I think you're confusing terms there. RTGs don't produce power, they produce electricity which goes right into the batteries. Also, adding batteries to an electric engine chain doesn't increase efficiency, efficiency is a setting on the electric engine that reduces power output in order to also reduce the amount of electricity used per power(much like turbochargers on a fuel engine reduce fuel per power).
I did a quick test to see if I could figure out the formula for total power output per battery. With the electric motor set to 100% output, every 1x1 battery block, which has an electricity storage of 2000, adds 80 power to the motor output when fully charged, meaning every 25 units of electricity gives 1 engine power from the motor. It also consumes 80 electricity per second at 100% output. Using larger battery blocks makes no difference - two 1x1 batteries has the same effect as a 1x2 battery.
Lowering output increases efficiency, but not at a linear rate - at 90% output efficiency is 105% and change, at 50% it's 133%, at 5% it's 190%. At those same settings, power produced/electricity-per-second numbers are 72/68.4, 40/30, and 4/2.1 respectively. Not entirely sure on this, but I think the game multiplies the stored electricity by the output value(100% is written as 1.00), and still gets 1 power from 25 electricity, and gets electricity-per-second by dividing the power output by the efficiency value.
Since the amount of engine power produced by the electric motor is based on electricity stored, power output goes down as the batteries deplete. As for why it's based on power stored and not number of battery blocks, I don't know. Real life batteries do have less voltage when under-charged.
PRE-EDIT: Ninja'd, but I had to do a bit of research on how to irl batteries first.