Here's a product that I just stumbled across that may be of interest
to those with large battery banks that are difficult to access:

http://www.batteryfillingsystems.com....asp?page=home

I have no experience with this system but it was recommended by
someone on a boating EMAIL list that I subscribe to, and looks like it
might solve a problem on my boat.

Basically it uses special replacement battery caps that are tied
together with plastic tubing and a remote filling device. Each cap
has a float valve which closes when the cell is full. They sell
different types of caps and floats depending on battery type.

It is being marketed to industrial and golf cart users but I can't see
any reason why it wouldn't work on a boat.