Add a new battery

[mic54]

Is it possible to add a third battery on a 2004 Maxum 2500, do I have to change the battery switch or the battery charger?

[seapuppy]

it depends on how you wire it up......if you wire it up parallel tied to your house bank....it will increase your time and not your voltage...ie you'd still be 12vdc but amperage load will be increased.....your charging system will work the same and it will be on #2 load on the sw....
if your just wanting to have another batt....then get the sw./charger for a 3rd bank.........

PS..welcome to the zoo!!

SP