How to Charge an AGM Battery and Why It’s Different

Absorbed glass-mat batteries deliver top-of-the-line performance, outperforming both regular and enhanced flooded batteries. They’re quickly becoming the preferred choice of battery for car makers. AGM batteries also work well on boats and in RVs because they can serve as both a starting battery and a deep cycle battery. AGM deep cycle batteries can run a long time between charges without ruining the battery itself.

That said, AGMs do need to recharge.

So, of course, charging one of the most advanced lead-acid batteries available would be different from charging a regular car battery.

How to charge an AGM battery

Before anything, take off any jewelry, put on safety glasses and go to a well-ventilated area or outdoors.

1. Plug in your AGM-compatible battery charger. Follow any other instructions for your specific charger.
2. Connect a backup power supply to the onboard diagnostics OBD-II Port.
3. Disconnect the car’s negative terminal and wrap it with a glove, rag or anything to prevent it from touching any metal. The negative cable is usually black, but you can also look for a minus (-) symbol on the cable as well.
4. IMPORTANT: Select AGM or Absorbed for the battery type on your charger. A smart charger may detect the voltage and battery type when you connect it, but don’t leave it to chance. Select 12 volts to match the voltage of your AGM battery.
5. Connect the charger to your car battery posts. Again, check the indicator lights and make sure they’re set to AGM or Absorbed.
6. Start the charger and wait two to eight hours to fully charge your AGM battery.
7. Disconnect the charger from the battery when it’s done. Your charger’s indicator light will signal when it’s done charging the battery. Again, avoid letting the charger’s clamps touch the battery’s loose negative clamp.
8. Reconnect the car’s negative terminal to the battery. Disconnect the backup battery. Now you’re ready to hit the road with a fresh start.

Charging your car battery will warm it up. If it gets too hot, the water inside the battery evaporates. In turn, the liquid inside gets more acidic. That means the battery’s insides corrode much faster. The solution is to charge your battery slowly. You want to raise its charge without raising its temperature.

What’s an AGM battery charger?

AGM batteries have special charging needs. Some battery chargers offer an AGM or Absorbed setting to meet those special charging requirements.

An AGM-compatible battery charger sends more amps into a lead-acid battery while keeping the voltage less than 14-15 volts. AGM chargers go through the three charging phases (bulk, absorption and float) just like a regular charger. However, a regular charger could exceed 17 volts when charging a battery.

AGM batteries can absorb more current than a regular car battery, which is why they can recharge faster. It takes time to charge regular batteries. They need slow, low-amp charging to prevent overheating — and they need more volts to push through their internal resistance.

AGM batteries don’t have nearly as much internal resistance, which is how they absorb 30, 40, even 50 amps at a time instead of a regular battery getting stressed out at more than 10 amps.

This is also why an AGM battery needs special charging.

Regular battery charging can break AGM batteries. Regular batteries need 15-17 volts to get the same amps. However, voltage greater than 15 volts can overheat an AGM and generate enough pressure to pop its safety valve. That one-way valve is supposed to relieve excess pressure from the harmless gases that come when a battery’s charging. Too much voltage becomes too much pressure, and pop. Without an airtight seal, an AGM will dry out in hours.

Then you’re in the market for a new AGM battery.

Bottom line: Do not use a regular battery charger for an AGM battery. Make sure you use the AGM or Absorbed setting. If you’re not sure, don’t risk it.

How long does it take an AGM battery to recharge?

About two hours to eight hours, depending on the AGM battery’s power specs, how drained it is now and the charger’s amps. AGM batteries usually will charge significantly faster than a regular, flooded battery.

A good rule of thumb: Divide a battery’s amps by your charger’s amps to get how many hours it’ll take to charge it.

AGM batteries tend to have more amps than a regular lead-acid battery. That’s why you have AGM deep cycle batteries or AGM dual purpose batteries. An AGM battery can hold more amps than a typical car battery. You can see that in the high amp hour (Ah) ratings an AGM battery has compared to a flooded battery of the same size. An AGM can also handle a high-amperage charge from a heavy duty battery charger.

The MTZ-48/H6 is an AGM battery with a 70 Ah rating. A small, 5-amp AGM-compatible charger could refill it in 14 hours (70 amp hours / 5 amps = 14 hours.) On the other hand, a 20-amp charger could charge it from zero to full in 3.5 hours. A regular car battery couldn’t handle charging at 20 amps for more than 20 minutes, let alone three hours.

Remember to only charge a car battery in a well-ventilated area and to always protect your eyes and hands. Take off any jewelry.

How do I know if my charger can charge AGM batteries?

Look for an AGM setting or instructions on how to use your charger with an AGM battery. Check the charger’s case, box and instruction manual.

If the display face of your battery charger shows a setting for AGM or Absorbed, then it’s compatible.

If you’re not sure, seek out an AGM-compatible charger, either online or from a trusted battery store.

An AGM battery can last years in your car, boat or RV, and keeping it charged with the right battery charger is the best way to make sure you get the most life from it.