by Scott Rosenthal
If you fly a state-of-the-art single-engine airplane, lucky you. You probably have dual alternators and perhaps even dual batteries, so electrical redundancy is all but a done deal. Even so, youre sure to carry plenty of battery powered devices in the airplane: several flashlights, a back-up portable GPS, a VHF radio. Redundancy or not, you still buy lots of batteries.
These days, there are more battery chemistries to choose from than just alkaline. In addition to the ubiquitous disposable alkaline, there are also rechargeable alkalines, lithium manganese dioxide, nickel-cadmium (Ni-Cd), nickel-metal hydride (Ni-MH) and lithium ion (Li Ion) cells. With so many choices, is it worth the trouble and expense to seek out other battery types or will you do as well with off-the-shelf alkalines? And if the latter is true, which alkaline is the best buy?
Modern portable electronics are power conservative and growing more so by the day. Not only can manufacturers cram more stuff into smaller spaces, but the integrated circuits in the devices require ever less wattage to operate, thus increasing battery life. Even flashlights are following this trend, with incandescent bulbs being displaced by brighter, less power-hungry LED-based flashlights.
For our testing, we set out to compare battery chemistries in different applications to see which types are best suited for which applications. We limited our testing to AA batteries, the most often-used size. We chose three applications for our testing-a flashlight, a portable GPS and a handheld VHF. The testing was originally done for our marine division so the GPS and VHF were marine units. However, these devices are similar in power requirements to aviation units.
Before starting our tests, we measured the battery current used by each of our test devices and we used this data to calibrate the GPSs onboard battery gauge. One interesting fact we learned is that the GPS power usage goes up (from 378 mW to 482 mW) as the battery voltage drops, whereas the VHF radio power usage went down (from 181 mW to 66 mW). This means that battery depletion accelerates as the battery voltage drops, at least for the GPS.
With this information, we designed a test jig with 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 has a 24-bit A/D converter with an accuracy of at least 0.002 percent. The PC recorded data to the nearest millivolt. Unlike other testing that just shows how long a battery lasts, our jig also allows us to make some inferences based on the battery voltage curve as the battery is used.
We bought all the name-brand AA batteries that we could find at local retail stores, including Duracell, Energizer, Radio Shack and Rayovac. In addition to alkalines, we also tested rechargeable alkalines, Ni-Cd, Ni MH and lithium technology. We tested four batteries of each type from each manufacturer and our jig ran two batteries in series, simulating actual usage. Running the tests twice allowed us to average out variability in the batteries.
Alkaline batteries are the baseline against which other battery technologies are compared. Alkalines are readily available, moderately priced and have excellent shelf live. We found seven models: Duracell, Duracell Ultra, Energizer, Energizer e Titanium, Radio Shack Enercell, Radio Shack Enercell Plus and Rayovac Maximum Plus.
Our testing showed some interesting data. The Radio Shack Enercell lasted the longest of all the batteries we tested, with the Duracell Ultra just behind it. The Rayovac Maximum Plus had the shortest lifetime, only 63 percent of the top Radio Shack Enercell.
But battery life is only part of the story; price fills out the picture. A shorter-lived battery with a lower price might be a good buy compared to a longer-lived battery with a higher cost. To examine this relationship, we calculated a cost-efficiency value we call dollars per thousand minutes ($/1000 min). We calculated this value by dividing the battery cost by the number of minutes to discharge to 0.8 volts times 1000. This number provides a means to compare batteries among-and between-chemistry types. Using this criteria, the last-place performer Rayovac comes in first place as the most cost-efficient battery. The downside is that youll be replacing batteries more often if you use the device regularly.
In our test, we didnt see much difference between the premium alkaline batteries-Duracell Ultra and Energizer e 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, but overall, our view is that premium batteries arent worth the extra cost. Based on our testing, Duracell alkalines deliver the best cost-efficiency while the Radio Shack Enercell, for a bit more money, delivers 5 percent more power than the Duracell.
The only example of rechargeable alkaline batteries we found at retail stores were from Rayovac. The rechargeable alkalines retain the qualities we like about alkaline batteries with the addition of recharging a finite number of times, according to the manufacturer. Speaking of which, for our testing, we obtained battery data sheets from each manufacturers Web site. Rayovac was the only vendor that doesnt seem to offer data sheets. We sent e-mails and left voice messages but never got a response to our questions.
During our research, we found references to the number of battery recharge cycles, ranging from 25 to 100 times. However, we couldnt get definitive information from Rayovac. Without hard data, we took Rayovacs up to 100 times and conservatively cut that in half to 50 recharge cycles.
Rechargeable batteries cost more initially than do disposable batteries. To compare disposable batteries with rechargeables, we divided our cost efficiency ratio by the expected number of recharge/discharge cycles. To balance the math, we assumed that disposable batteries have gone through only one recharge cycle. Using this data, the cost for each use of the rechargeable alkaline is about 20 cents compared to $2.29 for the non-rechargeable Rayovac alkaline.
