "which cells" I'm going to use and how many. I have 6V NiMH battereis (5 cells each).
Oh, you got it
Different chemistries have got different end-of-charge voltages, so knowing which you've got is important if you want minimum possible losses.
Are you implying that I can charge only certain cells, or just that I need to know what battery I want to charge?
As above, assuming you want to make a charger for your particular batteries without wasting too unnecessary power.
5 NiMH cells will have and end-of-charge voltage of ~7.40V to 7.45V when fast charging at around 1C (i.e. 2000mA for 2000 mAh cells) and it will take around 66 minutes to fill a totally depleted cell.
You'll have to allow for voltage drops in the current switching element, whether it be a transistor, a relay or whatever and you'll have to allow for mains voltage variations as well (assume a drop of ~10% worst case).
If your switching element have a drop of say 0.5V, you'll need a supply of around 9V ((7.45+0.5)*110% = 8.75V) for your 5 NiMH-cells.
Are you perhaps using a MAX712?
If not, at least read through the datasheet at http://www.maxim-ic.com/getds.cfm/pk/1666
, it will give you a better understanding on the subject.