It's best to use a charger that reads the battery charge and turns off when the battery reaches full charge, then turns back on to trickle at a certain charge, like 80%. My experience with my Formula 350 was they don't do that, but you may have a completely different set up. I would hope the manual for your charger would tell you how it works. The charger in my current boat turns off at full charge, back on at 80-85%. It helps to not prematurely fry the batteries.

Depends on the charger. My charger goes into maintance mode when the batteries are fully charged. When in maintance mode, once a month the batteries are discharged and recharged to help life of the battery.

Also depends on the batteries on board, if you have a mix of lead acid and AGM you cant run them off the same charger.

To clear it up the best way I know how is to do this. Post a picture of your batteries. Post a picture of your charger with the model number. One of us can look it up or may know off the top of our head what that charger and battery combo prefers.

Use the onboard charger. It's designed to maintain too. I had a 312 as well and I think it had a Charles Marine Products charger (currently discontinued making marine chargers).

Here's a manual for a unit the Series 5000SP chargers.

5000SPChargers.pdf

From the manual....

Proper Operation
When properly installed and connected, the SP series C-Charger will automatically charge and maintain the batteries. When power is applied, the SP series C-Charger will enter a bulk or “constant current” mode, during which a higher rate of charge is achieved to fully replenish the batteries. When the batteries near a full charge, the output current will decrease and the charger will enter a timed absorption mode. After four hours in absorption mode, the output will drop to a float rate to maintain the batteries at a temperature compensated finishing voltage (13.6 VDC @ 25C for lead acid).

Never heard of any chargers that do a rated discharge.