How long did it take US fleet submarines during World War 2 to charge their batteries?

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Run silent, run deep is about a WW1 S-class submarine. Looking at their specifications they all have 2 diesels, not 4.

Using a Group 1 sub as an example:

Propulsion: 2 Γ— New London Ship and Engine Company (NELSECO) diesels, 600 hp (448 kW) each; 2 Γ— electric motors, 750 horsepower (560 kW) each; 120 cell Exide battery; two shafts

Speed: 14.5 knots (27 km/h) surfaced; 11 knots (20 km/h) submerged

So with 2 diesels dedicated to charging, they put 900 kW into the batteries. 10 minutes gives 9000 kW.min or 150 kWh, enough to run one motor at 1/4 load for an hour. (yes, I ignored charging losses. I've no idea how efficient 1914-era electric systems were). 1/4 on one motor should be enough to get to 2 kts (drag increases with the square of speed).

The big question is what the non-propulsion loads were. If we take 30 kW (a little under 1 kW/person for the 40-man crew) that gives 5 hours of endurance if you're sitting still.

The later groups have more diesel power. For a Group 1 boat it really depends on the non-propulsion load, but the later groups should be able to do it.

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