Originally Posted by
Roelof
All chargers today are using a switched voltage regulator. Because these regulators do not make a loss into heat you can talk about power in watts. Charging 8.4v with 5A is 42 watt, on the 12v side the current will be 42/12 = 3.5A
Switching regulators still have losses. Typical efficiencies are somewhere around 80 to 90%. The rest comes out as heat.
The power supply should be rated at higher power than the charge voltage times charge current. An extra 30% is a good rule of thumb. In your example that would work out to 12V at about 4.5A.