Reviving an AGM Battery

An AGM (Absorbent Glass Mat) battery is a type of lead-acid battery that uses a fiberglass mat to absorb the electrolyte, making it spill-proof, maintenance-free, and vibration-resistant. AGM batteries are commonly used in classic cars, especially those with upgraded electrical systems or that sit idle for long periods, since they offer better charge retention, faster charging, and deeper discharge capabilities compared to traditional flooded lead-acid batteries. Although if they do become completely discharged, they can sometimes become difficult to revive using the same practices used on more typical flooded lead-acid batteries. We join Mark Simpson in the shop to explain the process of reviving an AGM battery.

Steps to Properly Recharge a Dead AGM Battery

If your AGM battery is completely dead (low or no voltage), follow these steps to safely recharge it:

1. Check Battery Voltage
Use a multimeter to measure the battery’s voltage. If the voltage is below 10.5V, many modern smart chargers may not recognize it, requiring a manual charging method.

2. Select the Right Charger
Use a smart charger or AGM-compatible battery charger that has a setting for AGM batteries. Avoid standard trickle chargers unless they specifically support AGM technology.

3. Precharge with Another Battery (if necessary)
If the battery is too low for the charger to detect, use a healthy 12V battery in parallel with jumper cables:
1) Connect the good battery to the dead AGM battery.
2) Attach the charger to the good battery and start charging.
3) Once the dead battery reaches 10.5V or more, disconnect the good battery and charge the AGM normally.

4. Slow Charge for Best Results
Set the charger to low amperage (1A to 10A max) to prevent overheating. AGM batteries prefer slow, steady charging to avoid damage.

5. Monitor the Charging Process
A fully charged AGM battery should read 12.8V to 13.0V at rest. If the battery gets hot or emits a rotten egg smell, stop charging immediately—it may be sulfated or damaged.

6. Perform a Final Voltage Check
After charging, let the battery sit for 12 to 24 hours and check the voltage again. A healthy AGM battery should hold its charge above 12.6V.

7. Maintenance Tips
Use a battery maintainer if the car is stored for long periods. Avoid deep discharges below 50% (≈12.0V), as AGM batteries degrade faster if fully drained repeatedly. By following these steps, you can safely revive a dead AGM battery and maximize its lifespan in your classic car.