I recently discovered these batteries and I am very pleased. Introduced 10 years ago by Sanyo and now sold under the Panasonic name. (where was I? Why did it take me a decade to discover them?)
These are rechargeable NiMH, nickel metal hydride, batteries. These are low self discharge NiMH. I used NiMH back in 2000 when we got our first digital camera. Back then their main fault was that when left sitting for a month they lost almost about 25% of their capacity due to high self discharge. Not these newer low self discharge ones. One thing that has always been good about NiMH is they can supply much more current than alkaline batteries. That has not changed.
Over the years the number of times the Eneloops can be recharged has increased from 1000 to 2000 or more. They have a capacity of 2000 mAh which is not bad. Maybe not as high as some good alkaline, but these are rechargeable. There are newer versions that advertise greater capacity.
There are some other brands out there but the Panasonic are recognized as being the best. maybe. The Amazon basics NiMH batteries are apparently re-badged Eneloops.
When you buy them they are precharged to between 60 to 80% charge. Using a solar powered factory. It takes several cycles to get them to supply the maximum capacity. They need a smart charger. Sanyo/Panasonic has a few; sometimes a charger and a pair of batteries is a good deal. LaCrosse makes several chargers which are smart, one can be used off 12 VDC. Some offer features that are useful. I have their "1000"model.
I've been using them in my newest high output AA flashlights. I am very pleased with the lights and the battery performance.