In every test we found of rechargeable batteries, the Sanyo Eneloop Rechargeable (Est. $15 for 4 AA) stands out as a top performer. The Eneloop is a nickel-metal hydride (NiMH) battery, the most common type available today. One problem with many NiMH batteries is that they tend to lose their charge quickly in storage, making it difficult to keep fully charged batteries on hand and ready to go. Sanyo was the first company to get around this problem with the introduction of the Eneloop, a low self-discharge (LSD) NiMH battery. It loses power more slowly, which means it can be sold pre-charged and ready for use.
Sanyo claims that Eneloops retain 75 percent of their charge after three years in storage. While no one has tested the batteries over quite this long a period, several reviewers at Amazon.com have found the batteries hold 80 to 85 percent of their charge after one or two years in storage. Much of the drop in power seems to take place over the first few weeks; electronics enthusiast Stefan Vorkoetter, who has conducted extensive comparison tests of rechargeable batteries, found that Eneloop batteries dropped to less than 90 percent of their charge after seven weeks.
Another plus of Eneloop batteries is their long overall lifetime. Sanyo claims its newest-generation Eneloop can be recharged up to 1,500 times. Again, no one has ever tested this claim fully, but the batteries made it handily through 200 charging cycles in one professional test with no discernible drop in voltage. In addition, reviews indicate these rechargeable batteries are strong all-around performers. Both professional testers and users say the Eneloop has no problem powering energy-hungry devices like cameras, and they can also run a fairly long time on a full charge.
Sanyo also makes a higher-capacity battery, the Eneloop XX (Est. $17 for 4 AA). The regular Eneloop has a capacity of at least 1,900 milliamp-hours (mAh); the Eneloop XX has 2,500 mAh. Its higher capacity means it should be able to run longer on a charge. Professional tests show this is indeed the case. In the one test that includes both Eneloop and Eneloop XX batteries, the Eneloop lasts 116 minutes on a charge, while the Eneloop XX lasts 145 minutes. However, in that same test, the Eneloop XX gets much worse ratings for endurance -- the number of times it can be recharged before failing. Regular Eneloops handle 200 charging cycles with no problems, while Eneloop XX batteries fail after an average of 62 charges. The higher capacity of the Eneloop XX may make it the better choice for devices that gobble a lot of power in a fairly short period of time, but for long-term use, the standard Eneloop is clearly a better value.
The Energizer Recharge Power Plus (Est. $20 for 8 AA) falls between the two Eneloop batteries in capacity, with 2,300 mAh. Although Energizer claims the batteries are pre-charged, two separate tests found that fresh out of the package, this battery has only around 300 mAh of energy -- less than 15 percent of its maximum capacity. Moreover, Energizer claims this battery will last only "up to 1 year" in storage, rather than the Eneloop's three years. However, because it has more capacity to start with, the Energizer Recharge Power Plus will have more juice after just a few weeks in storage than an Eneloop. This may make it more useful for powering high-drain devices.
Energizer does not claim as long a lifetime for the Recharge Power Plus as Sanyo does for the Eneloop. It says the battery can be recharged "up to 250 more times," as compared to 1,500 recharges for the Eneloop. Moreover, even this relatively modest claim may be an overstatement. In the same professional test that compared the two Eneloop batteries, the Power Plus batteries made it through just over 80 charging cycles on average before conking out. Some Amazon users complain that their batteries died even sooner, after anywhere from four months to two years. On the plus side, users say the Power Plus can run a long time on a single charge, and it's fairly quick to recharge.
Duracell, another well-known battery brand, also makes a rechargeable battery, the Duracell Rechargeable (Est. $9 for 4 AA). However, this battery isn't a particular standout in any professional tests. It lasts about as long on a charge as the Energizer Recharge Power Plus, but it's slower to recharge, and it doesn't hold its charge as well in storage. As for longevity, one test finds the battery typically lasts for about 100 charging cycles -- far less than the 400 that Duracell claims. Given the battery's middling performance, we see no reason to recommend it over other brands.
One problem with most rechargeable batteries, including all those mentioned above, is that they come only in AA or AAA sizes. We found only one rechargeable battery that has mostly good reviews and is available in a full range of sizes: the Powerex Imedion (Est. $14 for 4 AA). Every source we've consulted says this battery holds its charge very well, keeping anywhere from 65 percent to 85 percent after a year in storage. Most reviews also say the 2,400-mAh Imedion gives you plenty of power on a charge. In professional tests, it runs longer on a full charge than the original Eneloop, though not as long as the Eneloop XX. Several Amazon reviews also say the Imedion lasts longer than the Eneloop, and at a lower price.
When it comes to overall life span, however, the Imedion is less impressive. In one professional test, Imedion batteries didn't make it through the entire 200 rounds of charging and draining, dying after just 120 cycles. Several users at Amazon also report that the Imedion's capacity starts to drop off sharply after 10 recharging cycles or even fewer. Another problem some reviewers note with the Imedion is that the AA battery doesn't fit in some devices. Between this and the longevity problem, the Imedion is not as good a choice as the Eneloop for users who specifically need rechargeable AA batteries or rechargeable AAA batteries. If you need a C, D or 9-volt sizes, however, Imedion is the only reasonable choice.