George, if your battery can't be recovered and it is still under the original warranty, you should definitely return it to your original retailer for warranty service. What is the 10-digit serial number on the white 1”x3” “Non-Spillable” label on the side of the battery? Whenever multiple batteries are being used in the same application, whether they are wired in series or parallel, they should be identical in age, size and type. However, when you are just trying to recover a deeply-discharged battery with a standard battery charger, you can use a dissimilar battery, to allow the charger to deliver voltage to the deeply-discharged battery.

whoya, once any battery is discharged below about 11.2 volts, it's basically at a 0% state of charge. Because AGM batteries have very low internal resistance, they can continue operating beyond that point. However, the more deeply you discharge a battery, the fewer cycles you will get out of it. What size batteries are you using in your boat?

Jim McIlvaine
eCare Manager, OPTIMA Batteries, Inc.
www.youtube.com/optimabatteries