Features September 2003 Issue

Wet-Cell 12-Volt Battery Test

Two batteries from Deka and one from Interstate lead the pack in our test of eight deep-cycle batteries.

We last looked at batteries in our October 15, 2000 issue, when we reported the results of a test of the AGM (absorbed glass mat) subspecies. For this month's report we focused on flooded-cell, deep-cycle marine batteries, six in the popular Group 27 case size and two in Group 31 size. The two Group 31s, from Deka and West Marine, are actually the same battery—Deka/East Penn Manufacturing makes West Marine's batteries, which are private-labeled under the Sea Volt name. 

We tested eight wet-cell batteries. Top row (l. to r.): the Deka DC31DT, the Rolls 27 112 XJ, the Deka DC27 and the West Marine Sea Volt. Bottom row: the Trojan 27THM, the Douglas 27 DCM, the U.S. Battery 27TMX and the Interstate SRM 27B.

The Group 31 is physically a bit larger, so be sure it will fit your battery compartment. Also be aware that major battery-makers not only manufacture for private labels, but may sell the same battery under different brand names of their own.

All the batteries tested were either bought as new off the shelf or were supplied as new by manufacturers.

Battery 101
Flooded-cell (also called wet-cell) technology has been around since the 19th century, yet the electro-chemical reactions that drive the generation of flooded-cell battery DC electricity are generally unchanged. The case material, component quality, and material proportions have been optimized for a given battery function, e.g., a starting battery has a different internal structure than a deep-cycle battery.

Battery-storage capacity depends upon the area of the plates, while ability to handle deep discharge depends on plate thickness. Starting batteries have more thin plates; deep-cycle batteries have fewer, thicker ones. Output voltage for either is the same.

In the lead/acid battery reaction, each cell within the battery has plates made of sponge lead (called the active material). These are also the negative plates of the battery, which produce the electrons. The positive plates are made of lead dioxide. All plates are immersed in a strong electrolyte, which provides a mechanism for a charge to flow between positive and negative plates. There are separators (insulators) between the positive and negative plates to prevent inadvertent contact. The most common electrolyte for this type of battery is sulfuric acid or H2SO4.

In the discharging reaction, the electrolyte combines with the active material (lead) to produce lead sulfate) and water, which dilutes the acid. The voltage developed by this reaction is around two volts per cell. By combining (connecting) six cells, you get a 12-volt battery.

The rechargability of a battery depends on whether the chemical reaction that generates electrons during the chemical reduction is completely reversible. In the lead/acid cell, if you apply current to the battery (charging) at the right voltage, lead and lead dioxide form again on the plates by reverse reaction so you can use the battery repeatedly. However, there is inevitable deterioration that takes place as the cycles add up. Improper charging, delayed recharging, and excessive discharging can dramatically reduce cycle life and capacity of even the best batteries.

Hydrogen gas bubbles and lead sulfate build up on the plates coating them and insulating them from the electrolyte driving the reaction. When disconnected from the drawing source, the gas will somewhat dissipate, thus re-exposing plate surface area to the electrolyte. If a near-dead car battery sits for an hour, you can often get one more crank at starting. Beware of sparks around batteries, since the hydrogen is very explosive.

Deep-cycle batteries have thicker plates that are much better for prolonged low-to-moderate discharge. They are often reinforced to reduce shock and vibration damage, and are less susceptible to shedding of the active material in the plates. The addition of antimony to the lead mixture adds strength, but promotes self discharge and gassing, so it is a careful mix. Car batteries have thinner, often porous plates to maximize plate surface area. In the internal reaction, more plate surface area yields more starting current or cranking amps to turn over an engine.

Repeated deep cycling of a car battery weakens the plates and makes the active material shed from its grid. In repeated deep discharge and recharge, the car battery's capacity can drop below desired voltages in 50 cycles or less. Deep-cycle batteries, however, are designed to withstand this repeated charge and discharge with their hardier plate construction. This also means their plates have lower surface area exposed to the electrolyte, and hence lower cranking amps than a car battery. Some batteries attempt to split the difference with a plate design in between.

Battery ratings can help you determine what you're buying. The only problem is that the rating system is not completely standardized. We mentioned this problem in the last test story, and little progress has been made since. Many manufacturers stick to similar test conditions and ratings, but some do not. Keeping buyers confused is to the advantage of lesser quality manufacturers. We wish the industry would get together on this so that the consumer can compare apples to apples. We will briefly review what the ratings are and some of the differences you'll see.

