Okay, after poking around in Silent 3ditor I found this;
\Data\Submarine\NSS_s18\NSS_S18.sim
9:unit_Submarine\unit_Ship
That's what the default values look like. Testing in game I ran submerged until the batteries were 50% discharged, then stopped and surfaced, timed how long to full recharge. 36 hours. In another copy of the game I hacked that file;
(Tripled the drag on the rudder just because it bugs me that you can't reverse one prop while going ahead on the other, that value increases the rate of turn.) The horsepower of the engines apparently increases the recharge rate, jacking it from 1200 up to 9600 reduces the 50% recharge time from 36 hours to about 17 1/2 hours. Near as I can tell it doesn't affect anything else, altho I haven't done anything other than quick tests for speed, range, fuel consumption and battery discharge rate.