Bests & Worsts Reviews from Amazon

I have done extensive study on low-self-discharge (or 'pre-charged') rechargeable batteries, starting from early 2007 when the original Sanyo eneloop cells were introduced to the US market. Subsequently I have tested Sanyo 2nd-gen eneloop, followed by Sanyo 3rd-gen eneloop and finally these Panasonic 4th-gen eneloop. Every single eneloop cell I have purchased in the past 8 years, including the original ones, are still in good working conditions. That says a lot about the quality of eneloop batteries. But the question is: are there any REAL improvements in later generations of eneloop batteries? [Capacity] The advertised capacity ratings for all eneloop (1st to 4th-gen) batteries are identical: - AA cells are rated "Typ. 2,000mAh, Min. 1,900mAh" - AAA cells are rated "Typ. 800mAh, Min. 750mAh" Actual capacity measurement, using my old La Crosse BC-900 Battery Charger, also showed that capacity numbers are consistent from 1st to 4th-gen. In fact, my measured results are consistently about 4-5% higher than the advertised values. [Charge Retention Rate] With each new generation, eneloop's long-term charge retention rate gets a little more ambitious: - 1st-gen claims "85% after 1 year" - 2nd-gen claims "85% after 1 year; 75% after 3 years" - 3rd-gen claims "90% after 1 year; 80% after 3 years; 70% after 5 years" - 4th-gen claims "90% after 1 year; 70% after 5 years" Notice that those numbers are essential the same, just extending over longer period of time. In fact, I have tested a set of first-gen eneloop cells after 3.5 years in storage, and they still contain around 75% of rated capacity. To me, this shows that there are no real changes in the charge retention rate among different generations. [Cycle Life] The main difference between different generations is in the advertised lifespan: - 1st-gen: '1000 cyles' - 2nd-gen: '1500 cycles' - 3rd-gen: '1800 cycles' - 4th-gen: '2100 cycles' Let's consider how long it takes to actually test a rechargeable battery through 2100 deep discharge/recharge cycles: The standard test procedure calls for charging at 0.2C (~5 hours) and discharging at 0.2C (~5 hours), plus cooling period of one hour each between charge and discharge phases. So a complete cycle takes about 12 hours. To exercise a battery through 2100 cycles would therefore take nearly three years! However, 3rd and 4th-gen eneloop cells are both released less than 2 years from their previous generations. [Bottom Line] My theory is that all eneloop 2nd/3rd/4th gen batteries are in fact made with the same battery technology. It takes many years to collect the actual cycle life and charge-retention rate data. Therefore only later generations are able to advertise higher number of cycles and longer storage life. If my theory is true, then it follows that there's no reason to pay higher price to get the latest 4th-gen eneloop. Any earlier generation of eneloop would perform just as well. Finally, if you are extremely 'value conscious' like me, consider the following rebranded eneloop batteries and get the same quality for even less: - AmazonBasics Pre-Charged Rechargeable Batteries (in white wrappers) are rebranded Sanyo eneloop. - AmazonBasics High-Capacity Pre-Charged Rechargeable Batteries (in black wrappers) are rebranded SANYO eneloop XX batteries - Duracell Ion Core AA Rechargeable Batteries are rebranded Sanyo XX, while Duracell Ion Core AAA cells are rebranded eneloop AAA batteries [Update on Dec 22, 2015] For additional information on eneloop batteries in general, please refer to my newly updated "Panasonic/Sanyo eneloop FAQ": http://www.amazon.com/review/R3L93DRAE29PCG/