GE/SANYO eneloop 4 Pack AA NiMH Pre-Charged Rechargeable Batteries

I was really excited when I first heard about the Sanyo eneloop LSD (low self-discharge] NiMH batteries. The promise to "maintain 85% of original charge after one year" sounds amazing. However, after reading Sanyo's Nov 2005 press release carefully, I learned that the claim is actually based on "simulation test when stored at room temperature of 20 degree C".
The rechargeable battery is an electrochemical device. Since most chemical reactions happen faster at elevated temperature, it is reasonable to expect the self-discharge rate of the eneloop to be also faster at higher tempeature. The question is: by how much? The following is my experiment:
I purchased two eneloop AA 4-pack on Jan 3rd. The packages were dated "2006-07", which means they were probably last charged in the factor nearly six months ago. The initial voltage is 1.31V for all cells. I then tested a total of five AA cells using the "Discharge/Refresh" mode on my LaCrosse BC-900 charger.
The first discharge cycle (at 250mA) yields the residue charge in those cells. The average of five cells came out to be 1476mAh. The spread is between 1388mAh and 1531mAh.
Next, the cells were fully recharged (at 500mA) and discharged again. The second discharge cycle yields the freshly charged capacity. Amazingly, the numbers all came out to be within 1-2% of rated capacity of 2000mAh.
Therefore on average, those five eneloop AA cells I tested have lost 26% of their original charge after nearly 6 months of storage. This rate of energy loss is about three times higher than what Sanyo advertised. However, it is important to point out that a typical NiMH cells will lose around 30% of is original charge within one month (Some brands, such as the Energizer 2500mAh, will loss 30% in one week). So the self-discharge rate of eneloop is still 6-7 times slower. Just remeber that they are best stored at temperature of 20 degree C or lower, if you hope to get the advertised self-discharge rate.
[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.