[Jimbo1959]

Is there a general rule of thumb concerning battery charger output (amps) vs. battery bank size?

[FISHTIGUA]

Try this guide from West Marine.

The West Advisor: Battery Charging

[Capt J]

Not just battery size, but you want to size the charger based upon the amperage draw you can put on the battery bank under normal usage if you're staying on board the boat. You basically want a charger that can keep the bank charged with all of the lighting on, water pump, shower sump pumps, refrigerator and anything else DC that you might run while living on board.......

[K1W1]

Hi,

This is extracted from the article linked to above.

What size charger do I need?

Deciding how big your charger needs to be is determined by the size and type of your batteries, and whether your boat has a continuous or intermittent source of AC power to run your charger. Boats that spend most of their week at a dock, constantly hooked up to shore power, require smaller chargers. You need enough capacity to run the continuous loads on your battery system, like DC refrigeration (frequently the biggest user of battery power) and lights, plus enough power to float-charge your batteries. A good rule is to have enough amperage to equal the sum of the DC loads plus 10 percent of the amp-hour capacity of the batteries.

If you’re cruising or anchored out, and aren’t plugged in except intermittently, you will want enough capacity to recharge in the time you have available, if possible. You need enough juice to replace the power consumed by all the DC loads, as above, plus an average charge rate equaling the amp hours required divided by the hours available. Maximum amperage that batteries can accept during the Bulk Phase of the charging cycle vary depending on battery chemistry: flooded batteries can accept a charge rate of up to 25 percent of C; gel batteries have a higher acceptance rate of as much as 30 percent; AGM batteries accept the highest charging amps, as much as 40 percent of C.