Ask PYS: Power

Bill – Battery Rate of Charge, on Jul 14, 2014 asked:

I have three gel batteries on board my 32 foot Catalina sailboat. To my understanding these are very good batteries. I moored the boat out for the last two days with the fridge on. Today I started the diesel engine and ran up the coast for about 2 hours. Will that have recharged the batteries? I understand all the batteries are wired together with no separate battery for a starter battery. I am concerned as it is moored out again tonight with the fridge on. It is not plugged into the dock. The battery gauge showed about 11.5V today after running it for two hours. It usually shows over 12V.

The rate at which you charge your batteries with the alternator depend on the size of the alternator and if it is internally or externally regulated. Assuming you have a stock alternator of 55 amps with internal regulator you can expect a charging amperage of about 35 amps. This charging amperage will be offset by any loads currently running on your boat. Therefore the net charge rate will be anywhere between 25 to 30 amps.

The capacity of 3 gel batteries, assuming they are group 31, is about 300 Ahrs. This 300 Ahrs of capacity is assuming the batteries are in perfect condition, which is unlikely. Realistically you might have 70% to 80% of this capacity. Let's settle with 75% ideally and you have 225 Ahrs of realistic capacity. Depending on how deeply you discharge your batteries, let's 50% of capacity, or 112.5 Ahrs it will take you over 3 to 4.5 hours of charging at 25 to 30 amps to recharge (one day away from shore).

Assuming your batteries were fully charged, after a day of discharge a voltage of 11.5V is a cause of concern. Batteries which are not discharged below 50% and under little load should never get to 11.5V. Looks like you might have a battery problem.