Features July 15, 2004 Issue

Battle of the Rechargeables

Our test of rechargeable AA batteries reveals that nickel- metal-hydride is the optimum technology, and Rayovac's 15-Minute Rechargeable I-C the top performer.

How many battery-powered devices do you routinely carry onboard? We did a quick check on a friend's boat and came up with the following devices: eight flashlights, one portable GPS, two handheld VHF radios, a digital camera, a clock, kitchen timer, binocular compass light, digital caliper, digital multimeter, portable CD player, portable MP3 player, icebox fan, deck vent fan, and a cell phone. Most of these devices use disposable batteries (really, they're throw-away since they don't dispose of well). In fact, on his extended summer cruise he characteristically stocks 48 AA batteries, 24 AAA batteries, 20 C batteries, 24 D batteries, three 9V batteries, and three 123 camera batteries. None of these is rechargeable, but our friend says that will likely change soon. In anticipation of that change, we decided to test AA rechargeable batteries: 

The AA test specimens—the disposables on the left and the rechargeables on the right—stand ready for action.

All the batteries mentioned above are alkaline cells, but as readers know, there are many more battery chemistries to choose from. Battery chemistries available today include not only the ubiquitous alkaline, but also rechargeable alkaline, lithium manganese dioxide, nickel-cadmium (Ni-Cd), nickel-metal hydride (Ni-MH), and lithium ion (Li Ion)—(see PS June 2001, page 24). All the names, chemistries, and abbreviations are cute, but what we want to know is how well the different battery types work in typical equipment on our boats. But first, a little history.

Background
Carl Gassner of Germany invented dry cell batteries, similar to our conventional zinc-carbon batteries, back in 1888. By 1896, the National Carbon Company (later to become Union Carbide and then Eveready) sold the first dry cell battery for consumer use, which was based on zinc-carbon technology. Waldmar Jungner of Sweden invented the nickel cadmium battery (Ni-Cd) in 1899, but the technology was not commercialized until the early 1950s. Around 1909, Thomas Edison invented the alkaline battery, based on a technology similar to our modern-day alkaline batteries. By 1920, the standard battery sizes we use were being sold to consumers.

Beginning in the 1960s, alkaline batteries—based on alkaline manganese dioxide chemistry derived from Edison's nickel-iron alkaline invention—became available for consumer use. By the late 1970s, lithium primary batteries became available. Since then, other battery chemistries have been developed and commercialized, such as nickel metal hydride (Ni-MH) and lithium ion (Li Ion).

Modern day electronics are very power-conservative. Not only can manufacturers cram more stuff into smaller spaces, but the actual integrated circuits in the devices run at lower voltages. The lower voltage reduces the power consumption and can either increase battery life or allow smaller batteries to be used in the same application. Even flashlights are following this path with incandescent bulbs beginning to be displaced by LED-based flashlights.

Testing
For our testing, we wanted to compare the different battery chemistries in different applications to see if we could draw some conclusions regarding which battery types (i.e., chemistries) are suitable for different applications. We limited our testing to AA batteries, which in our experience are those most-often used.

We chose three different applications for our testing—a flashlight, a portable GPS, and a handheld VHF. The flashlight was a two-cell incandescent, the GPS was our older Garmin GPS 12XL, and the VHF was a Standard Horizon HX460S.

Before starting our tests, we measured the battery current used by each of our test devices. Rough calculations showed that a set of batteries would last about four hours in the flashlight and more than 24 hours for the GPS. With all the batteries we wanted to test, the time involved would have been prohibitive. Instead, we used the built-in battery-state indicators to try to learn something about the test devices and different battery voltages.

We used an adjustable lab power supply to power the GPS and VHF. As we varied the voltage, we recorded the current used and the battery-state indicator on the devices. These tests allowed us to gauge the battery voltage at which the GPS and VHF become unusable. We discovered that the VHF will use more of the batteries' available power than the GPS. (Both battery-state indicators showed "dead" when the voltage reached 4.0.) One interesting fact we learned was that the GPS power usage goes up (from 378 mW to 482 mW) as the battery voltage drops, whereas the VHF's power usage went down (from 181 mW to 66 mW). Regardless of the reasons, this means that battery use accelerates as the battery voltage drops. Or, using a battery chemistry with a lower starting voltage will have more power used than one with a higher voltage.

