Features October 15, 2000 Issue

AGM Batteries: Optima Our Choice

If weight is not a consideration, however, we also recommend the 81-pound GNB Stowaway multi-purpose sealed AGM battery.

The basic lead-acid storage cell was invented by Thomas Edison in the late 1800s, and the fundamental chemistry hasn’t changed since. If you suspend two pieces of lead in a sulfuric acid solution and apply a charging current to them, one piece becomes coated with brown lead oxide, a process that absorbs electrical energy. If you then remove the charging source and connect a load between the two pieces of lead, the process is reversed, and the battery delivers electrical energy.

The Optima deep-cycle battery
starts quickly and is lighter than
the GNB Stowaway.

What has changed is the physical structure of the battery itself. Without going into excessive detail, batteries have evolved into two main design classes—sealed and unsealed—and into two major application areas—starting and deep-cycle. Unsealed batteries—also referred to as “flooded” batteries—have vent caps that are removable and open to the outside air. This is the oldest design, and one that permits hydrogen and oxygen gases formed during charging to escape; it also permits (and requires) periodic replenishment of the water lost. Sealed batteries force these gases to recombine, eliminating the need for refilling. Sealed batteries were once limited to “gel cells” in which the liquid sulfuric acid was replaced by a jelly-like semisolid. Over the past few years, however, other types of sealed batteries have been introduced.

Traditionally, a battery was designed for one of two applications—starting or deep cycle. The two applications present different design problems to battery makers. A starting battery must be able to deliver very high current for a short period of time; a deep-cycle battery must be able to deliver a more moderate current on a long-term basis. A starting battery is seldom fully discharged; a deep-cycle battery must withstand frequent deep discharges and recharging.

Some newer batteries, however, do an admirable job in both capacities. We predict that the old unsealed battery is on its way out.

Flooded Technology
The chemistry of a basic flooded cell battery has remained essentially unchanged for more than 100 years. The battery cases have gone from rubber to the more-durable and less-electrically conductive polyethylene, and separator materials are better. But it’s still essentially the same technology.

Flooded cells are cheap, can be recycled, and (with TLC) can last more than several seasons. You can get four or five years out of top-quality brands—if you maintain them diligently. In fact, the best brands and designs—such as Trojan, Rolls and Surrette—are not expensive if you look at life-cycle costs; they are actually cheaper than any other battery because they have the potential to last up to 1,000 cycles in real-world use.

Those are the pluses; now for the minuses.

• They emit explosive gases (hydrogen) and can spew corrosive elements that can wreak havoc on surrounding structures.
• They need frequent maintenance—or at least checking—to avoid having to replace them seasonally.
• They need to be kept recharged to fulfill their projected life since they lose nearly 1% or more of their charge per day.
• You need good access (something some boats fail to provide) to check electrolyte levels.
• If you fail to recharge them promptly, the sulfation process (change of active material to inactive material) takes a toll on capacity. In fact, while conducting this round of tests, we found flooded cells that had been minimally maintained with full recharges every four months during idle storage had still lost up to 30% of their capacity in only a year.

It boils down to this: Keeping flooded batteries charged is critical to both capacity and life.

Early Maintenance-Free Batteries
As we said at the top, there are two types of batteries: sealed and unsealed. Some unsealed “maintenance-free” batteries are sold with removable caps for replacement of electrolyte, so they’re really not “maintenance-free.” The important thing to remember about any unsealed maintenance-free battery is that it emits hydrogen gases.

Unsealed maintenance-free batteries are basically flooded batteries in which the antimony—an additive designed to make battery grids resistant to flaking of their active material—has been replaced with calcium, which helps slow the battery’s self-discharge.

Flaking is the result of the normal charge/discharge process as well as vibration effects. Overcharging greatly accelerates electrolyte loss and flaking. Calcium does the same job as antimony, but not as effectively, so the battery is not as resistant to vibration and doesn’t have the cycle life of antimony-enriched battery grids.

Other changes to these batteries include more room in the bottom of the case for shedded grid material to prevent shorting of the cells. They also have more electrolyte than normal flooded batteries to lengthen the time before the loss reaches a critical point. Capacity and cycle life is generally lower than a (normal) antimony-based flooded cell. Thus, these usually fall into the low-cost end of the battery market. They’re perfectly fine as a car starting battery but fail to hold up for much more than one season in deep-cycle (defined as any usage other than starting) or marine-starting environments.

