To AGM, Gel, and Flooded Battery Technology Glossary Up to AGM, Gel, and Flooded Battery Technology Index Next AGM, Gel, and Flooded Marine Battery Page

Battery Types: Flooded versus AGM and Gel

On the kinds of batteries we may use on board:

The most common kind of battery in Marine use today is the lead acid battery. Using an electrolyte consisting of sulphuric acid, these cells can store impressive amounts of electrical energy in a relatively small space. This energy is stored in chemical form within lead grids mounted inside the battery. The reliance on lead grids and paste explains the great heft of lead-acid batteries.

The battery universe is further divided along the lines of battery construction. Currently, there are three common lead-acid battery technologies: Flooded, Gel, and AGM.

Gel and Absorbed Glass Mat batteries are relative newcomers but are rapdily gaining acceptance. There are some very compelling reasons to use VRLAs:

Thus, there are some significant differences between battery types in terms of features and construction. However, there are also some very important figures to consider when it comes to choosing the right battery: Various capacities, cost, warranty, etc. The following table tries to summarize across brands using batteries as close to the 8D Group Size as possible

Comparing physical attributes between VRLAs and Flooded Cells VRLA Flooded
Lifeline AGM (8D) West Marine Gel (8D) Inexpensive Trojan (2xT105) Premium Surrette 400 (HT8DM) Premium Surrette 500 (12CS11PS)
Amp-hour capacity (20hr rate) 255 225 225 221 342
Warranty (Replacement/Pro rated) 1/5 Years 1.5/5 Years 0.5/3 Years 2/5 Years 3/7 Years
Life Cycles (@ 50% DOD) 1,000 500 500 1,250 3,200
Initial Purch. Cost (USD/12V set) 387 449 152 246 683
Initial Purch. Cost (approx. $/Ah) $1.52 $2.00 $0.68 $1.11 $2.00
Energy Density (Ah/in^3) 0.111 0.098 0.136 0.097 0.076
Weight Factor (Ah/lb) 1.614 1.424 1.815 1.348 1.257
Max. net replenishment during bulk charge, accounting for charge limits, efficiency and assuming a 400Ah battery bank 1550A* 177A 85A 85A 85A

I tried to level the playing field by selecting as many group 8D batteries as possible. The two exceptions are the Trojan T105's and the Surrette 12CS11PS (no series 500 Group 8D battery is manufactured by Surrette for the marine market). The larger battery size is to the advantage of the Surrette, although it does not impact results greatly. The Trojan T105's were used because I was not able to find ready pricing on the Trojan 8D. I would expect results to be somewhat comparable.

*Concorde Batteries used to claim no charge limit on its web-site, while claims 4x amp-hour capacity. I limit charge current in the cost model to 100% of amp-hour capacity just to be on the safe side.

Comparison of Battery Types using several different measurements

Energy Storage per unit Weight and Volume

Comparing Marine Batteries: Absorbed Glass Mat (AGM), Gel, and flooded

Here is one of the classic comparisons that people like to make: How much charge the battery can store per unit weight and per unit volume. As you can see, the Trojan T105 comes out ahead in both departments due to its low weight and compact construction. However, this construction technique will also lead to a lower cycle and overall life.

Purchase Cost per unit Weight and Volume

Comparing Marine Batteries: Absorbed Glass Mat (AGM), Gel, and flooded

As we can see from this chart, the purchase cost per amp-hour and purchase cost per cycle still make the Trojan T105 look like the most attractive battery. Thus, if you are strapped for weight, space, and cash, such a battery might be ideal. The Trojan product has thin lead plates that make these batteries lighter but also shorter lived. Rolls advertises very long pro rata warranty replacement periods for their premium line that are indicative of the confidence they place in their product.

Premium cells are handicapped by lower energy storage density but offer longer lives and greater resistance to the self destructive habits of lead acid batteries: Thicker lead plates and a more complicated product make it possible. Hence, premium cells usually have a higher resistance to vibration, are easier to service, and have higher cycle lives than their budget competition. Many boat owners are willing to put up with the initial purchase price in return for reliability and not having to replace them every few years.

So what is a "Marine" Battery?

Perhaps it's shocking condiering their retail prices, but most batteries sold through marine hardware stores do not qualify as premium batteries. Pay close attention to what you're buying. Batteries are not created equal and brand or price are not the primary indicator for quality. For example:

Thus, Caveat Emptor! Try to get as much information about your prospective marine batteries before you buy or you'll be sorry. Furthermore, consider that premium batteries usually only exist in non-standard form factors. For example, you will probably have to make some custom modifications to properly mount/restrain the tall and heavy Rolls/Surrette 500 series (18"+ high, min. 128 lb+ each).

However, life cycle costs are not just a function of the initial purchase costs. You should also consider the fuel/engine wear savings of using VRLAs over flooded cells. AGMs offer the highest charge acceptance, efficiency, and a reasonably long life which makes them generally a better bargain (see results in cost model section). Unfortunately, there are fewer shapes and sizes of VRLAs to chose from (relative to the flooded cell universe anyway), and less familiarity and presence world-wide. On the other hand, VRLAs can be shipped anywhere by air. Flooded cells have to be bought locally or delivered by surface transport.

I used DEKA gel cells in the past for comparisons, but West Marine recently brought out a private label 6V gel cell series that they claim will sustain over 1,000 "full discharges". Given that reputable brands never claimed more than 600 cycles in the past, the West Marine claim may be a bit dubious. Due to West Marine's return policy, I'll give them the benefit of the doubt. West Marine also released a set of private label AGMs. Unless I missing something, these are very expensive and have a much lower cycle life than the Lifeline competition manufactured by Concorde Batteries. Thus, I don't see why anyone would want to buy a West Marine AGM.

Can I Mix AGMs and Flooded Cells?

While several sources state that you can mix AGMs with regular flooded cells, I would not recommend it (gel cells have sufficiently different set points to make them totally incompatible with flooded cells or AGMs). Ideally, your house bank would consist of a number of identical batteries wired in series and/or parallel that were manufactured on the same day.

So how can I save money with AGMs?

There are many attributes that determine the true cost of a battery technology. Much like incandescent versus compact fluorescent light bulbs, your choice of battery technology may cost you less up front but will cost you more over the life of the product. For example, the faster, more efficient bulk charging that AGMs and gel-cells allow will lead to reduced wear and tear on your charge source (engine, gen-set, etc.). More on all that later down. Suffice to say that I do not believe the T105 to be a bargain.

How about Nickel-Cadmium Cells?

They have their place. Usually in power plants where there is lots of excess energy, etc. Learn more about them on my Nickel-Cadmium page.

Anyway, onwards to sizing and charging requirements! This is where Lifeline AGMs really start to shine, assuming your charging system can take advantage of them.