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A Type XXI with its standard battery comprised of 372 elements, lead-acid accumulators weighting 620Kg each could travel for about 400Nm underwater at 5Kts

The heavvy duty batteries of the day could hold about 35Wh/Kg of energy. Modern Lead acid accumulators nearly double that figure with 60Wh/Kg for long discharges.

A type XXI equipped with modern Varta-Hagen batteries designed for submarines would be able to travel for 700Nm at 5 Kts.

If we decide to use more effective accumulators like Silver-Zinc or Silver-Iron (employed by some russian subs but very expensive) the total underwater range of the XXI might be extended to some 1200 (!) Nm at 5Kt. That means 10 days underwater. Naturally we would have to install some air recycler apparatus as the air onboard a type XXI lasts only for 48 hrs.

Just for comparison, a modern fuel cell or AIP submarine like the Gotland, U-214 or Agosta 90B can travel for about 1500Nm before having to resurface. And since the AIP system needs special fuel, once the stores are exhausted its underwater range is much lower, about 300Nm

Would the XXI still be a kickass submarine ?. Probably not. Too much noisy with its conventional design propellers. A damn fine design for its time and for the following 20 years, though.