True Maintenance-Free Lead-Acid Batteries
The first generation of these batteries was limited in size, expensive and failed to provide long life due to very narrow charging parameters. They were valve regulated, lead acid (VRLA) batteries, meaning the innards are actually sealed from the atmosphere via a one-way valve.

The battery’s internal pressure is greater than the atmosphere’s, so it helps force the freed hydrogen and oxygen, which occurs during battery recharging, to recombine with the battery’s water instead of escaping into the atmosphere.

One of the earlier versions of sealed technology (as far back as the 1960s) employed a new way of packing together the active ingredients of the battery. Traditionally, the flooded battery consisted of the lead and lead dioxide plates, suspended in a bath of electrolyte-dilute sulfuric acid, with insulated separators to prevent shorting of the grids.

The new sealed technology got rid of the sloshing liquid, as well as the antimony. Instead, the battery was made of an active paste, similar to soft candle wax, with a mixture of calcium and electrolyte and lead dioxide paste. Based on this paste-like active material, they were called “gel cells.”

Some interesting benefits resulted. For starters, the gel cell had great resistance to loss of charge and could go several months between charges without loss of capacity or sulfation. Some versions could charge more rapidly than their flooded counterparts, while others were very sensitive to the levels of charging currents. These varying charging characteristics led to some confusion over gel cells and resulted in broad—but not necessarily correct—criticism.

Additionally, a gel cell was capable of operating on its side—a convenience—although with a 10% capacity loss. The gel cell had good resistance to vibration damage even with the calcium because there was not enough liquid inside to slosh around or vibrate.

The downside was greater expense, lower cycle life (200 cycles at best) and fewer case sizes. But the biggest problem was an extreme sensitivity to excess charging voltages.

In the mid-1990s, some brand name gel cells had an absolute charging limit of 14.2 volts. This meant dealing with them separately from the flooded batteries on board, or settling for undercharged flooded cells. Some people were unaware of these requirements and ended up cooking their gel cells.

Absorbed Glass Mat (AGM)
In the 1980s the US military issued a request for a sealed battery for its aircraft that could take a lot of punishment, needed no maintenance, had the ability to emit tremendous amounts of energy and yet would recharge very quickly. It also had to work in any position.

The result was the development of the absorbed glass mat (AGM) battery. It’s very similar in design to the gel cell, with even less electrolyte. “Starved electrolyte battery” describes the AGM very well.

These batteries were also considered VRLA and still required well-controlled charging regimens for maximum life. Intelligent and adjustable—but high-priced—multi-stage chargers were available to do just that.

The Tests
Our test group of four sealed lead-acid batteries included the GNB Stowaway PowerCycler, the Exide Orbital starting, the Optima deep cycle and the Optima starting. We also tested an unsealed Deka Marine Master flooded, deep-cycle battery for comparison purposes.

Testing was a bit tricky because we had a mix of batteries—starting, deep cycle and both Group 27 and Group 34. Manufacturers are pushing the Group 34s as universal replacements for both Group 24s and Group 27s.

Another wrinkle was the differing voltages of AGM cells, which varied from 12.8 to 13.2 volts open circuit, fully charged. So we had to use some windage from past testing because we only have voltage tables from the Battery Council on flooded cells regarding voltage versus state of charge.

With some practice runs, and holding procedures to strict timing and current levels, we feel confident that we got a good idea of their capability. We tied the test input/output data to a laptop computer for accurate recording and used a Fluke graphical multimeter for the recording interface.

All test loads were purely resistive. We limited the maximum discharge current to 83 amps or about 1,000 watts because we were looking at performance under high discharge current conditions that could be accurately controlled. Considering the dozens of tests we performed on the batteries to assure we were getting the true picture of their capabilities, 1,000 watts was our equipment limit.

Click here to view the Batteries Value Guide.

The bottom line is this: AGM cells clearly have greater high-discharge capability compared to flooded cells. Their low-current discharge is equal to or better than flooded cells. Recharging capability can be up to twice as fast, but only with a high-current charger.

Results: AGM Today Is Even Better
Our test group represented another generation of AGM batteries, one that has improved on the previous group’s shortcomings but retains the desirable qualities. The new GNB AGM battery has blown away the low cycle life problem. The brand new Stowaway PowerCycler multipurpose sealed AGM battery has gone over 625 cycles in GNB lab tests, according to the company. This is a conventional design in the sense of a typical straight grid/separator. But one unusual aspect is the weight.

The secret of high-cycle capability is lead and this baby has plenty: 81 lbs. in its Group 27 design. What you get for the weight is fantastic cycle life and unbeatable reserve capacity.

