The problem is most battery chargers have built-in safety features. A traditional battery that's at 10.5 volts or less is seen as defective, which means a short, a bad cell, or some other problem. Most battery chargers are designed not to work with a battery that appears to be defective because that could result in an unsafe situation up to and including an explosion. But in many such cases the AGM battery is fine; it has simply slipped below the minimum voltage threshold of the charger and the charger doesn't know what to do with the battery.
Here are three options for bringing that fine AGM battery back to life.
Recovery Option #1: The Best Solution - AGM-Specific Chargers
The best method for recharging a deeply over-discharged AGM battery is to purchase a modern charger that has kept up with battery technology. Many chargers now have AGM-specific settings and de-sulfation steps that help recondition and recover deeply over-discharged AGM batteries. These are becoming more common, and they work well for all lead acid batteries. They have the additional capability of doubling as a battery "maintainer" for vehicle storage. Some come with additional wiring to permanently attach leads from your battery to an accessible spot on your vehicle. This makes it easy to hook up when you store your car, truck, boat, or RV.
Recovery Option #2: The DIY Solution
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 charger into charging the deeply over-discharged AGM battery.
Here's what you need:
A good battery, preferably above 12.2 volts. (It can be an AGM or flooded battery, it doesn't matter.)
The seemingly dead, deeply over-discharged AGM battery
A watch or timer
Now, here's what you do:
Hook up the good battery and the deeply over-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. Now, hook up the good battery to the charger. Turn on the charger. The charger will "see" the voltage of the good battery, and start providing a charge.
After the batteries have been hooked up for about an hour, check to see if the AGM battery is slightly warm or hot to the touch. Batteries naturally become warm during charging, but excessive heat may be an indication that there really is something wrong with the battery. Also discontinue the process if you hear the battery "gassing"-a hissing sound coming from the safety valves. If it's hot or gassing, STOP CHARGING IMMEDIATELY!
Check back every hour to see if the AGM battery has charged to 10.5 volts or above. If it has, disconnect the charger from the wall outlet and remove the good battery from the charger. Now, connect only the deeply over-discharged AGM battery to the charger. Turn on the charger and continue until the AGM battery reaches a full charge, or until the automatic charger completes the charge process. In most cases, the AGM battery will be recovered.
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 guy, this is the option for you.
Take the battery to a professional battery specialist who knows AGM technology. Most specialists are willing to provide "charge and check" procedures free or for a small fee. Auto parts stores are typically not capable of accurately determining an AGM battery's condition, and many use conductance testers that don't provide correct readings. Battery specialists like Interstate Batteries and other independent battery distributors are experts who can help determine if your battery is recoverable or not.
Battery Safety Tips
Removing and Replacing
Sooner or later you'll have to remove or replace a battery and like everything else there's a right and a wrong way to do it.