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

Absorbed glass-mat (AGM) batteries deliver exceptional performance, outdoing both regular and enhanced flooded batteries. They are rapidly becoming the preferred battery choice for car manufacturers. AGM batteries are also highly effective on boats and in RVs due to their ability to function as both starting batteries and deep cycle batteries. AGM deep cycle batteries can run for extended periods between charges without damaging the battery itself.

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AGM batteries, however, do need recharging.

Recharging these advanced lead-acid batteries is distinct from charging a standard car battery.

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How to Charge an AGM Battery

Before beginning, remove any jewelry, put on safety glasses, and move to a well-ventilated area or outdoors.

1. Plug in your AGM-compatible battery charger. Follow any specific instructions for your charger.
2. Connect a backup power supply to the onboard diagnostics (OBD-II) Port.
3. Disconnect the car's negative terminal and cover it with a glove or cloth to prevent it from touching any metal. Look for a minus (-) symbol or a black cable.
4. IMPORTANT: Select AGM or Absorbed for the battery type on your charger. A smart charger may detect the voltage and battery type automatically, but always verify manually. Select 12 volts to match your AGM battery.
5. Connect the charger to your car battery posts, ensuring the indicator lights are 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 when completed. The charger's indicator light will signal completion. 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. Your vehicle is ready to go with a fresh start.

Charging warms your car battery. If it gets too hot, the water inside evaporates, making the liquid more acidic, which accelerates internal corrosion. Charge your battery slowly to increase its charge without raising its temperature significantly.

What’s an AGM Battery Charger?

AGM batteries have specific charging requirements. Some battery chargers include an AGM or Absorbed setting to meet these needs.

An AGM-compatible charger sends more amps to a lead-acid battery while maintaining a voltage below 14-15 volts. AGM chargers go through the three charging phases (bulk, absorption, and float) akin to regular chargers. However, a standard charger may exceed 17 volts, risking battery damage.

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AGM batteries can absorb more current than regular batteries, enabling faster recharging. Regular batteries require slow, low-amp charging to avoid overheating and need higher voltage to overcome internal resistance.

AGM batteries have lower internal resistance, allowing them to absorb 30, 40, or even 50 amps at a time, whereas regular batteries struggle at over 10 amps. This demands special charging for AGMs.

Standard battery charging can damage AGM batteries. Regular batteries need 15-17 volts to achieve the same amps, but voltages beyond 15 volts can overheat an AGM and generate excessive pressure, potentially triggering the safety valve. Overpressure can cause an AGM to dry out quickly.

For more details, visit What is an AGM battery?.

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Now, you know what it takes to charge and maintain an AGM battery correctly.

Bottom line: Do not use a regular battery charger for an AGM battery. Ensure you use the AGM or Absorbed setting when charging. If uncertain, do not take the risk.

How Long Does it Take an AGM Battery to Recharge?

It takes approximately two to eight hours, depending on the battery's power specifications, its current state, and the charger's amperage. AGM batteries recharge faster than standard flooded batteries.

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Bring your AGM battery to an Interstate All Battery Center or any retailer that sells Interstate products for a quick, accurate test.

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As a rule of thumb, divide the battery's amp hours by the charger's amps to estimate charging time.

AGM batteries often have higher amp ratings than standard lead-acid batteries. They can handle higher amperage from heavy-duty chargers, making them suitable for various applications, including deep cycle and dual-purpose use.

For example, the MTZ-48/H6 AGM battery boasts a 70 Ah rating. A 5-amp AGM-compatible charger could recharge it in 14 hours (70 Ah / 5 amps = 14 hours), while a 20-amp charger could do it in just 3.5 hours. In contrast, regular car batteries cannot endure 20-amp charging for prolonged periods.

Always charge car batteries in well-ventilated areas and use protective gear for eyes and hands. Remove any jewelry before starting.

How Do I Know If My Charger Can Charge AGM Batteries?

Look for an AGM setting or refer to the charger’s instructions to confirm compatibility with AGM batteries. Check the charger’s case, packaging, and user manual.

If the display on your battery charger shows an AGM or Absorbed setting, it's compatible.

Choose a Battery Charger from the Battery Experts

Interstate Batteries offers smart battery chargers, perfect for maintenance and full-power recharging. Find them now on Amazon.

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If unsure, invest in an AGM-compatible charger from reputable sources, either online or at trusted battery stores.

An AGM battery can last years in your vehicle, boat, or RV. Using the correct charger ensures you get the most out of its lifespan.

Contact us to discuss your specific needs for What is the difference between an AGM battery and other batteries?. Our experienced sales team is ready to help you find the best solutions for your requirements.