|
Foreign Service
By Dan Marinucci | May 2009
Absorbed glass-mat batteries (AGMs) are becoming popular on European and some Asian cars. As always, following proper procedures when servicing them is the safest path to profitability
Because you may be seeing more absorbed glass-mat (AGM) batteries in the near future, let’s take a look at how they differ from conventional batteries and how you’re supposed to test and recharge them.
Most techs know what a flooded battery is. This is the conventional battery that contains liquid electrolyte. When you carry one of these across the shop, you may hear or feel the electrolyte sloshing around inside. A flooded battery is vented to the atmosphere and releases hydrogen and oxygen during charging (commonly called gassing). A traditional flooded battery gasses whether the device charging it is an alternator or a battery charger. The first major concern with gassing is that it creates a potentially explosive, gaseous mixture in and near the battery. The second is that it eventually lowers the electrolyte level in the battery. Over the years, improved designs have reduced gassing but have not eliminated it.
Usually, a conventional flooded battery is located under the hood. But some automakers—especially European makers—have stowed a flooded battery somewhere inside the vehicle. Here, the battery usually has a venting system connected to a hose or tube that routes battery gases outside the vehicle body again.
Unlike a traditional flooded battery, an AGM battery doesn’t contain liquid electrolyte. Instead, its electrolyte is suspended or absorbed in spongy cell separators. A good analogy for these electrolyte-laden separators would be a paper towel that has absorbed just enough liquid to be damp but not dripping wet.
Furthermore, an AGM battery doesn’t gas like a flooded battery does. To grossly simplify the chemistry, an AGM retains and “recombines” those gases traditional batteries vent to the atmosphere. The recombined gases form water, so the AGM battery literally waters itself and doesn’t lose electrolyte. One engineer said that the AGM battery design still can and does gas occasionally, so the potential for a battery-related fire or explosion hasn’t been totally eliminated. However, the overall risk on an AGM is very low compared to that of a flooded battery.
Industry sources agreed that AGM battery advantages are no reason to take shortcuts. To the contrary, they urged all techs to follow standard safety practices such as wearing eye protection and keeping ignition sources away when working on all batteries—including AGMs.
The first important service tip is to verify the kind of battery you’re about to work on. Always consult an accurate application guide before jumping to conclusions. For instance, a vehicle could have a traditional flooded battery or an AGM under the rear seat. It could have a traditional battery under the hood for starting purposes and an AGM somewhere inside the car to power the on-board electronics. Or it could have two AGMs—one underhood for starting and another in the trunk for electronics.
Never, ever attempt to add water to an AGM battery. If you somehow manage to force the battery open, exposing it to extra oxygen will “poison” it or upset its chemical composition. If this happens, the battery’s ruined!
When it comes to diagnosis, battery makers told me that either a traditional load test or a conductivity check is okay with them. However, their premise is that you perform either test properly and don’t compromise it with problems such as loose or dirty connections, damaged test leads, etc. One engineer told me that although the conductivity test is easier to perform and therefore widely used, he still swore by the load test as the better approach. Another engineer simply commented, “Neither technique is foolproof.”
The engineers I interviewed agreed that first, charging an AGM battery is similar to charging a traditional flooded battery. Second, it’s more difficult to fully charge an AGM—and do it reasonably quickly—without the correct equipment. So I urge you to update to an AGM-type battery charger if you haven’t already done so. After all, these batteries are becoming more popular. All my homework indicates that using an AGM-capable charger is the safest, fastest, most accurate way to do the job.
In a pinch, you can use a conventional battery charger on an AGM battery if you faithfully follow some rules. Yes, these rules may be too inconvenient for some people. First, connect a digital voltmeter to the AGM battery before you turn on the charger. Then turn it on and charge away as long as battery voltage doesn’t exceed about 14.40 volts. I’m intentionally being conservative here; some manufacturers put the limit at 14.40 to 14.60; others state 14.80 volts. Where necessary, turn the charger setting down until the voltage is within limits. Then monitor the voltage to be sure it remains within limits during the charging process. For your information, a trusty old roll-about charger can push that battery to 16.00 to 17.00 volts or greater.
Second, regularly monitor battery temperature while you’re charging with a conventional charger. If you’ve practiced this, you know that the battery feels warm under normal conditions. But stop charging if that battery becomes uncomfortably hot to the touch. Basically, hot batteries are bad batteries!
If an AGM battery doesn’t overheat but remains within the voltage range during charging, continue charging and periodically take the battery off the charger and retest it. Although this approach can get the customer back on the road again, it may recharge that AGM to only about 80%.
Last but not least, you could encounter an AGM battery that’s so discharged—voltage is so low—that the battery charger won’t even turn on. You can circumvent this by connecting a good spare battery to it in parallel (positive terminal to positive terminal, negative to negative). Now connect the charger to the good, spare battery and begin charging. Then patiently monitor the dead AGM battery to see if it gets hot during the charging process. Stop charging right away and discard that battery if it gets hot.
If the badly discharged AGM battery doesn’t heat up, slowly recharge it through the spare battery until its voltage is high enough to satisfy the charger—probably 10.50 volts or greater. Then shut off the charger and disconnect it and the spare battery. Now reconnect the charger directly to the AGM battery and continue charging.
|