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In another forum their is a brief discussion of the Optima battery, a spiral cell absorbed glass mat type battery. In it, Dave C (Owen Dyneto) makes this comment:

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Instead of just responding in that forum, I thought it might be interesting to have a dedicated thread to battery types and choices, making searching and linking a little easier for a very often asked series of questions.

To start, I give below my thoughts on AGM versus wet-cell batteries, along with my conclusions as to the best battery for our cars.

Here is my two cents on the benefits and drawbacks of absorbed glass mat (AGM) versus wet-cell (WC) batteries. These comments come from my marine experience as well as my understanding of the chemical and physical properties of lead acid batteries.

Pros of AGM
1) Very very low self-discharge. A typical WC battery self-discharges about 1-3% per day, a typical AGM (like an optima) 0.5-1% per month. If you are absolutely scrupulous about your battery maintenance (trickle charger, maintaining electrolyte levels), not a big advantage. I am not the worlds greatest battery maintainer, and my cars are stored in a location without power. Plus, I like being able to keep a spare battery around for years without having to worry about keeping it on a trickle charger.

2) AGMs are safer batteries for the following reasons. They are sealed and contain very little electrolyte relative to WC batteries. In a collision, if dropped, or tipped-over they are very unlikely to leak or internally short. Thus, significantly reduced chance of acid burns or fire. AGMs also emit very small amounts of hydrogen during charging versus a WC battery. Most hydrogen recombines with oxygen within the electrolyte, due to the slow rate of migration of hydrogen through the glass mat. Thus, little chance of explosion, even if the battery is subjected to over-current.

3) AGMs do not heat up as much during charging, which adds significantly to their longer service life. The heating and cooling of a WC battery causes the lead plates to expand and contract. In a discharged battery, with significant lead sulfate present, the lead sulfate can be sloughed off the plates, falling to the bottom of the little baggies that contain them. This reduces the capacity of the battery. This sloughing does not occur in an AGM battery.

4) AGMs are much less sensitive to current (amperage) during charging, because they don't heat up, and they don't discharge hydrogen.

5) AGMs will survive being frozen much better than WC due to their construction.

6) AGMs have a higher charge-acceptance rate versus a WC battery. Thus, they will come back to full charge sooner. This is a nice benefit for those who tend to take short trips in their cars. Note: you want your batteries to be fully charged to prevent permanent sulfating between uses.

7) Per pound, AGMs have much higher Ah capacities and significantly greater efficiency of discharge. Thus, you get more bang for your pound.

Cons of AGM batteries
1) AGMs are much more sensitive to over-voltage charging. This will cause gassing, through a safety vent on the side of the battery. Since the battery is sealed, the lost electrolyte will reduce the life of the battery. If the voltage cut-out is working well, this should not be a problem. If you are working on the charging system, or suspect that there is a fault in the regulator, then it is a good idea to swap in a WC battery until the problem is identified and solved. Thus, a complete shop would have a WC battery around for testing. I don't have a complete shop.


2) AGMs are about 2X the cost of WC batteries.

The drawbacks are, in my experience, offset by their other positive characteristics like longer life and greater safety. Thus, unless I have a car where originality of both look and design of the battery is important I will always choose a quality AGM battery. Right now, I am using Optima batteries, but there are other AGM batteries that may work (golf cart type batteries come to mind).

Thanks,

Tom

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"AGMs are much more sensitive to over-voltage charging."

So then would a battery charger putting out something like 14 volts (for 12v agm battey) or say 8 volts (for 6v agm battery) cause problems???

I've found that relatively new battery chargers tend to put out as high as 16v (on 12v setting) and as high as 9v on 6v setting. I currently have a rather NEW so-called trickle charger (12v only) that plugs into cigarette liter and it puts out a little over 16 volts.

Nice review, Tom. Thanks for taking the time.

PS - Tom, if you have one in your 34 or prior car, do you drive in daytime with the lights on to sop up the overcharge? I find that with a fully charged (lead acid) battery, even with the 3rd brush turned to minimum output, it's still a bit too much charge in prolonged daytime driving.

Tom, nice write-up. Obviously, you have been seriously thinking about this. Thanks for doing this.

Here are a couple of additional data points for consideration.

