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Location: Mauston, Wisconsin | One last bit of advice. When I say resting that means the battery has been on charge for ~24 hrs previously and they have been setting without charge or load (resting) for ~8 hrs.
All deep cycle batteries we typically use in our boats should be fully recharged after 24hrs of charging, depending on the charger and ampere rating. The typical recharge time varies with the charging current - never use a 75A boost charger on a deep cycle battery if you don't know what you're doing....
With respect to state of charge or battery capacity in the battery. The voltage state of charge chart in this webpage is a good guideline. If your battery reads 12.4 or lower after resting for 8 hrs, its probably time for a replacement battery.
http://www.emarineinc.com/pages/Batteries-Maintenance-101.html
My experience is that once they reach this 80% SOC (12.4V), its time to replace them. For what its worth, my TM wet-cells are now at the 12.4V level after ~12 years. I've pushed them to the edge. They will be replaced for next season, I'm just hoping I can put it off for the rest of this season.
Have fun!
Al |