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What Is "Low Self-Discharge"?Sanyo first introduced low self-discharge technology in 2006 in response to a shortcoming of normal NiMH rechargeable batteries, namely that they suffer from a very high self-discharge rate. In plain English, this means that the battery will run down in a matter of weeks or months even when it's just sitting there in the package or a switched-off device. This characteristic restricted NiMH batteries to applications where high currents are needed for a relatively short period of time, such as digital cameras. With the introduction of the Sanyo's Eneloop battery, NiMH batteries could be used in devices that were traditionally restricted to disposable alkaline batteries, such as wall clocks and TV remote controls. And, when used in gadgets like digital cameras or flashlights, they can be left installed for weeks or months on end and will still work when you pick up the camera for that spontaneous snapshot or reach for the flashlight during a power failure. A happy side-effect of low-self discharge is that the manufacturer can pre-charge the batteries before they leave the factory and they will still be charged when you buy them many months later. This is great, but in my opinion, the battery manufacturers' marketing departments over-emphasize this on the packaging (marking them "pre-charged") while glossing over the underlying low self-discharge characteristics that make these batteries pre-chargeable, and more importantly, so much more useful than ordinary rechargeables. Comparison MethodologyThis review compares the tested batteries in four different scenarios:
In each scenario, we'll look at the total capacity provided in milliAmp-hours (mAh), total energy provided in milliWatt-hours (mWh), and average voltage under load (V). In the one-week and seven-week tests, we'll also look at the charge retention (remaining capacity as a percentage of just-recharged capacity). The brands being compared are (in alphabetical order):
As I get my hands on other brands, I will test those too and update this article with the new information. (If you are a manufacturer or distributor of low self-discharge AA batteries and want to see yours in this review, please contact me to arrange to have them tested.) All the tests were done using my BattMan II Computer Controlled Battery Manager. Discharging was done at 1200mA, which is on the high end of what these batteries would be expected to deliver in an application such as a digital camera. Charging was also done at 1200mA, using the −ΔV peak-detection method. The charging graphs were inspected manually to ensure no false peaks occured. The initial recharge (after the fresh-out-of-the-package test) was repeated until the capacity stopped increasing. The ResultsThe table below contains all the results for the batteries I've tested so far (here is a large-font version). The batteries were tested in groups of four cells, but the results are for an individual cell (i.e. one AA "battery"). Additional comments appear after the table.
Notes
AnalysisAs mentioned in the notes above, the out-of-the-package capacity is really not a very meaningful measure because it depends on too many things besides the battery itself. However, for what it's worth, the Powerex Imedion battery had the highest pre-charged capacity and the Panasonic R2 had the lowest, delivering only 1/3 of the rated capacity. The Rayovac Hybrid batteries and the Panasonics had the lowest voltage under load, at only 1.05V, meaning that many digital cameras would consider them "dead" fresh out of the package.
A week after recharging, things evened out somewhat, with all the capacities falling into a fairly narrow range. Those batteries that exhibited the highest freshly-charged capacity generally dropped the most, the most significant example being the Rayovacs, which retained only 90.3% of their capacity. The President's Choice Rechargeables on the other hand retained 94.7%. The range of voltages was also quite narrow, with a low of 1.08V for the Panasonic and Rayovac models and a high of 1.12V for Sony. After seven weeks, the performance started to spread out again. A real distinction began to emerge between the Chinese-made batteries and those from Japan. The former retained between 85.5% and 88.8% of their capacity while the latter ranged from 91.3% to 91.8% (a very narrow range). The made-in-Taiwan Powerex Imedion was in the middle, with 89.5% retained capacity. In terms of absolute retained capacity (instead of percentage) though, Powerex came out on top. The range of voltages also spread out after seven weeks, with a very low 1.06V from the Rayovacs and a respectable 1.10V from Sanyo, Sony, and President's Choice. Voltage under load can be critical in devices which show a low-battery warning at 1.1V and consider a battery to be "dead" when it drops down to 1.0V. These higher voltage brands will far outlast the others in those applications.
Which brand to choose is a matter of what you are using them for. If a really long life at low currents is important (e.g. as a clock battery), a brand with a high retention percentage in the seven-week test is the best choice. Those will hold their charge the longest, regardless of the initial capacity. If you will be using them over a relatively short period such as a few months in higher current applications (e.g. a digital camera you keep in your pocket or purse), choose one with a high capacity and voltage in the seven-week test. RecommendationsTo choose the best low self-discharge battery for your application, you have to know something about that application. If your device has a relatively high voltage (above 1.0V) at which it considers the battery to be dead, choose a brand with a high voltage under load. The Sanyos came out tops in the tests but the Sony and Duracell offerings were right up there with them. In selecting between these three, you may well decide based on other factors such as price, the fact that Sony uses the least amount of packaging material, or that Sanyo includes a reusable battery storage case. If you're interested in shorter term capacity, the Rayovacs do well right after being charged but quickly fall behind. They also have the lowest voltage under load, which may make their apparent capacity in some gadgets even lower. On the other hand, they can often be found locally at lower prices than the other brands.
What I did not test is how well these batteries will hold up after being
recharged hundreds of times over the course of many years, and there's really
no way to know this without waiting those years to find out. The only data
point I have in that regard is that the Sanyo Eneloop batteries that
I've been using in my Nikon camera for Other Articles of InterestFor more information on NiMH rechargeable batteries and devices to charge them, you may be interested in the following articles:
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