Amp-Hour Rating. This applies primarily to deep-cycle batteries. It is considered in a context of the number of amp hours that can be drawn from a battery at 80°F at a relatively slow rate until it reaches 10.5 volts. Moreover, the rate of discharge is non-linear, so a faster discharge will not be a simple division problem, because the battery becomes unable to efficiently discharge at higher rates. Example: A 200- amp-hour battery would not be able to sustain a 20-amp load for 10 hours—it would be more like 6 hours, because of greatly reduced discharge efficiency. (There is a complicated formula to accurately estimate time remaining, which expensive, chip-driven electronic discharge meters use.) Our testing evaluated the discharge time at various loads to gain a realistic picture of capabilities—something impossible to find consistently among manufacturers.

Marine Cranking Amps (MCA) is a common rating used on marine deep-cycle batteries. This is the number of amps of load that a battery can deliver for 30 seconds at 32°F before it drops to 7.2 volts (1.2 volts per cell). A starter motor is designed to operate at lower voltage because of the huge current demands. This gives you an idea of how much power the battery can supply to start your boat's engine.

Cold Cranking Amps (CCA) is more typically used with a car-type battery, and is usually measured for a 30-second duration at 0°F—a tougher standard than MCA. Some makers use Cranking Amps (CA), which is the same as MCA.

We were a bit confused with the data on the Douglas Battery website. Douglas appears to define a Marine Cranking Amp (MCA) rating as the number of cranking amps available for a 10-second duration at 80°F. This rating was given to marine starting type batteries. Their data sheet also states that the "Battery Council International" (BCI) no longer recognizes the 32°F rating for cranking amps. Under this body, cranking performance is now a calculated rating as opposed to tested. It also appears that the amp-hour rating is a calculated value on the Douglas data sheet. Perhaps this is the reason why the Douglas data we derived in our testing did not match its stated performance well. Note that there are both international and national standards that manufacturers can apply to their ratings, so read carefully.

Reserve Capacity is the time in minutes that a new and fully charged battery can supply 25 amps of draw at 80°F. This is another test that we ran. By definition, this test is considered over when battery voltage drops to 10.5 volts. This rating allows estimation of the time that your battery can operate essential equipment when your engine or charging system fails.

These ratings should be checked carefully to understand what loads were placed on the battery to determine the rating. Amp-hour ratings can be designated for different rates. We also noted that some manufacturers rate reserve capacity at 23 amps instead of 25 amps. Again, it pays to remember the definitions and read labels carefully.

Diligent maintenance is the key to a long battery life. If you allow your battery to be stored for a long period of time without charging, sulfating can occur, resulting in a build-up on the battery's plates. This insulates them from the electrolyte, reducing performance. The best way to tell if your battery is charged is by using a hydrometer. (A voltmeter is second-best.) As the reaction in the battery takes place, water is generated, diluting the electrolyte. This changes the specific gravity, which is what the hydrometer measures. Care must always be used in testing not to get a spark near the open cells (explosion hazard).

A well-charged battery would return readings on a voltmeter in the range of 12.4 to 13.4 volts. Around 12.7 or 12.8 volts is a good average.

A battery should rest a minimum of 30 minutes after charging before measuring to get a more accurate reading, and allow the surface charge to dissipate. Use of a "smart" chip- controlled charger will optimize both battery performance and battery life, and will pay for itself over the long haul. Also, recharging should occur within 24 hours of discharge to reduce sulfation. Also note that a discharged battery can freeze because of the higher concentration of water in the cells.

Overcharging is not good for deep-cycle batteries, and fast, shop-style boost chargers are not recommended.

Initially, we planned to use common charging equipment that the average boatowner might use. We tried a Schumacher automatic 10/2-amp charger. These can typically be purchased at Wal-Mart for about $40. We quickly found that after a heavy deep cycle (below 10.5 volts) a higher amperage charger was required to burn off the build-up on the plates. A 25-amp "smart" computer chip-controlled charger setup with timer or automatic switch is optimal. A 10-amp "smart" charger will also work for this capacity battery because it's capable of higher, but controlled voltages and current than the simple, cheap chargers. Always make sure that battery plates are covered with electrolyte, and be sure to fill the battery with distilled water—never more battery acid.

How We Tested
To determine battery performance right out of the box, we promptly measured reserve capacity after logging baseline measured values. We also noted general condition, markings, and the condition of the posts. All batteries were maintained at room temperature. After our initial reserve capacity tests, we placed each unit through five charge/discharge cycles. This was done to condition the batteries. While this type of battery does not reach its peak performance until it has been cycled 25-50 times, we felt a series of five cycles (each taking about 6 hours to complete) at least got them started evenly, and allowed us to continue before getting any grayer.

After determining that our 10/2 amp charger was not up to the task, we used a 40-amp automatic charger to bring each unit up to full charge. For load simulation, we constructed a bank of 12-volt light bulbs wired in parallel to simulate our amp draw or load, but it was purely resistive. (Some motors can have very high starting loads.) Very simply, the amp load is equal to the total number of bulb watts divided by the voltage. We used six 50-watt (12 volt) bulbs to approximate the 25-amp load test. These are common in larger boats and RVs.

