Have you noticed how batteries are moving into the trunks and under the seats of more and more late-model vehicles? Chances are, if the battery is not under the hood, it’s an absorbent glass mat (AGM) or gel cell battery.
Packing a lot of power for their size and weight, these batteries offer many benefits. They are sealed, so in the event of an accident, acid won’t spill out. They can be installed at odd angles. And their lighter weight allows manufacturers to shave off a few pounds from the vehicle.
Because they are relatively new to the market, I’d like to dispel a few AGM battery service myths:
MYTH: You can use your regular battery charger on AGM or gel cell batteries.
False. These batteries like to be charged slow and low. Many AGM/gel cell battery chargers have microprocessors that collect information from the battery and adjust the current and voltage accordingly. Some have different settings for charging flooded, gel and AGM batteries. Overcharging can kill these batteries.
Also, alternators are not chargers. Don’t rely on an alternator to do the work of a charger. If a battery is discharged to the point that it cannot start the vehicle, use a charger as soon as possible to make sure the battery gets fully charged.
MYTH: AGMs and gel cells can be tested the same way as conventional batteries.
False. These types of batteries have lower internal resistance than flooded batteries. Older capacitance battery testers/analyzers may not be able to accurately read these batteries. Most new battery analyzers have a special mode for AGM/gel cell batteries. Old-school load testers might not provide conclusive results.
MYTH: AGM or gel cell battery replacement is the same as flooded battery replacement.
True and false. While the installation of the battery may be the same for the two battery styles, some vehicles require an extra step to tell the vehicle that the battery has been replaced. Newer GM vehicles have a battery sensor module on the negative battery cable, while Ford has its Battery Monitoring System. Other manufacturers have similar systems. These systems require recalibration with a scan tool if the battery is replaced. If the system is not recalibrated, the alternator might overcharge the new battery, causing the battery to fail soon after replacement.
MYTH: Voltages are the same when the vehicle is running on late-model vehicles.
False. Automakers are trying every trick in the book to increase fuel efficiency. The latest strategy is regulating the operation of the alternator. This strategy includes determining when and under what conditions the battery is charged.
A late-model vehicle could be running at idle with less than 13 volts at the battery, and the alternator light will not be illuminated. If the tech revs the engine, the voltage may not rise. But, when the right throttle, cruise speed or electrical load criteria are met, it will control the alternator and produce enough energy to accommodate the load and the battery’s state of charge.
These late-model vehicles can calculate what’s happening in the battery. Some systems are so sensitive that they can detect if the owner has attached wires to the battery posts to power aftermarket accessories. This is why a scan tool or modern, dedicated battery tester might be required to test and replace batteries in the future.