Traditional lead-acid (flooded) batteries have been with us for decades. The basic technology has been improved and refined, but still remains the same chemical reaction. The choice of which lead-acid battery to buy has been between starting, deep-cycle and combination types. Now there are sealed lead-acid gel cells and AGM (absorbed glass mat) batteries to consider for deep-cycle needs. Both of these sealed battery types offer very interesting capabilities.
Lets look at the different types of batteries available today.
A battery is a sterling example of compromise. No one design works best in every application. For example, a flooded (descriptive name for a common wet cell battery) starting battery has many thin, low-density, active material plates with great surface area for providing the hundreds of amps needed to start an engine. Only a small percentage of the battery capacity is normally used, and its quickly replenished. However, its thin, low-density plates can’t take the heat and chemical stress caused by repeated deep discharge cycles, which would cause it to quickly shed its active material.
More versatile is the combination deep-cycle/starting battery. This design has thicker plates, but not too thick, so relatively high starting current is still available. These designs are less expensive than deep-cycle batteries and are intended for people looking for an inexpensive battery with more modest deep cycling demands than, say, a full-time cruiser. With care, this type of battery can survive 150-200 deep cycles. Flooded, low water loss, maintenance-free variants of these batteries are also popular, and primarily differ by containing substantially more electrolyte. To reduce loss of electrolyte, the better ones use a calcium additive in the plate design that is not as strong as antimony (the normal plate grid-strengthening alloy).
A true deep-cycle flooded battery has several significant differences, though its more a matter of degree than radically different technology. Unfortunately, marketing terms have replaced true specifications, so it has become more difficult to recognize the good from the so-so. First, the plates are much thicker and plate grids are reinforced with antimony for long cycle life and resistance to physical disintegration from vibration forces as well as chemical cycling stresses. Thick plates yield lower peak starting current demands due to lower plate surface area, but are excellent designs for prolonged, low-to-moderate discharge cycles. There is no reason they cannot be used for starting chores, but have to be sized to meet cranking needs. Properly cared for, top quality deep-cycle batteries such as a Rolls, can last 900 charges or more when mostly cycled to 50% capacity (the best combination of discharge vs. cycle life). The active material may be twice as dense on quality batteries, thus, greatly contributing to cycle life.
Sealed Lead-Acid Technology
Gel electrolyte batteries have been around for more than 50 years, but are becoming more popular because of their unique benefits, as well as lower prices and better availability. Also, highly regulated and sophisticated charging systems that can prevent overcharging-the Achilles heel of this type of battery-are readily available now and not too expensive. Electrolyte, however, cannot be added if sealed batteries are subjected to overcharging, which leads to an early death. By the same token, sealed batteries are truly maintenance-free.
The construction of gel and AGM batteries is somewhat different than flooded batteries in that there is no liquid electrolyte per se, but rather a thick paste which is applied to thin plates and separators which are then tightly packed together. In the charge/discharge process there is no real fluid migration. Instead of strengthening the plate structure with antimony to resist shedding, calcium is used, which is far less subject to promoting self-discharge. Calcium doesn’t provide the physical strength of antimony, but the paste construction of gel-cells is inherently stronger and more vibration resistant than flooded cells.
Some of the other advantages of gel batteries are the absence of explosive or corrosive gasses being vented during charging and there is no fluid to spill. They have very low self-discharge rates-on the order of .1% per day vs. .7% per day for flooded cells. Unlike flooded batteries, they can be left in a discharged state for days with no ill effects. They can be installed on their side, but with a 10% capacity loss. They can even tolerate a complete dunking, whereas flooded batteries can produce deadly chlorine gas when mixed with seawater. Some gel cells can be charged significantly faster with the proper equipment, and they can be shipped via UPS for easy acquisition.
Absorbed glass mat (AGM) batteries, sometimes called starved electrolyte batteries, are a newer variant of the gel-cell. They were originally developed for the demands of military aviation. In terms of construction, they have even less electrolyte than gel cells, but have all the advantages of gel cells and more. They can even be installed upside down with no loss of capacity. And, because of their extremely low internal resistance, they can be effectively and quickly charged at very high rates of current. This low internal resistance also allows a better discharge profile; AGMs (and gels) maintain a higher terminal voltage during discharge. This gives better equipment performance for high current-draw equipment such as large inverter loads (e.g., microwave ovens).
