Always open the sub to charge the batteries ,no need so far with Lipos which is nice on my Robbe XXI ,Lipos run far longer and are lighter than the standard battery,love them !

In my XXI the lipos are in a semi air sealed compartment in the forward section that is vented automaticly to the ballast tank each time it surfaces which is another nice feature of this designe.
If you where to look at my threads on the modified Type XXI the cats Meow hear on RCU you can get an idea of what I did,I charge the lipos without having to open the sub up ,there are 2 wirtes red and black that when the sub is turned off become hot directly from the lipos mains,the lipo charger is programed to ballance charge them and so far all is good,after extensive testing of course.

If you went with lipos you should get the same benifits that I do ,my lipos are 2 5500mah bats.

Dave