Our current draw was not regulated (as makers sometimes do to optimize test results) to match the voltage of the battery. In other words, when the voltage drops to 11 in the battery, we are still powering the same wattage of bulbs. Of course the lights are dimming, so the difference is not huge at these current values.

Most of our tests started with battery voltage at or above 12.5 volts. This somewhat balances the increased load later in the tests, and resulted in slightly lower duration values than would be determined in regulated load tests. Hence our data should have been slightly lower than that supplied by the manufacturers. Since we are comparing brands and their value, our data is useful for comparison but not completely accurate for rating. During our conditioning cycles, the batteries were discharged at the 25-amp rate and fully recharged five times.

We tested all units using 10-, 20- and 25-amp loads while monitoring voltage, cranking amps, impedance (internal resistance; a lower number is better) and minutes of run time. All battery data was measured using a sophisticated MicroVAT battery tester from Snap-on Tools.

US Battery 27TMX
This battery arrived from the manufacturer with an initial voltage that appeared average: 12.63 volts. Available cranking amps seemed low at 581 amps compared to the 875-amp rating claimed for the unit. We note that of all batteries tested, including the big guys, the US Battery's 875-amp rating was the highest of the bunch, but its performance leads us to question that value. This battery also had one of the highest impedance levels, which may indicate that it was old. Further, in our preliminary "out-of-the-box test," its reserve capacity was measured at only 130 minutes (claimed 202 min.). Perhaps it was the fact that we hit the battery with a heavy load without a top-off charge, but both initial voltage and hydrometer values indicated a full charge.

We found recharging difficult. We noted the expulsion of water from each cell during recharge (forced out). All cells seemed to be overfilled (completely topped off). After a full recharge and five cycles, we re-tested the reserve capacity and noted that it had dropped to 120 minutes. Our MicroVAT indicated that the battery was bad based upon input cranking amp data.

The US Battery arrived with the poorest labeling of the bunch, with only a small label indicating battery model number and an amp-hour rating. No other information appeared anywhere. Both the positive and negative battery terminals were dirty.

Bottom Line: This battery didn't perform well in our tests. Not to condemn it, because it has an attractive price, but this sample bore the hallmarks of a battery that had been left too long in storage.

Rolls 27 112XJ
Based on its reputation, higher cost, and prior Practical Sailor test data, we expected excellent performance from the Rolls battery (sold under the Surrette name in Canada)—although we tested a different model and size before. In these tests, however, we found the Rolls to be a mid-level performer.

This battery provided some unusual information in testing. Its label data provided a cold cranking amps rating along with an amp-hour, weight and reserve capacity rating. A big blue #1 sticker was affixed to it. The battery we received did not match the description provided online with respect to color. Our unit was all black, while the website suggested that the cover should be red. In our "as received" observations, we noted dirty terminals, the lowest voltage as received (12.34), but a very high available current of 800 amps. Only the Interstate was higher at 820 amps. With a claimed 190-minute reserve capacity, we measured only 143 minutes as received. After conditioning and a re-test, we measured reserve capacity again at 166 minutes.

Bottom Line: At $99, we expected a bit more. Rolls claims 350 cycles, which is tops for this type of cell. Others performed as well or better for less.

Interstate SRM 27B
We were surprised by the performance of this battery. It arrived with clean terminals, 820 of the stated 840 cranking amps (the highest of all units) and the lowest impedance of all as received (3.98 m-ohms). Perhaps this was the freshest battery we tested.

In our initial reserve capacity test, this unit outperformed its rating without even a trickle charge. We measured a reserve capacity of 184 minutes at 25 amps, and it has a rated capacity of 180 minutes. Its performance in the 10- and 20-amp tests was also the best of the Group 27 batteries. As a matter of fact, this unit beat the West Group 31 unit in reserve capacity and was close to the Group 31 Deka.

The design of the carry strap on this unit could use some reconsideration. It's too wide in the gripping area, causing palm and finger pain when lifted. This may seem minor but with the suggested use and required maintenance for these types of batteries, you could be doing a lot of lifting to and from a charger.

Bottom Line: With a 30-month warranty, good labeling, excellent performance ratings, and a price tage of $83, this is an excellent battery. We can easily recommend it.

Deka DC31DT & DC27
Deka submitted both its DC31DT and its DC27 for our evaluation. The 27 arrived with clean terminals, but the 31 with a dirty positive terminal. Initial voltages looked good at 12.6 and 12.8. Both measured about 10% below their rated cranking- amp capacity as received. The 27 also seemed to have slightly higher impedance than other Group 27 units (5.07 m-ohms). Only the US Battery and Trojan units possessed more internal resistance.

