Battery connection for bilge pump

[Ross Gard]

My batteries for the bilge pump are just under floor, sitting on bottom of bilge. So if water comes in, I only have the battery depth, before the batteries are covered, and no power.
So I want to place an extra battery above floor level.
So how do I connect the bilge battery so it is continually charged, along with rest of boat power. But remains separate from shorting if water covers house battery, but remaining connected to house battery bank?
Regards Ross.

 

[arf145]

Something like the Xantrex Echo Charger would do that. Hooked to main batteries and the sump backup, it would trickle some power to the backup whenever the mains were charging.