I think I've managed to crack the code of the numbers we see for U-Boat Batteries.

27 MAK 740

I think it means:

[Number of Positive Plates in Cell] + [Type of Battery] + [Watt-Hours per Plate] + [Descriptor]

I can't find anything to prove it, but I have a hunch that German acronyms for battery types were:

MAK = Marine Akkumulator Kalzium – Marine Battery, Calcium-Type
MAL = Marine Akkumulator Lead? – Marine Battery, Lead-Type (Not Sure)

Descriptors may have been:

W = Unknown – Used on Type IXC Batteries. (example: 44 MAL 740W)

E = Elektroboot – perhaps optimized with slightly different battery chemistry/plate construction to handle the repeated chargings/dischargings from near continuous underwater travel? (example: 44 MAL 740E)

If my hunches are correct; the 44 MAL 740 cell designation deciphers to:

44 positive plates
Marine Battery with Lead Plates
Rating of 740 watt hours per positive plate

leading to the following rating:

44 plates × 740 watt-hours/plate = 32,560 watt-hours ÷ 2.1 Volts = 15,504.76 amp-hours per cell at lowest possible discharge rate.

The train of thought that led to the above was the discovery of the existence of some experimental batteries which were under test at the end of the war which had the following designations:

AFA 70 MAL 760 (9600 Ah @ 6000 A discharge – 12 months' lifetime)
AFA 60 MAL 760 (slightly lower capacity – 15 months' lifetime)

It's no coincidence that the American Sargo I wartime batteries were 47 plates, while the post-war Sargo II batteries had 77 plates – the same rough numbers used in the 44 MAL and 70 MAL battery designations.

Additionally, a key clue to designations is that in lead-acid battery construction, the number of plates per cell will always be an odd number – a 13-plate battery will have seven negative plates and six positive plates per cell. The use of even numbers in German battery designations indicate they were measuring by positive plates; while the US measured by total plates.