With this information, we designed an experiment to simulate the flashlight load, but with enough information to try to make some predictions for comparing battery usage in the GPS and VHF units. Our test rig was a two-cell battery holder, a 5.9-ohm resistive load, a custom electronic device for measuring the battery voltage, and a PC logging voltage measurements once a minute. The custom device uses a 24-bit A/D converter, with an accuracy of at least 0.002%. The PC recorded data to the nearest 0.001 volts (1 millivolt), saving it in an Excel spreadsheet for future analysis. Unlike other testing that just shows how “long” a battery lasts for a test, our jig also allowed us to make some inferences based on the battery voltage curve as the battery was used.

The Batteries
We went out to our local stores to obtain all the different name-brand AA batteries that we could find. We were not interested in searching for exotic batteries, nor for small-brand batteries that might only be found in isolated areas. We selected batteries from Duracell, Eveready (Energizer), Radio Shack, and Rayovac.

In addition to alkaline batteries, we also tested rechargeable alkalines, Ni-Cd, Ni-MH, and Lithium. We could only find one example each of rechargeable alkaline, and Ni-Cd. The lithium battery we found was a disposable battery.

The testing involved four batteries of each chemistry type from each manufacturer. Our rig ran two batteries in series and we ran two sets of tests on each battery type and brand. Since most devices use multiple batteries, running them in series would better simulate battery usage. Running the test twice allowed us to average out some variability in the batteries.

Alkaline
We found seven different batteries to test: Duracell, Duracell Ultra, Energizer, Energizer e² Titanium, Radio Shack Enercell, Radio Shack Enercell Plus, and Rayovac Maximum Plus. Alkaline batteries have a lot of advantages over other battery technologies, like long shelf life, low cost, and high power. The downside is that they are thrown away after every use.

Alkaline batteries are the baseline that all other portable battery technologies get compared with. Our testing showed some interesting data (see Alkaline Longevity/Cost Efficiency, page 5). The Radio Shack Enercell lasted the longest of all the batteries we tested. The Duracell Ultra was just behind it. The Rayovac Maximum Plus had the shortest lifetime, only 63% of the top Radio Shack Enercell.

We also developed another measure of the battery cost. A shorter-lived battery with a lower purchase cost might be a good buy compared to a longer-lived battery with a higher cost. To look at this relationship, we calculated a cost-efficiency value we called “dollars per thousand minutes” ($/1000 min.). We calculated this value by dividing the battery cost by the number of minutes needed to discharge to 0.8V, times 1,000. This number gives us a way to compare batteries among, and between, chemistry types. Using this criteria, the last-place Rayovac comes in first place as the most cost-efficient battery. The downside is that you will be replacing batteries more often.

In our particular test, we didn’t see much difference between the “premium” alkaline batteries—Duracell Ultra and Energizer e² Titanium—compared to their standard versions. Interestingly, the premium Radio Shack Enercell Plus was significantly shorter-lived than the standard Radio Shack Enercell. We have no explanation for this. Overall, we would make a blanket statement that the premium batteries aren't worth searching for at the store.

Based on our testing, Duracell batteries deliver the best cost efficiency. The Radio Shack Enercell, for a little bit more money, will deliver about 5% more power than the Duracell.

Rechargeable Alkaline
The only example of rechargeable alkaline batteries we found at the stores we searched were from Rayovac. Rechargeable alkaline batteries retain the qualities that people like about alkaline batteries with the addition of recharging a finite number of times. 

For all our battery testing, we tried to get battery data sheets from each manufacturer's website. People don't normally check these data sheets, but they contain a wealth of information regarding how the batteries will work in different applications. Rayovac was the only vendor from which good data sheets were not available. We sent e-mails, left voice mail messages, and we never got a response to the questions we had.

During our research, we found many different references to the number of battery recharge cycles. We came across numbers from 25 to 100 times. However, we could get no definitive information from Rayovac. Without hard data, we took Rayovac's "up to 100 times" and conservatively cut that in half to 50 recharge times.

