Piston Slap: Proper resuscitation for finicky AGM batteries?

Hagerty Community user “hyperV6” writes: 

When charging an AGM battery, what is the proper way to do it? If I don’t have an AGM charger, is there a way to charge it with a conventional charger?

(Not necessarily asking for myself, just because I have a lot of experience with them at my job. Most so-called failed batteries are just a battery too low to work with a conventional charger. Optima has a lot of good tech on this, this may be a good one for your audience.)

Sajeev answers: 

I agree on the value of this subject one million percent. No wait, make that eleventy billion percent!

You are correct about Optima’s fantastic tech info on absorbant glass mat (AGM) batteries, although it’s buried in an FAQ instead of a unique page. There’s probably a good reason for it, as search engine public relations is a smart move in today’s highly connected, highly judgmental age. Since it’s hard to find the information, I’ll paraphrase it below.

According to Optima, here are three options to recharge a deeply discharged (under 10 volts) AGM battery:

1. Option #1: Buy a charger designed for AGM batteries. Old fashioned ones lack the processing power to monitor the battery’s vitals and charge it accordingly. Consider these units as possessing the computing power of an Atari 2600 early electronic fuel injection system, analyzing inputs and adjusting the power output accordingly.
2. Option #2: Use an old-school charger and DIY computer processing. Who needs a computer when you got one resting on your shoulders, right?
   1. Parts needed:
      1. Battery charger (under 15 amps)
      2. Jumper cables
      3. Any good car battery, preferably above 12.2 volts.
      4. A voltmeter or multimeter
      5. Watch, timer, or countdown timer on your phone
   2. Steps to take:
      1. Connect the good battery to your “bad” AGM battery in parallel (positive to positive and negative to negative). Hook up the good battery to the charger, but ensure the charger is powered off. Once connected, turn on the charger.
      2. After about an hour, see if the AGM is warm or hot to the touch. Warm is acceptable, but stop charging IMMEDIATELY if the battery is hot or if you hear a hissing sound. Optima makes a big deal about this, therefore so should you!
      3. With your multimeter, “Check back often to see if the AGM battery has charged to 10.5 volts or above. This generally takes less than two hours with a 10-amp charger.” If it has, disconnect everything and reconnect the (formerly deeply discharged) AGM battery directly to the charger. Treat it like you would any car battery at this point, as Optima suggests, “in most cases, the AGM battery will be recovered.”
3. Option #3: Get a professional to help. This is not the well-intentioned folks at the parts store, rather a dedicated battery store. I did a near-me Google search and found two companies that could assist; hopefully that search nets something good for you! (If not, try changing “near me” in the search to the name of the nearest city instead.)

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