Charging an AGM at 10.5 V or greater: No problem, under normal starting conditions, an Optima battery should never experience “at rest” voltages below 12 V. Most 12V chargers and alternators have no problem recharging an Optima if it has an “at rest” voltage of 10.5 V or greater.

How to Resuscitate a Deeply Discharged AGM Battery

One of the benefits of AGM is having incredibly low internal resistance. The internal resistance, to a large extent, determines the battery performance and runtime. The lower the resistance, the less restriction the battery encounters in delivering necessary cranking power. Lower resistance also enables the battery to accept a charge more quickly.

Today, most battery chargers have built-in safety features. A charger that immediately recognizes a defective battery may not attempt to charge it for safety reasons. A traditional battery that has 10.5 V or less is typically considered defective, which could be from a short, a bad cell, or other defect. However, in most cases, a discharged AGM battery has simply slipped below the minimum voltage threshold of the charger and can easily be recovered. Absorbent glass mat battery failures are commonly caused by use in improper applications. If either occurs, Optima recommends the following three options for bringing a capable AGM battery back to life:

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 incorporating the latest battery technology. Many chargers now have AGM-specific settings and desulfation steps that help recondition and recover deeply over-discharged AGM batteries.

Recovery Option 2: The Do-It-Yourself Solution

This DIY recovery method using the equipment you may have in the garage requires a battery charger, jumper cables, a good battery (preferably above 12.2 V) and the drained AGM battery. This step-by-step method will allow the deeply discharged battery to be charged:

1. First, hook up the good battery and deeply overdischarged 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.

2. Second, hook up the “good” battery to the charger cables. Turn on the charger. The charger will recognize the voltage of the good battery and start providing a charge.

3. 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 there is something seriously wrong with the battery. Also beware of a hissing sound coming from the safety valves. If either occurs, immediately discontinue the charging process and refer to Recovery Option 3.

4. Check back every hour to see if the AGM battery has returned to 10.5 V 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 drained AGM battery to the charger. Turn on the charger and continue until the AGM battery returns to full capacity, or until the automatic charger completes the charging process. In most cases, the AGM battery will be recovered.

Recovery Option 3: Enlist the Professionals

If you do not own a battery charger and do not want to make the investment, this is the option for you. Take the battery to a professional battery specialist who is knowledgeable about AGM technology.

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Black & Decker
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