Servicing absorbed glass mat (AGM) batteries isn't difficult, but the sophisticated AGM design—which is becoming more popular—does require the proper equipment and a little care.
Here are the key points you should know.
First, look before you leap. When in doubt about the type of battery in a vehicle, always refer to a current battery application guide or accurate parts source.
You may encounter combinations you might never expect, such as a conventional battery under the hood as well as AGM type under the rear seat or inside the trunk of the same car. The underhood battery starts the vehicle while the AGM unit powers, for instance, vital onboard computers.
Second, always replace an AGM battery with the appropriate AGM model. Don't take shortcuts that may cause unexpected complications and shortchange the customer.
Third, never try to add water to any AGM battery. Not only will this maneuver not rejuvenate an AGM battery, it will ruin the unit by exposing its innards to unwanted outside oxygen.
Chemically, outside air poisons the contents of an AGM battery. If you discover that someone else has forced open an AGM battery, replace that battery. Period.
Remember that conventional batteries normally create and then vent off gases to the atmosphere.
To grossly simplify, “gassing” lowers the battery's liquid electrolyte level. Hence, the need to periodically add water to it.
However, AGM batteries literally water themselves by holding and recombining gases that traditional batteries discharge. The AGM format has not eliminated battery gassing completely, but it has reduced gassing to a level that's miniscule compared with that of a conventional battery design.
An AGM battery's low-gassing trait makes it well suited for filling dead space under a car's rear seat or in a corner of its luggage compartment. Besides taking advantage of unused space, this approach also helps by getting the battery out of that extremely harsh underhood environment.
Battery experts explained that an AGM battery has a pressure-relief valve on it and this battery design occasionally may “pass some gas.” What's more, an AGM located under a seat or inside the trunk typically has a vent hose or tube that's supposed to carry any gasses outside the vehicle.
For one thing, always use the correct battery for this kind of application. For another, always reinstall its vent plumbing exactly the way the vehicle manufacturer intended.
Fourth, some technicians favor the familiar load test when diagnosing batteries. Others prefer the simpler conductivity-measuring technique because it doesn't require the finesse and practice that a load test does.
Regardless, battery engineers told me that either technique is adequate for an AGM battery.
Fifth, be sure you've got an up-to-date battery charger that's AGM-capable. You won't regret the investment. Charging an AGM battery is similar to charging a traditional battery, but it's difficult to charge an AGM unit completely—let alone do it quickly—without the appropriate battery charger.
A modern, AGM-capable charger also is better suited to recharging badly discharged AGM batteries than outdated chargers are. Outdated equipment such as those large, roll-about battery chargers may easily raise the voltage of a discharged AGM battery to 16 volts or greater.
Suppose this is the only charger you have handy at the moment.
If so, connect a digital voltmeter to the discharged AGM battery. Then connect the charger to the battery and adjust its output so battery voltage doesn't exceed about 14.5 volts.
Now, you must watch that battery voltage doesn't exceed 14.5 volts; second, the battery cannot overheat. These steps are both inconvenient and time-consuming compared with using a modern, AGM-capable charger.
Finally, remember that battery gasses are both noxious and flammable. So whether you're working on traditional batteries or AGM units, always follow industry-recognized safety precautions—beginning with wearing proper eye protection and keeping potential ignition sources away from all batteries at all times.
During freezing weather, do not attempt to charge any battery that you suspect has frozen. The remnants of electrolyte in a severely discharged battery can freeze in winter weather. Typically, a frozen battery is more than unsalvageable. It's also a potential bomb. Discard it.