Does this sound too good to be true? Well, lets look at the downside. First, is the matter of cost-up to $200 for a Group 27 battery. Next, is the sensitivity to overcharge voltages (as distinguished from high current). Even slight overcharging can cause permanent loss of capacity, because these cells are sealed and nothing can be added to replace lost electrolyte. Fortunately, well-controlled charging equipment can negate this problem…providing it is properly used. AGM batteries have a potential cycle life of 500+ charges (gels 400-500), which is only about half that of the best quality flooded varieties. Lastly, they are not as energy dense. Group 27 AGMs have 85-95 amp-hour capacity vs. 105-115 for flooded cells.
So many people have trouble with short battery life or far less capacity than expected that the causes need to be examined. As mentioned above, flooded cell plates are often strengthened with antimony, helping to reduce the shedding of plate material. The downside of antimony is the promotion of self-discharge when the battery is left idle.
During charging, antimony also promotes gassing, which contributes to shedding when overcharging occurs. And over time, battery cycling produces shedding of active plate material, which gradually reduces the ultimate capacity of the battery or causes sudden failure by the build-up of plate debris on the base of the interior of the case, eventually shorting out the cells. Thin plate, flooded, low-density starting batteries are much more prone to this problem if deep-cycled.
Sulfation is the next foe of batteries. It is the formation of lead sulfate on the plates during discharge (a natural byproduct of the process). If the battery is fully charged soon after discharge, the lead sulfate is soft and easily converted back to active material. If not promptly recharged or incompletely recharged on a regular basis, the lead sulfate gets harder and harder to convert back to active material, reducing capacity and leading to early death. Gel cells and AGMs have a much greater resistance to sulfation.
Overcharging a battery is just as bad as undercharging as it leads to gassing (in flooded cells) and loss of the electrolyte-a particularly destructive process to gel cells and AGMs. If lost electrolyte is not replenished promptly in flooded cells (not possible in sealed types), the area exposed to air dries out and is permanently lost. Additionally, during the overcharge process, galvanic action attacks the positive grids, leading to deterioration and weakening of the grid.
Heat damage can occur from excessive rates of charge (for the capacity of the battery). Batteries have a small but meaningful internal resistance, which causes heat to build up during the charging process. If excessive charging is left unchecked, particularly in a hot environment such as an engine compartment, heat build-up causes more resistance, causing more heat, which buckles the plates until thermal runaway begins and the battery destroys itself. In extreme conditions, a battery can actually explode. Any temperature over 80F shortens battery life.
When comparing batteries it is vital to understand the terms as well as the rating criteria used by manufacturers. One manufacturer may give projected cycle life at 40% discharge, another at 50%, and yet another at 70%. The projected life would be dramatically different for what may be the same battery. The best balance of service life and utility on a cost basis is with 50% average discharge.
Manufacturers may also rate battery death differently. Some call it dead when the battery can be recharged to no more than 80% of its original capacity, while others say 60%.
Amp-hour ratings are among the most misunderstood concepts. For example, a battery may be rated to deliver 5 amps for 20 hours-this is called the C20 rate. You may think, then, that it would yield 10 amps for 10 hours (twice the load, half the time). Not necessarily so, because of the limitations of electrolyte diffusion through the plates. The higher the rate of discharge, the lower the overall capacity before falling to a given threshold discharge value. Sealed batteries in general, and AGMs in particular, tend to have better performance in this regard. A gentleman named Peukert figured out this variation in discharge efficiency years ago, and developed an equation for this phenomenon. Advanced amp-hour meters that use microprocessors and this equation (as well as a value for a given battery called the Peukert exponent), can calculate the time remaining in a given battery with reasonable accuracy-providing you enter the correct data for your battery and it is in top shape.
Amp-hour ratings define capacity in terms of how many amps can be drawn from a battery at a relatively low rate before reaching a predetermined cut-off of useable voltage (e.g., 10.5 volts for a 12-volt battery under load at a fixed temperature).
Another common rating is reserve capacity, which is the number of minutes a battery will last and not fall below 10.5 volts with a 25-amp load at a temperature of 80F.