This is an impressive battery. In the tests it bettered the competitors, but in fairness the GNB is a Group 27 and the other AGMs are Group 34, a slightly smaller size (and much lighter) that is intended to fill the gap between Group 24 and Group 27 batteries.

Two problems of sealed batteries—price and sensitivity to higher voltages—have also been solved. Today, the prices are competitive with premium flooded batteries on a cycle-life basis, and the charging limit voltages are up to 15 volts so a regular charging system can be used.

Jelly Rolls
The appropriate term is spiral-wound AGM battery. The Optimas, on the market for several years, are made in Colorado and the Exides in Spain. Both employ the spiral technology, but the new Exide Orbital is not an exact clone of the Optima. At least it’s different enough to avoid infringement on the Optima patent.

There are two Optima marine models, one starter and one deep cycle, as well as six other options for land vehicles. All different designations are coordinated with top and case color codes: red, yellow and blue tops, dark and light blue bottoms. The idea is to tailor the battery technology to match the use. Nearly all models are in Group 34 sizes, except for one 6-volt version.

The red, yellow and blue case tops designate the terminal types, and the light blue and dark blue case bottoms designate starting versus deep cycle. Optima is emphatic about using the proper battery for the proper application. The company only projects a life of 50 cycles if you use one of its starting batteries as a deep cycle—about what you can expect from any other brand if you use it in a deep-cycle role. Most other companies are not so forthright. The deep-cycle versions have plenty of juice to act as a starting battery (more so than flooded deep-cycle batteries), so the only advantage of a pure starting model is a lower price and a bit more cranking capacity (see the Value Guide).

Exide has only just come out with its Orbital marine starter model in Group 34 size, so considerations have to be made on fitting this different size cell to much more common Group 24 and 27 battery boxes (as with the Optimas). It’s available in an automotive version with top and side terminals, and with an even greater two-year replacement, 84-month prorated warranty. See the Value Guide for dimensions and warranty details. The weights of the Exide and Optimas are quite nice, 34-44 lbs., depending on specific model, so handling is a breeze.

The marine Optimas have a slight edge over the Exide in terms of energy output, but the Exide has the edge in price and warranty. Based on our experience with them, as long as you have no problems fitting them to a given application in their Group 34 case design, the Exide and both Optima batteries are first-rate choices.

Exide’s deep-cycle version was scheduled to be on the market by late last summer.

If you can handle the weight, the GNB Stowaway PowerCycler sealed VRLA is your best choice, in our opinion. It beats the competition by a wide margin on all counts: cycle life, reserve capacity and amp-hour ratings. This battery can handle most starting and deep-cycle needs.

In current form, there are no side terminal provisions. However, this is easily solved with an adapter for less than $5.

If weight is a factor, we’d recommend the Optimas—proven in the field for several years.

The Optimas offer a lot of energy density on a pound-for-pound basis—about 25% greater energy by weight. At half the weight of the GNB, they are easy to tuck into especially tight spaces—just like the Exide, which also acquitted itself well.

In spite of slightly lower amp-hour and cranking ratings than the Optimas, the Exide did as well as the equivalent category Optima starter on our charging and discharging tests. Moreover, it has a lower price and a much better warranty (on paper at least)—50% longer for full exchange.

We’re confident this Exide will be an outstanding performer based on our tests, but it has to prove itself in broad consumer use before we recommend it over the Optima.


Contacts- Deka, East Penn Mfg., Deka Rd., Lyon Station, PA 19536; 610/682-6361. Exide, 645 Penn St., Reading, PA 19601; 800/523-8954. GNB Battery Technologies, 375 Northridge Rd., Ste. 100, Atlanta, GA 30350; 800/523-4622. Optima Batteries, 17500 E. 22nd Ave., Aurora, CO 80011; 303/340-7440.

Comments (3)

Time to update this article! 13 years technology might be similiar but the market has changed significantly.

Posted by: Thomas G | May 28, 2013 4:06 PM    Report this comment

One of the things that is confusing and critical to any long distance sailor is recharge rates. How rapidly can they be brought back to charge. In essence how long, assuming alternator capacity is large enough How long do I have to run my engine or genset to top them off?

Posted by: David S | April 7, 2012 4:56 PM    Report this comment

One of the things that is confusing and critical to any long distance sailor is recharge rates. How rapidly can they be brought back to charge. In essence how long, assuming alternator capacity is large enough How long do I have to run my engine or genset to top them off?

Posted by: David S | April 7, 2012 4:29 PM    Report this comment

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