Fixed cell chargers being infinite energy sources.

Looks like someone modified the cell chargers without understanding that the units of power are not the same as the units of charge. The result is that cell chargers would give their power cell much much more energy then they used, so theoretically you could run power-consuming machinery forever by swapping the APC between two power cells.

I changed the power used to preserve the old charging rate, so power cells will still charge in the same amount of time. Incidentally, the value for CELLRATE might be far too low.