Yelling down the wrong voicepipe, it's the eng power. Stock game I submerge to periscope depth and run all ahead standard, about 7 knots, goes 10 miles before the battery is down to 50%. Stop and surface, takes 36 hours to recharge.
Change the eng power from 1200 to 9600 and it depletes the battery 50% over the same time and distance, but takes half the time to recharge. Another test I changed that value from 1200 to 49600.0 and it recharged from 50% in 10 hours. Again near as I can tell that eng power number doesn't affect anything except the recharge time. If we had an idea what the recharge time SHOULD be we could probably get it pretty close just by changing the eng power number.