I read your posts about Power over Tether solutions not being ready. For my application it would actually be enough to trickle charge the batteries over the Tether. Operation would be normally battery powered, but the batteries can not be removed from the vehicle for charging (has to stay under water even though inactive).
Has somebody already tried that? Is a small enough charger module available to be included subsea, maybe instead of one of the batteries, that can be driven using a low DC voltage down the Tether.