The bigger 31 was rated with a reserve capacity or 25-amp rating of 185 minutes and delivered 190 after our conditioning cycles. The 27 was rated at 175 minutes and delivered 171 minutes after conditioning. Both these units performed well in all tests and matched their rated performance. The Deka website offers some of the most comprehensive information and specifications of all the brands tested.

Deka even supplied both 23-amp and 25-amp reserve ratings so they could be properly compared to the manufacturers who decided to rate at 23 amps. Again, this is one rating area where we caution the consumer to check the rating values carefully.

Bottom Line: Solid performers that can be found for reasonable prices. The Group 31 Deka is our Best Buy.

West Marine Sea Volt 105
As noted earlier, this battery is made for West Marine by Deka/East Penn Manufacturing, and is the same as the Deka DC31DT. While initial readings were fine, test results indicate that the battery we bought as new off the shelf from a West store had sat in stock too long, and maybe in tough conditions. We understand that West has a program to cycle batteries through their stores in a timely way. This one must have missed the boat.

West will replace a warranteed battery free for one year, and offers a pro-rata warranty for three years. This may account for the price being higher than Deka's.

Bottom Line: The test battery started life as the same good battery as the Deka Group 31, but apparently fell on hard times. Lesson: Off the shelf doesn't necessarily mean fresh.

Douglas Aquatroll 27 DCM
The Aquatroll was rated at 725 cranking amps and measured 744 out of the box. It had dirty terminals. Its initial voltage was OK: 12.39 volts. Its claimed reserve capacity is 155 minutes but it only delivered 123 minutes as received. After conditioning and a re-test we noted little improvement in these values. The reserve power increased to 140 minutes but still fell short of the 155-minute rating.

The performance information appearing on the label was minimal, although a comprehensive booklet was attached explaining warranty and safety information. The warranty was confusing and seemed to indicate only 90 days and then it was up to the dealer or sales facility. It was a bit vague.

We really liked the Douglas lifting strap design. This was the only unit tested that was comfortable to carry without palm pain. Further on that note: We were concerned when lifting some of the other batteries because their nylon straps were so small. You could actually feel the elongation in some of them. Both the big Deka and the West Marine Sea Volt are guilty of this. The tensile strength of the cordage is probably adequate, but it felt like the cord was going to fail.

Bottom Line: We like its lifting strap, but performance was mediocre.

Trojan 27 TMH
Trojan's product specifications indicate 720 cranking amps at 32°F. Upon arrival, we measured only 625 amps of available current at 12.55 volts.

This battery arrived with the highest impedance of all units tested at 6.42. Perhaps the high impedance was also an indicator of an aged battery. As received, the Trojan only delivered 128 minutes of reserve power at a 25-amp load. The manufacturer claims 200 minutes. This makes the Trojan the lowest-performing unit we tested.

After five charge/discharge cycles, the reserve power performance was not much improved, coming in at 137 minutes until the unit dropped to 10.5 volts—still well below the 200-minute rating. This battery seemed to have a higher amount of cranking amps left at this voltage than some of the other units. The Trojan may be engineered as more of a dual-purpose battery than just for deep cycling.

Bottom Line: The Trojan is priced about 10% higher than other batteries in its class, while in this test it performed at a lower level.

Taking good care of your battery provides as much value as shopping hard for one. All things being equal, finding a good price is the next most important factor. After calling around to a variety of sources nationwide, we found a wide range of pricing for the same batteries. The list prices found online were substantially higher than those offered by "point of sale" representatives.

Prices have not increased to any great degree since our last series of tests.

We suspect that each battery comes with its own history and character. Some may be better than others, even for the same brand and model. Their character may be shaped by the history of the unit—when it was manufactured and how it was stored prior to shipment. Our guess is that the supplier that has the best control of these factors also delivers the best performance at the consumer level. In this case it was Deka and Interstate.


Also With This Article
Click here to view "Value Guide: Wet-Cell Batteries."
Click here to view "How to Maximize Battery Life."
Click here to view "Die Hard Lives Up to its Name."

• Deka/East Penn Mfg., 610/682-6361, www.eastpenn-deka.com
• Douglas, 800/368-4527, www.douglasbattery.com
• Interstate, 800/541-8419, www.interstatebattery.com
• Rolls, 800/368-4527, www.rollsbattery.com
• Trojan, 800/423-6569, www.trojanbattery.com
• U.S. Battery, 888/398-7871, www.usbattery.com
• West Marine, 800/BOATING, www.westmarine.com

Comments (0)

Be the first to comment on this post using the section below.

New to Practical Sailor?
Register for Free!

Already Registered?
Log In