The great thing about AGM batteries, including OPTIMA REDTOP® and YELLOWTOP® batteries, is that they have very low internal resistance. This allows very high amperage output so that the battery can power your accessories longer and deeper than a traditional battery, but at the same time deeply discharging it.
An
AGM battery with its low internal resistance may stump car guys because sometimes it doesn't work like a traditional flooded lead-acid battery.
Here's the problem: Most battery chargers have built-in safety features that may prevent chargers from recharging deeply discharged batteries. A traditional battery that's at 10.5 volts or less is seen as defective, having either a short, a bad cell or some other defect. Most analog chargers are binary and are either on or off. If they don’t come on, it may be because the charger thinks the battery is “bad.” Turning on to charge a “bad” battery could create an unsafe scenario. But the fact is that the
AGM battery may be just fine; it has simply slipped below the minimum voltage threshold of the charger to turn on, and the charger doesn't know what to do with the battery, so it does nothing.
Recovery Option #1: The Best Solution – AGM-Specific Chargers
The best method for recharging a deeply discharged
AGM battery is to purchase a modern charger that has kept up with battery technology. Many chargers now have AGM-specific settings and desulfation steps that help recondition and recover deeply discharged AGM batteries.
Recovery Option #2: The DIY solution for charging a deeply discharged battery.
This is a recovery method for the do-it-yourselfer using the equipment you've got in the garage. With this option, you're going to trick your traditional charger into charging the deeply discharged
AGM battery.
Hook up the good battery and deeply discharged
AGM battery in parallel – positive to positive and negative to negative. Do not have the charger connected to the battery or turned on at this stage. (There's more here about how to tell if it's going to explode or when it's got a charge)
Recovery Option #3: Enlist the Professionals
If you don't own a battery charger, you don't want to make the investment, or you're not the do-it-yourself kind of person, this is the option for you.
Take the battery to a professional battery specialist who knows AGM technology.