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Review: Pre-Charged (Low Self-Discharge) Rechargeable Battery Comparison

Three popular brands of pre-charged (low self-discharge) rechargeable NiMH AA batteries.

It's been about two years since I first published my review of Sanyo's Eneloop low self-discharge rechargeable batteries. Since that time, other manufacturers have jumped on the bandwagon, and comparable batteries are now available under many different brand names. This review compares the performance of several of these batteries (including Sanyo's).

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 Methodology

This review compares the tested batteries in four different scenarios:

Fresh out of the package.
Immediately after being recharged.
One week after being recharged.
Seven weeks after being recharged.

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):

Duracell Rechargeable Pre-charged
EasyPix Pre-charged Rechargeable
Panasonic R2 Technology Rechargeable
Powerex Imedion
President's Choice Rechargeable
Rayovac Hybrid
Sanyo Eneloop
Sony CycleEnergy

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 Results

The 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.

Duracell⁽⁵⁾
Rechargeable
Pre-charged E8ge 1000
Ready to Use
Rechargeable EasyPix
Pre-charged
Rechargeable Panasonic
R2 Technology
Rechargeable Powerex
Imedion President's
Choice
Rechargeable Rayovac
Hybrid Sanyo⁽⁴⁾
Eneloop Sony
CycleEnergy

Made in... Japan China China China Taiwan Japan China Japan Japan

Fresh Out of the Package⁽¹⁾

Capacity⁽²⁾
(mAh) 1383 1708 1443 682 1573 1387 1489 1439 1377

Energy
(mWh) 1509 1818 1565 719 1697 1502 1557 1588 1500

Voltage⁽³⁾
(V) 1.09 1.06 1.08 1.05 1.08 1.08 1.05 1.10 1.09

Immediately after Recharging

Capacity
(mAh) 1888 1967 2019 1781 2014 1871 1962 1928 1874

Energy
(mWh) 2181 2183 2268 2037 2240 2131 2216 2217 2167

Voltage
(V) 1.15 1.11 1.12 1.14 1.11 1.14 1.13 1.15 1.16

One Week after Recharging

Capacity
(mAh) 1770 1807 1880 1667 1881 1771 1772 1796 1757

Retention
(%) 93.8 91.8 93.1 93.6 93.4 94.7 90.3 93.2 93.8

Energy
(mWh) 1946 1973 2071 1795 2056 1949 1905 1979 1964

Voltage
(V) 1.10 1.09 1.10 1.08 1.09 1.10 1.08 1.10 1.12

Seven Weeks after Recharging

Capacity
(mAh) 1729 Coming 2010-Feb-06 1780 1581 1803 1718 1677 1763 1711

Retention
(%) 91.6 88.2 88.8 89.5 91.8 85.5 91.4 91.3

Energy
(mWh) 1871 1922 1686 1931 1891 1780 1943 1876

Voltage
(V) 1.08 1.08 1.07 1.07 1.10 1.06 1.10 1.10

Notes

Out-of-the-box capacity is provided for interest's sake only. It is not a very useful measure because it really depends on how long the batteries sat on the shelf before you bought them, and the conditions under which they were shipped and stored.
The capacity was measured at a 1200mA discharge rate, which is about three times the current at which the manufacturers measure the capacity. As a result, all of these batteries test lower than the advertised capacity. However, the results are more realistic for high-current applications like digital cameras.
This is the average voltage that the battery provided during a full discharge at 1200mA. This is not the same voltage you would see if you just measured it with a multimeter, but it is typical of the instantaneous voltage a digital camera might see while snapping a picture.
The results reported in this chart for Sanyo's Eneloop batteries are not identical to those in my original 2007 Eneloop review because I have tested a new set for this comparison, using the same new BattMan II battery analyzer that all the other brands were tested with. The differences are due to the normal variation among batteries of the same brand, and differences in the equipment used.
The Duracell Rechargeable Pre-charged batteries that I tested were the ones with a white ring around the positive terminal. Since then, Duracell has switched to batteries made in China. These can be distinguished by a black ring around the positive terminal. I have not tested those, but reports from others suggest that their performance is more like that of the Rayovac Hybrid batteries.

Analysis

As 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.

Immediately after recharging, the batteries with the highest capacity were the EasyPix and Powerex offerings, although all the brands provided an acceptable capacity. The lowest voltage, only 1.11V, was provided by the Powerex batteries, with the highest coming from the the Sony CycleEnergy brand, followed closely by Sanyo's Eneloop and Duracell's Rechargeable Pre-charged.

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.

Charging

All of these low self-discharge batteries can be charged in any good quality AA NiMH battery charger. I would not recommend any charger that advertises a charge time of one hour or less, nor would I suggest using a "dumb" overnight charger. These batteries can also be purchsed with an appropriate charger included. For more information on charging, please refer to my article, Choosing and Using NiMH Rechargeable Batteries.

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.

Physically, the Duracell, Sanyo, Sony, and President's Choice batteries look identical (ignoring the artwork on the sleeve of course). They are also all made in Japan. The Rayovac and Panasonic batteries (from China) look and perform similarly to one another but quite differently from the Japanese brands. Likewise, the Powerex (Taiwan) and Easypix (China) batteries are similar to each other in appearance and performance, and different from the others. My theory is that these eight brands of batteries are made by only three different manufacturers.

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.

Recommendations

To 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 two years are still going strong, averaging 300 or more photos and three months between recharges.

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