Keep em charged, all the time!
The 4-pack of Panasonic AAA eneloop 2100 Cycle rechargeable batteries with charger is down to $18.76 on Amazon. This pack normally sells for around $23 to $25, and this is the lowest price we've ever seen.
Each battery has 800mAh and a 2100 cycle, which means they can be recharged up to 2100 times. They will maintain their charge for a long time (up to 10 years) when not in use. This pack comes with a charger that can fit up to four batteries and works with both AA and AAA batteries. It will automatically turn off when the batteries are fully charged. Users give these batteries 4.7 stars based on 2,883 reviews.