Rechargeable batteries initially cost more than disposable batteries. To be able to compare disposable batteries with rechargeable batteries, we took the cost-efficiency number and divided it by the expected number of recharge/discharge cycles. For the sake of balancing the math, we made the assumption that disposable batteries have gone through one cycle. Using the data on page 6 (Rechargeable Alkaline Batteries), the cost for each use of the rechargable alkaline is about 20 cents compared to $2.29 for the non-rechargeable Rayovac alkaline battery.

Another measure is the return on investment. Rechargeable batteries cost more than disposable ones. Ignoring the battery charger cost, we can compare the $/1000 minute values for the Rayovac alkaline ($2.29) comparedto the Rayovac rechargeable alkaline ($9.95). By the fifth use ($9.95/$2.29 = 4.34), you will be saving money with the rechargeable alkaline batteries, and there are still 45 more uses left in the battery. At 47 cents per Rayovac alkaline battery, the cost savings over the life of the rechargeable alkaline battery will be (45 x 47 cents) $21.15. That's not a bad return for your pocket, nor a bad return for the environment as well, which is something we sailors should always favor.

Lithium
Rechargeable lithium batteries are very popular in laptops, cell phones, etc., but we could not find rechargeable lithium AA batteries in any of the stores that we checked. Too bad, we would have really liked to try some.

For the last few years we’ve been using disposable Energizer e² Lithium (lithium/iron disulfide) AA cells in our digital camera. The batteries were expensive, but they seemed to last a long time, so we decided to subject these to our tests.

The lithium batteries tested almost 100 minutes more than the median minutes of the alkaline batteries tested. However, a 34% increase in battery longevity will cost 2.1 times as much in our $/1000 cost-efficiency number.

There are a number of applications that can benefit from the long lifetime of the lithium cell. For example, an underwater flashlight or camera has needs for a long-life battery, since changing batteries underwater is not an option. Likewise, photographing once-in-a-lifetime events may not wait for one to change batteries. Another reason for considering this battery is that the lithium battery's flat discharge curve may allow one to use some electronic equipment much longer than our battery test would indicate. More on this later.

Ni-Cd
Ni-Cd rechargeable batteries have been around a long time. They're found in all sorts of rechargeable products, like Dust-Busters, and electric razors. Sailors will likely recognize them from their use in Marinco Nicro Solar vents. Past advantages included their high power density, small size, and sealed construction. A significant problem with them, however, involved their recharging characteristics. They were finicky rechargeables, picky about charge rates, incomplete discharges, and even incomplete charges (some referred to this as "memory" effect).

For our testing, we could only find one Ni-Cd AA battery at Radio Shack, though we looked elsewhere as well. We decided to test this battery type along with our others to see how Ni-Cd batteries stack up to the newer rechargeable battery types.

Our testing (see Ni-Cd Cost Efficiency sidebar) confirmed our suspicions from long use of Ni-Cd batteries—they don't hold as much of a charge as alkaline batteries. With rechargeable alkaline and Ni-MH batteries readily available, and given the problems of cadmium disposal and recharging issues, we cannot find a good reason to use AA Ni-Cd batteries in portable devices.

Ni-MH
Ni-MH is one of the newest rechargeable battery technologies on the market for consumer AA use. All the major battery manufacturers have Ni-MH products, but their charging systems are not standardized from company to company.

Our testing showed that Ni-MH batteries have a flat discharge curve similar to a lithium cell and the longevity of an alkaline cell (see Battery Discharge Rates and Lifetimes sidebar). We found conflicting data as to their number of recharge cycles, anywhere from 500 to 1,000 times, which is similar to the number of recharge cycles of a lead-acid battery. For our analysis, we used 500 recharges to be on the conservative side. Together with the tremendous cost savings of reusing these batteries, it's hard to find a reason to not use them.

We first stumbled on the Radio Shack I-C³ batteries and their 15 minute charger. The batteries need charging coming out of the package. After 15 minutes of charging, we took the batteries and placed them into our test rig and got impressive results. And a 15- minute recharge time is wonderful compared to the many hours needed to charge rechargeable alkaline and Ni-Cd batteries. However, not all is wonderful.