I have found that if my WC batteries are disconnected (either removing the ground cable or switching the battery cut-off switch) that the batteries will hold enough of a charge for many months, without any attention, to start the engine. One is relatively new at about 18 months, but one is approaching its 5th year. The age of the other one, a 12-volt, is not known, but guess at least 4-years. Whether 6- or 12-volt does not seem to make any difference. I am somewhat surprised at this, but glad to have it.

Russell, (I believe that is his name), an acquaintance of the Flackmaster, made some comments about AGM batteries at one of the Tech Sessions in Salado. If I recall correctly, he said that if your 6-volt AGM battery goes dead, and you get a boost to get going, you will need a new battery by the time you drive it home. He did not go on to explain what was happening, nor did he get any questions as to the why, but was recounting an experience.

(o[]o)

Are AGM batteries used in any late model modern production cars????

I don;t know. But if they are then maybe significant voltage regulator changes were made by the car manufacturer to accomodate the production use of the AGM battery.

I don't recall reading of AGM batteries used OEM in modern cars but if not, think the reason is probably due more to cost than performance. One thing I'd be curious about is JWs recollection of replacing if jumped. Hadn't heard that but something to keep in mind. Wonder if it was a 12v to 6v jump as several have mentioned is doable with wet cells but not recommended.

I've heard of several Optima failures, most of which seem to be related to charging issues, but one I'm quite familiar with was quite dramatic. Of course there were extenuating circumstances. I know I've related this incident before, but in a nutshell the vehicle was a 1956 Executive doing Interstate travel. For whatever reason the T/L decided to run constantly, see-sawing the car up and down repeatedly. At some point the ignition cut out, but as the car was then coasting in direct drive, fuel continued to be drawn into the cylinders as it slowed. Then the ignition recovered and the resulting blast blew nearly the entire exhaust system off the car and onto the highway. The battery was found to have exploded. To the vendor's credit, they replaced the battery and covered the entire cost for a new exhaust system.

Just a few comments and my from years of using Optima batteries in import cars with largish stereo systems (aka misspent youth). The Yellow Top ones were what everyone used because you could run the stereo system for a long time with them cause they are deep cycle batteries. But they don't like supper high instant draws (like starting a car), and the fast charges. The Yellow Tops are like good BBQ....Low and Slow.

The point is that I would see people get 12-18 months max out of a Yellow Top Optima cause they are not really designed to be "starter" batteries. They are made a Aux batteries where the draw is even, and so is the charging.

Now the Red Tops are the opposite, they are specifically made as Starter batteries and are designed as such. Once I switched over to Red Top I was getting 36-48 months on one of them. But then again, unlike the Yellow Top which can be drained down to empty, and recharged back up over and over, the Red Tops are much more like a traditional WC. They don't like to to be drained all the way empty, as this kills their lifespan.

Red Top = Starter Battery.
Yellow Top = Deep Cycle, Trailer Battery. Also used in multiples for electric/hybrid cars.

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If you have a large enough capacity WC battery, it will probably retain enough charge to start a car. The problem with self-discharge is that during that time you are also creating the perfect conditions for permanent sulfation of the plates. The lower self-discharge of AGMs reduces that potential.

I am not certain why Russell would say that about AGM batteries. I have jumped AGM batteries in the past, and they did not suffer catastrophic failure. As Big Kev pointed out, some types of AGMs will experience reduced life as a result of being severely drained. It depends upon the design and the batteries prior history. Also, you could kill the battery by trying to jump with a 12V battery.

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I have my gen set to charge at it's lowest rate. However, I also have a voltage/current regulator in the system. Cheating, I know...

Regarding the failure of the Optima, I would say that an explosion from any battery under those conditions would not be unexpected!

Other comments (sorry, can't easily quote them as they don't show up below the text entry area-weird).

Maximum charging voltage for an AGM depends upon the particular battery design. For an Optima 6V, the maximum is voltage is 7.5V. specs for all their batteries can be found here.

There is no particularly special set-up required for installing an AGM battery in a standard car, as far as I know. However, spending the money for an AGM for a modern daily driver is probably not worth it because of the cost. By the time most folks are ready to replace their third battery, they are getting rid of the car.

Of course, if you have bitchin stereo needs like Big Kev, that might be a different story...

I am not an expert on batteries, I just have a lot of experience from my offshore boating days. So, please, take what I say as a point for discussion and further research.

Cheers,

Tom

Tom, the above mentioned link doesn't work, perhaps a side effect of permanent sulfation?

--> This way please.