Wet Cell versus AGM batteries for old cars

Posted by Thomas Wilcox On 2011/11/21 8:07:31

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:

Quote:

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