Battery voltage measurements can be very confusing. Is a battery dead at 10.5 volts or 11.7? It depends on the context. Most US companies define a battery as dead when it cannot sustain 10.5 volts under load at the C20 discharge rate, at a temperature of 80F. For a 100-amp-hour Group 27 battery, this works out to a 5-amp load for 20 hours before reaching the 10.5-volt level. Once the load is removed, the battery voltage immediately begins to climb. Without any loads on the battery, one can measure open-circuit voltage, which is a reasonable and easy way to determine a batterys state of charge. It is the only easy way for sealed batteries. A battery discharged to 10.5 volts at C20 should bounce back to approximately 11.7 volts, open-circuit. When access to the cell electrolyte is possible, a good hydrometer is the most accurate way to check state of charge and for cell imbalances. For an accurate open-circuit voltage measurement, the battery must have had no loads for a minimum of several hours (24 is better). See the open-circuit chart on page 17 for nominal voltage charge levels after a 24-hour rest. Note that the difference between fully charged to fully discharged is less than a volt, so accurate meters and measurements are important. Also note that sealed batteries have slightly higher values at comparable charge levels.
Two other ratings are commonly seen, particularly for combination or starting batteries. One is cold cranking amps (CCA). This is the maximum amperage delivered to crank a sluggish engine for 30 seconds at 0F, to an end voltage of 7.2 on a 12-volt battery. Marine CCA values are similar in concept, except that the test is done at 32F, so the amperage will be higher for a given battery.
Acceptance rate is the rate a battery can accept a given current and voltage for maximum charging without damage. It is based on the diffusion of water and acid through the plates. If you jack up the voltage to higher than normal, say 16 volts, the battery will begin to heat up to levels that can cause damage, as explained above. However, controlled overcharging of flooded batteries, called equalization, is very beneficial. Acceptance rates vary from battery to battery (thin plate cells have a greater acceptance rate), and type of construction. It also varies with the state of charge.
Seven Group 27 batteries were tested, including three from West Marine. The West Marine Sea Volt flooded cell is made by Trojan, the Sea Gel by East Penn, and the Lifeline AGM by Concord. The Rolls Battery Engineering Co. was founded by John Surrette; these batteries are marketed every where in the world as Surrette, except in the US, where they are marketed under the Rolls name.
The seven batteries shown on the chart included both flooded and sealed lead-acid, combination and deep-cycle models. They were tested to see how well they performed (how many minutes they lasted) at three discharge levels and two ending voltages. We chose 10-, 20- and 25-amp rates as being representative. A microprocessor-based tester called an EMROL BATTEST 12-20-3 was used to subject each battery to a constant current discharge (automatically adjusting for voltage drop over the course of the test). Each test was saved to memory, downloaded and displayed on a computer as a graphic chart or minute-by-minute voltage listing. A 50% discharge level was selected for the 10- and 20-amp loads, and the industry standard 10.5 volt cutoff for the reserve capacity tests. Each test was performed three times after five discharge/charge conditioning cycles. The average of the three tests was used for comparisons among brands.
Additionally, recharge times were tracked to see if there was any significant difference. We were primarily interested in checking the gel and AGM batteries, because faster recharging is a listed advantage. A Fluke 867 graphical scope-meter with a logging function was used for the entire charge process. Ambient temperature was corrected to the standard of 80F.
The cutoff voltage for the discharge test was tailored to the particular discharge current under test, although each battery received the same computer-controlled cutoff voltage for each discharge level. It was targeted to result in a rested open-circuit voltage equivalent to 50% discharge, a level that most manufacturers use for predicting cycle life. We stuck with open-circuit voltage measurements instead of using a hydrometer because some of the batteries were sealed and we wanted to use the same methodologies for each. Because each test subjected the batteries to the appropriate cutoff voltages for that given test cycle, comparisons between batteries are relevant.
For recharging, a high quality Statpower TRUEcharge 20+ multi-stage charger was used. This type of device is a must have for maximum deep-cycle battery performance because it precludes the sulfating that often occurs from single-stage charging, which results in chronic undercharging. The Statpower charger also has special charging profiles for gel cells or AGM batteries that require slightly different voltages. A separate equalization cycle is also available and was used effectively. There are three built-in temperature adjustments and an optional temperature probe to place on the battery for extra control. There is even a constant 13.5-volt setting to act as a 20-amp power supply.
Results and Conclusions
These tests did not measure cycle life. Figure roughly 150-200 cycles for low cost combination batteries, 250-350 for moderately priced flooded deep cycle cells, and more than 900 for the best flooded cells. A gel battery will last roughly 500 cycles, and the AGM more than 500 cycles. These numbers are based on proper maintenance-abuse will swiftly kill even the best battery.
