Trolling Motor Battery Charging

by Jack
(Essex England)

I need to know if it is possible to run a 24 volt trolling motor and to be able to charge 3 batteries at the same time or with a switch, from the main engine charging output. I am familiar with how a) voltage readings and b) specific gravity readings are used to measure battery state of charge. My query is this: how does one know that a 400a/h battery bank is full, i.e. has 400a/h available? I have recently fitted a BEP battery monitor which, amongst other things, tell me the battery capacity (as a % and in a/h). My batteries are brand new, I have a new Sterling 40amp charger (Adverc on 90 amp alternator on engine) and, even after a week on (shorepower) charge, can't get close to 400 a/h (100%) charge. I have very carefully installed and calibrated the BEP monitor. Is there another way to measure capacity, other than voltage & specific gravity?