[Updated Oct 24, 2008]
My original review on the Sanyo eneloop NiMH cells was written nearly two years ago. Since then, I have learned a lot more about the characteristics about eneloop and other low-self-discharge cells. So it is time to clear up some inaccurate information I wrote about eneloop's self-discharge rate.
1. I was told that the Sanyo eneloop cells were not 100% fully charged when they leave factory. This makes sense because new cells are fast-charged in the factory to save time. In order to avoid problem with heat and gas built-up, they cannot be charged to 100%. I have tested some eneloop cells that were manufactured 20 months ago, and they still maintained about 70% charge. On the other hand, cells manufactured 6 months ago contain about 75% charge. This confirmed that the discharge rate is much lower than I previously estimated.
2. The self-discharge rate of eneloop (and other LSD cells) is not linear! My own testing showed that a fully charged eneloop may lose 10% of its initial charge during the first month of storage, but in the second months it may lose just 2-3%. The rate becomes even slower after three months. In fact, my 5-month self-discharge test using various brands of LSD cells gave nearly identical results as my previous 3-month test.
3. When an eneloop cell is freshly charged, its capacity actually measures at about 5% higher than its rated capacity (2100mAh vs. 2000mAh). In my previous tests, I used measured capacity as base to calculate the percentage loss. This results in apparently higher percentage loss. Had I used the rated capacity as base value, the self-discharge rate would be about 5% lower.
Based on the above revelation, I believe Sanyo's claim about eneloop (maintain 85% charge after one year, 80% after two year) to be accurate. Therefore the title of my previous review ("The News of LSD Has Been Slightly Exaggerated") has to be replaced.
Over the past two years, I have tested several different brands of LSD cells available from Amazon.com, including Rayovac Hybrid, Kodak Pre-Charged, and Duracell Pre Charged. They have all performed very well - practically no difference from the Sanyo eneloop. So the bottom line is: just pick which ever brand of LSD cell is on sale, and you can't go wrong.
[Update on April 21, 2011]
- After testing and using various brands of LSD cells over the past four years, Sanyo eneloop cells have proven to be the most reliable and consistent of them all. I now have to recommend Sanyo eneloop over all others brands.
- Recently Amazon started to offer the SANYO NEW 1500 eneloop cells, at around 25% higher price. In my testing, the improvement of the new eneloop over the original is hardly noticeable. So you'll have to decide whether it is worthwhile to pay the extra cost. Sanyo Eneloop 8 Pack AA NiMH Pre-Charged Rechargeable Batteries
[IMPORTANT NOTE]
My original review below was written back in 2007, when the Sanyo eneloop first became available
through Amazon.com. Part of my initial estimation about eneloop's self-discharge rate turns out to be inaccurate. Please see my other Amazon spotlight review for updated information.
[Original Review follows]
According to Sanyo, the new eneloop LSD (low self-discharge) NiMH batteries can maintain 85% of its original charge after 1 year of storage. This claim is slightly misleading, because it is based on simulation test at 20 degree C. At higher temperature, the self-discharge rate is likely to be much higher.
I have tested five of those AA cells (details are given in my review for the eneloop 4-pack). The average energy loss is about 26% after less than 6 months of storage, based on manufacturer date codes. This self-discharge rate is about 3 times higher than what Sanyo claimed. However, it is still 6 times lower than that of ordinary NiMH batteries. Therefore I'm in the process of replacing most of my existing rechargeable cells to the Sanyo eneloop.
Thanks to lower self-discharge rate, you'll discover a lot more applications for eneloop cells in your house, such as in clocks and remote controls. Do NOT use those cell in smoke detectors, since their discharge voltage profile is very different from that of alkaline cells. Also, don't use them as emergency flash lights batteries in your car, because the higher temperature during summer time will probably nullify the advantage of LSD.
[Update on Jan 13, 2007]
I have tested six new eneloop AAA cells, dated "2006-06'. The average residue charge is 589mAh, and the freshly charged capacity is 827mAh. This implies a self-discharge rate of 29% in 7 months, which is consistent with the rate for AA cells (26% loss in 6 months).
[Update on Jan 29, 2007]
The Rayovac "Hybrid" rechargeable NiMH batteries are now available at Walmart, priced at only $[...] for 4-pack of AA or AAA cells. It is also advertised to have low self-discharge rate, but not as low as that for eneloop. The eneloop is supposed to retain 85% charge in 12 months (when stored at 20 degree C), whereas Hybrid is supposed to retain 80% charge in 6 months (no mention of temperature).
[Update on June 3, 2007]
In my 2-month self-discharge test using four different brands of NiMH cells. Hybrid and eneloop came up neck-and-neck!
Room temperature: 60-62 degree F (16-17 degree C). All capacities measured are average of 2-cells.
- Sanyo eneloop 2000, purchased Jan 2007.
Initial capacity measured: 2070mAh
Capacity after 62 days: 1796mAh (-13.2%)
(Note: my previous result for eneloop showed -18%, but I repeated the test and it did better this time)
- Rayovac Hybrid 2100, purchased Jan 2007.
Initial capacity measured: 2155mAh
Capacity after 62 days: 1859mAh (-13.8%)
- LaCrosse 2000, purchased Jan 2006.
Initial capacity measured: 1902mAh
Capacity after 62 days: 1417mAh (-26%)
- SONY 2300, purchased Sep 2004.
Initial capacity measured: 2210mAh
Capacity after 62 days: 1309mAh (-41%)
So neither eneloop nor Hybrid did as well as advertised, but they are still significantly better than previous generation of NiMH cells. So you can't go wrong with either brand. - Eneloop - Battery - Aa - Nimh'
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