Another measure is the return on investment. Ignoring the battery charger cost, we can compare the $/1000 minute values for the Rayovac alkaline ($2.29) compared to the Rayovac rechargeable alkaline ($9.95). By the fifth use ($9.95/$2.29 = $4.34), youll be saving money with the rechargeable alkaline batteries and theres still at least 45 more uses left in the battery. At 47 cents per Rayovac alkaline, the cost savings over the life of the rechargeable alkaline battery will be at least $21.15 (45 X 47 cents). Thats not a bad return for your wallet and the environment. (The charger, by the way, costs about $15; the payoff comes with lots of battery use.)
Rechargeable lithium batteries are the power source of choice for laptops and cell phones but we couldnt find rechargeable lithium AA batteries in any of the stores that we checked. For the last few years, weve been using disposable Energizer e Lithium AA cells in our digital camera. The batteries are expensive but they last, so we decided to test them.
The lithium batteries tested almost 100 minutes more than the median minutes of the alkaline batteries we tried. However, that 34 percent increase in battery longevity cost 2.1 times as much in our $/1000 cost-efficiency number.
Some applications can benefit from the long lifetime of the lithium cell. For example, an underwater flashlight or camera needs a long-life battery, since changing batteries underwater isnt an option. Another argument for lithium batteries is that their flat discharge curve may allow you to use some equipment longer than our battery test would indicate.
Nickel-cadmium rechargeable batteries are found in all sorts of products such as cordless vacuums and electric razors. Advantages include high power density, small size and sealed construction. A significant problem with Ni-Cd batteries is their recharging characteristics. Theyre picky about charge rates, incomplete discharges and incomplete charges, sometimes called the memory effect.
Our testing of a Ni-Cd set bought at Radio Shack confirmed our suspicions-they dont take as much charge as alkaline batteries do. With rechargeable alkaline and Ni-MH batteries readily available, we recommend against Ni-Cd batteries for portable devices.
Nickel-Metal-Hydride or Ni-MH is one of the newest rechargeable battery technologies on the market for consumer use. All the major battery manufacturers have Ni-MH, but their charging systems arent standardized across manufacturers. Our testing showed that Ni-MH batteries have a flat discharge curve similar to a lithium cell and the longevity of an alkaline cell.
We found conflicting data as to the number of recharge cycles, anywhere from 500 to 1000 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, its hard to find a reason to not use them.
We bought Radio Shack I-C batteries with a 15-minute charger. The batteries need charging right out of the package and after 15 minutes, we placed them into our test jig and got impressive results. A 15-minute recharge time is wonderful compared to the hours needed to charge rechargeable alkaline and Ni-Cd batteries.
However, not everything about these batteries is wonderful. We obtained Ni-MH batteries from Duracell, Energizer, another Radio Shack type and Rayovac. All the brands had their own fast chargers: Duracell and Energizer at 30 minutes, Radio Shack at over an hour and Rayovac at 15 minutes. We learned that the Radio Shack I-C is a private label branding of the Rayovac 15-minute rechargeable I-C, 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 high rate, while foreign batteries are charged at a slower rate. Instead of buying different chargers, we stayed with the Radio Shack 15-minute charger (model 23-039, $39.99), which charged the Radio Shack I-C and the Rayovac I-C in 15 minutes. We charged the Duracell Rechargeable Accu, Energizer and Radio Shack overnight as recommended in the chargers instructions. Based on our data, the Rayovac 15-minute rechargeable I-C is a clear winner. We picked it over the Duracell because the Duracells custom charger takes 30 minutes to recharge and … why wait?
So, are these the perfect batteries? No, unfortunately. The batteries will self-discharge when stored and after a month at room temperature, theyll retain about 75 percent of their power, says Duracell. Self-discharge is critical if you seldom use the device, less so if you use it regularly.
Ni-MH technology has one other quirk. Battery state indicators in GPS units base their measurements on the battery voltage and not on the amount of power remaining in the cell. With alkaline batteries, the battery voltage tracks remaining power well down to about 0.9 volt, at which point the battery voltage drops quickly. However, Ni-MH and lithium cells have flat discharge curves. These cells maintain almost a constant voltage until just before the cells have exhausted their power. This lower starting voltage will cause the battery state display on the GPS to show a not-full battery, an obviously erroneous indication.
Which battery to buy? If the devices the batteries will power are seldom used-say a portable GPS for back-up navigation or a VHF comm that lives in your flightbag for months at a time without being touched-alkalines remain the best-value choice.
In our view, Rayovacs Maximum Plus and Duracells delivered the best performance for the buck, even though Radio Shacks Enercell outlasted them. We prefer the Duracells because youll need fewer of them and thats better for the environment. Our tests showed that premium alkaline batteries dont seem to deliver any cost performance gains over the less expensive standard types so our view is to skip paying extra for the premium brands; theyre just not worth it.
If you use your portable devices constantly-say you have a GPS which you dont run on ships power or you have an old tail dragger with no electrical system-rechargeables are a compelling purchase. The more batteries you use, the bigger the payoff in rechargeables over disposables.
Of all the rechargeable types, we liked the Ni-MH option best, specifically Rayovacs 15-minute rechargeable I-C. The batteries charge fast and retain a good punch. The only downside is self-discharge when stored, something you can address by always having a freshly charged set available and/or a pack of alkalines readily at hand for emergencies. You shouldnt need them often, however, if you keep the Ni-MHs charged.
Scott Rosenthal is a contributing editor to Practical Sailor, Aviation Consumers sister magazine. He works in the electrical/electronics field.