We obtained Ni-MH batteries from Duracell, Energizer, another Radio Shack type, and Rayovac. All the different brands had their own fast chargers (Duracell and Energizer at 30 minutes, Radio Shack at over an hour, and Rayovac at 15 minutes). We suspect that the Radio Shack I-C³ is a private label branding of the Rayovac 15 Minute Rechargeable I-C³, which allows them to use the same fast charger.

From what we could gather, the chargers can identify the "allowed" brand of battery and charge it at a very high rate. Foreign batteries are charged at a much slower rate. Instead of buying all different chargers, we stayed with the Radio Shack 15 Minute Charger (model 23-039, $39.99). This charged the Radio Shack I-C³ and the Rayovac I-C³ in 15 minutes. We charged the Duracell, Energizer, and Radio Shack overnight as recommended in the charger's instructions.

Based on our data displayed above (see Rechargeable Comparison) the Rayovac 15 minute Rechargeable I-C³ is our winner. We picked it over the Duracell because the Duracell's custom charger takes 30 minutes to recharge. To us, 15 minutes to recharge becomes a non-issue in those times that you need a battery and don't have a charged one.

With all the good news about Ni-MH batteries, there is a downside. The batteries will self-discharge when stored. After a month at room temperature, a Ni-MH battery will only retain about 75% of its power (according to the Duracell website). This information regarding self-discharge becomes important if you seldom use a piece of equipment. For example, a handheld VHF kept in a ditch bag would do better with alkaline orlithium disposable batteries than with Ni-MH.

Conclusions
As we mentioned earlier, electronic devices such as a GPS or handheld VHF have cut-off voltages at which they refuse to operate. Sometimes this is a limit of their internal circuitry and sometimes it is an end-point set in the software to prevent discharging the batteries below a certain point where outgassing or leakage could start.

Using the data we collected from the different battery types and the voltage measurements we made on our test GPS and VHF units, we have tried to come up with a table projecting relative operating times of these devices and different battery chemistries. We have chosen to use the median data for each battery type to try to minimize outlier data. The minutes will not be right if we actually ran these devices, but instead serve to indicate relative times based on the battery discharge curves. 

As mentioned, our two test devices have battery state indicators. Typically, these indicators base their measurements on the battery voltage and not on the amount of power remaining in the cell. With alkaline batteries, like lead-acid batteries, the battery voltage does track the remaining power very well until about 0.9V, at which point the battery voltage drops quickly. However, Ni-MH and lithium cells have quite flat discharge curves. These cells maintain almost a constant voltage until just before they've exhausted their power. For devices like the test GPS that cut off at around 1.0V, almost 30% of the power in the alkaline cell is wasted. This isn't the case with lithium or Ni-MH cells.

The Ni-MH charged voltage is less than the fully charged alkaline battery voltage, 1.38V versus 1.61V. This lower starting voltage will cause the battery state display on the GPS to show a "not-full" battery. Obviously this is incorrect and, as we can see in GPS Battery Use above, the Ni-MH batteries will power the GPS for more time than alkaline cells.

The VHF Battery Use table is interesting. It shows that alkaline and Ni-MH will both power the VHF for about the same period of time, but that the VHF will always show a battery alarm with the Ni-MH batteries.

The days of just using disposable batteries for flashlights are gone. Many of today's electronic devices need a minimum voltage level to function, which may leave a lot of power still in an alkaline cell. We were very impressed with the Ni-MH batteries and how—even with a lower starting voltage—they will last as long as alkaline batteries in our boating applications. They are not perfect, but until a better choice comes along, we recommend using the Rayovac I-C³ AA batteries. Of course you'll want to keep some standard alkaline batteries around for emergencies.

 

Also With This Article
"Alkaline Longevity/Cost Efficiency"
"Rechargeable Alkaline Batteries"
"Battery Discharge Rates and Lifetimes"
"Ni-Cd Cost Efficency"
"Rechargeable Comparison"
"VHF and GPS Battery Use"

Contacts
• Duracell, 800/551-2355, www.duracell.com
• Eveready Battery Co., Inc. (Energizer), 800/383-7323, www.energizer.com
• Radio Shack, 800/843-7422, www.radioshack.com
• Rayovac, 800/237-7000, www.rayovac.com

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