Art161

I was reading a technical book about electronics, and am interested in what it has to say about absorbent glass mat (AGM) batteries. While the book says that AGM batteries are an improved lead acid battery, there was one item that concerns me.

The book says that while AGM batteries can be charged to 2.40 volts per cell without problems, the float charge should be reduced to 2.25 to 2.30 volts per cell and perhaps less in the summer. It further states that automotive charging systems designed for flooded lead acid batteries often have a fixed float voltage setting of 14.40 volts (2.40 volts per cell), and that this could result in overcharge on a long drive.

Does anyone here have comments or an experience with overcharging?

Derf00

I searched and found a similar article. From what I can tell AGM and what I take away from it, is not ideal for DD cars. Gel Cell is still the best.

http://batteryuniversity.com/learn/a..._glass_mat_agm

"AGM and other sealed batteries do not like heat and should be installed away from the engine compartment. Manufacturers recommend halting charge if the battery core reaches 49°C (120°F). "

That means trunk mount and even then, on a hot summer day a trunk in temperate climate could easily surpass that, in AZ, ambient temps could run that hot.

" Automotive charging systems for flooded lead acid often have a fixed float voltage setting of 14.40V (2.40V/cell), and a direct replacement with a sealed unit could spell trouble by exposing the battery to undue overcharge on a long drive."

http://batteryuniversity.com/learn/a...d_acid_battery

Again, not worth it. Go with a gel cell battery if you want better performance for your car unless it's a special application that is designed to handle an AGM battery.

Art161

The book I am using is the 2015 ARRL Handbook, and it doesn't have much to say about gel cells. It does state that "Heat can be a problem for AGM and other gelled electrolyte batteries." I'm not prepared to discuss the advantages and disadvantages of gel cells, because I don't know much about them.

Nuke

We use AGM batteries in various UPS applcations and, as far as heat goes, they have a rather narrow band of preferred operating temperature and can experience "thermal runaway" faster than other battery types. I havent researched it enough to be able to say that it would prevent or limit their use in an automotive application.

157dB

My voltage regulator "follows" the charge demand of the battery
and adjusts the voltage accordingly. When the battery is charged,
the voltage goes down (from charging voltage) low enough to not
overcharge my battery.
Lead Acid, Gel Cell or AGM, it still works the same way.
The fully charged voltages for the different type of batteries
listed above vary greatly. The OEM regulator is set up for
Lead Acid batteries.

On a side note, taking your Lead Acid battery up to 15Volts
once in a while will "equalize" the individual cells and bring
up any "weak" cells up to full charge.

Look at it like 6 glasses that hold a total of 6 cups of water
and one or some never get fully "filled" when they are only
filled with 5 cups of water (14 Volts) at random.
Increase that to 6 cups (15 Volts) and all are sure
to be "filled".

jz78817

I think the issue with AGM is thermal expansion; with the fiberglass mats taking up the space between plates, any excessive thermal expansion might run the risk of breaking plates or swelling/cracking the case.

Gel cells are still flooded lead acid like a conventional car battery, but with silica added to make the electrolyte more viscous.