The reserve capacity test results were generally 5% to 15% lower than manufacturer data, even adjusting for standard temperature. We double-checked the computer tester with real-time, manual amp load measurements, as well as separate, manual discharge tests with a sine wave inverter. The computer tester seemed right on. Further, one sealed battery actually exceeded its ratings, which would lead us to believe our protocols were correct. The true deep-cycle flooded cells were the most affected by slight under-performance and will benefit most from the conditioning effects of additional cycles and equalization. Another 25 to 50 cycles would probably bring them close to predicted values.
In the reserve capacity test, there was approximately 25% charge remaining at the standard 10.5-volt cutoff in the flooded cells due to the inherent inefficiencies of relatively small Group 27 batteries at a 25-amp load. The acid cannot diffuse through the plates rapidly enough to keep up with the load. The gel and AGM cells were slightly more discharged than the flooded cells at a 10.5-volt cutoff due to their slightly higher voltage at comparable discharge points. The lower internal resistance of the sealed batteries also was a factor in more efficient discharge curves at relatively high loads. This higher discharge voltage trait is particularly beneficial for those boat owners who use large inverters under high current loads.
The two sealed batteries gave excellent performance right out of the chute. Their more efficient internal design allows them to have reserve capacity ratings on a par with flooded cells, even though at very low discharge rates they have slightly lower amp-hour ratings than flooded cells. The best example of this was the West Marine Sea Gel that has only an 86 amp-hour rating at C20, yet significantly exceeded its reserve capacity rating in our tests. The West Marine Lifeline AGM has higher specifications, but did not quite make its reserve capacity rating (which incidentally was second only to the Rolls in having the highest capacity rating).
With regard to recharging, the Sea Gel was just a bit faster than the flooded cells, while the Lifeline AGM was at least 20% faster due to extremely low internal resistance. These are both impressive batteries.
Among lower-priced batteries, the Exide was a good performer, nearly meeting its reserve rating. It also consistently recharged a bit faster than the other flooded cells due to its slightly greater acceptance rate inherent in combination cell construction. For the same reason it needed less cycle conditioning. The three-month replacement warranty, however, is very low. For about 50% more money, you can get four to six times longer warranty coverage with an Interstate or West Marine Sea Volt flooded cell; both also have significantly greater cycle lives. This fact alone makes them the most attractive in the sub $100 category for most boat owners.
The Rolls batterys internal construction and active material density makes its extremely high cycle rating very realistic. Indeed, in the June 1 and October 15, 1996 issues, we cut open a half-dozen batteries and observed Rolls robust construction. While it fell a little short of its rating in the reserve test, note that it also had the highest listed reserve capacity of those tested. The other flooded batteries also fell short close to the same amount. Thus, the Rolls excellent warranty makes it the best choice for intensive cycling , where it is clearly the cost/cycle champ.
The sealed gel and AGM batteries are excellent, but they are the most expensive. You have to ask yourself how good you are about maintenance. If you hate fooling around with distilled water, periodically checking on the battery even when its not being used, and promptly charging your battery all the time, then the life cycle cost of a sealed cell makes sense and is in line with quality flooded cells. The one catch is that you must be meticulous about having a well-regulated charging system to assure that a sealed cell is not overcharged. The additional benefit of any-position mounting may be desirable.
For the occasional user, a discounted Exide is a good bet. What really cinched the Exide in the low price category was its ability to hold up its voltage level better than the GNB. Just plan on taking care of it and replacing it every few years, even with light service.
For the more frequent user, go with the best cycle life you can afford, as it will be the most cost effective and likely be trouble-free due to better construction. The Interstate is a good compromise, with excellent performance, price, cycle life and warranty. The West Marine Sea Volt is a similar performer in all categories. The Rolls is initially more expensive, but for the heavy user, it will work out to be a Best Buy. It can be found at boat shows discounted up to 25%. Flooded cells also can take the abuse of overcharging better than other types.
Lastly, if you are considering flooded cells, check out 6-volt golf cart batteries. They are taller and are normally used in pairs of two wired in series. They have good cycle life, excellent capacity, and are very cost effective. West Marine has golf cart batteries as well as the battery boxes to fit them.
Contacts- Exide Corp., 645 Penn St., Reading, PA 19601; 800/523-8954. GNB Technologies, 375 Northridge Rd., Ste. 100, Atlanta, GA 30350; 800/523-4622. Interstate Battery, 12770 Merit Dr. Ste 400, Dallas, TX 75251; 800/872-4001. Rolls Battery Engineering, 8 Proctor St., PO Box 671, Salem, MA 01970; 978/745-3333. West Marine, PO Box 50050, Catalog Division, Watsonville, CA 95077; 800/262-8464.