How quickly does an alternator charge a marine battery?

[bass_case2]

I have a new to me 07 22ve with two deep cycle batteries and a 1/2/all combiner. Last time we went out we ran audio on #2 for hours and got a low battery warning.

It was about time to head in so I switched to all and drove back to the marina.

it’s been a couple weeks and now I’m wondering if the 15 minute drive back to the marina charged it at all.

how long do you generally have to run the boat to get a decent charge back on a battery?

[okiepilgrim]

Not an expert here, but depending on what alternator you have and what is pulling power while you're underway (e.g. radio, ballast pumps, lights, heater, blower, etc.) your batteries should be receiving voltage in the 12.6 -14.5 range. Similar to if your batteries were hooked up to a charger, the amount of charge they receive will depend on their size, the voltage they're receiving, and time. I don't know what level triggers a low battery warning on your boat, but if we assume it was 80% discharged, then it would likely take several hours to charge that back up to near 100%. My group 31 batteries can take 12 hours or more to charge on a 10 amp charger if they're fully discharged. I have no idea how many amps make it to the batteries, but higher amps would charge faster.
I would guess your 15 min drive didn't get you much on that house battery in "both" mode. I have an onboard charger and typically charge mine every time I pull it off the water.