About eight years ago, a lot of manufacturers, especially the hybrid makers, switched to Absorbed Glass Mat (AGM) batteries on many of their models, to get the following benefits over traditional liquid filled lead acid batteries:
§ Shorter recharge times since AGM batteries accept a faster charge rate.
§ No risk of the batteries freezing when the car is unattended since AGM batteries have a very low self-discharge rate. Emphasis on self discharge. (A fully charged battery won’t freeze, but a flat one will).
§ About 10% more capacity in the same size battery.
Now, for some background. While the spill-proof batteries currently on the market are often referred to as gel batteries, most are AGM batteries-short for absorbed glass mat. Optima and Odyssey batteries are common examples. Unlike a gel battery, in which a silica agent is added to the electrolyte to form a semisolid, an AGM battery uses an ordinary sulfuric acid solution like any standard automotive battery (about 60/40 water and acid at full charge). However, here the electrolyte is absorbed and retained by layers of boron-silicate glass matting between the lead plates. While both battery types can rightly be considered leakproof and spill-proof, AGM is currently considered the superior technology, especially for automotive use. Some AGM batteries employ spiral-wound, cylindrical cells (Optima), while others use flat plates and box cells (Odyssey) like conventional batteries. Also, sealed is something of a misnomer-lead-acid batteries generally are capable of venting when necessary.
At Japanese Motor Works, our experience with AGM batteries has not been as good as we’d once hoped for: We have been through many batteries since the switch to AGM. When you consider that AGM batteries are as much as double the price of liquid filled batteries, that stings!
A basic problem is that AGM batteries are fully charged only when the current (amperage) that the battery is absorbing at the acceptance voltage (typically about 14.4 volts) reaches 1-2% of the battery’s total capacity. In other words, a typical AGM battery is fully charged when the charge current drops below about 2.5 amps. Most typical driving patterns for most hybrids are not going to provide this type of battery charging scenario!
So, We’ll just hook up the old trickle charger, right? No, wrong. Unless you know some tricks and have a shop full of extra batteries and other stuff, (like we do) it requires an AGM specific charger. This type of charger will make sure that the charger really does provide its full output (amperage) until the voltage at the battery reaches the correct adsorption (typically about 14.4 volts) since, if it does not, charging will take longer than it should. Worse still, battery testing at an independent company known as LifeLine has shown that their batteries actually last a shorter time when charged at low rates.
Lifeline Batteries has earned global recognition as a premium sealed AGM battery technology company. Lifeline Batteries are manufactured in West Covina, California by the Concorde Battery Corporation and is a US family owned and operated company. The Lifeline Battery technology is a direct spin off from Concorde’s industry leading aerospace battery lines. I consider the guys who develop and make these types of new technology more of an authority than the guys who put them into cars. Here, I’m referring to the engineers at the auto companies who deny everything most of the time if it makes them look bad.
So, perhaps AGM’s aren’t the best way to go for a hybrid, but far be it for me to re-engineer a hybrid!
To close, I’m not sure just which type of drive cycle is best for the health of the AGM in your hybrid car or van. Just consider this when we call you and tell you that the low voltage battery in the hybrid you brought me needs to be replaced at about 5 years, and it is 2 to 3 times the cost of the last time you put a battery in your Corolla.