Brum, the rule of thumb I learned for battery charging is

( 1.5 x batt. capacity ) / charge rate

This assumes that battery is fully discharged. It always takes more energy to charge a battery than you get back out. The extra energy shows up as heat (battery gets warm).

For 2300 mah NiMH cells, and a charge rate of 700 mah, that's 4.9 hours. 3 hours may be for charging some old 1500 mah cells or possibly NiCd cells.

As far as converting your 12V, 1A output to what you need - there are many variable switching power supplies available. They take 12V and convert it to any other standard DC voltage. They're small and light - look at the charger for a cell phone for an example.