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Message: <blockquote data-quote="nickt" data-source="post: 547439" data-attributes="member: 4923">I have one 9v rechargeable battery, radio shack, I think. I've had it for years and it keeps going. I forget what tool I have it in, it moves around. I have a few tools (meters, laser levels, etc) that I might only use once a year, but when I do, I'll kill the battery in a weekend. For smoke alarms, I would go with a normal alkaline battery, never a rechargeable. There are fancy lithium 9v batteries now. Not needed for most things, but there are some devices that would benefit from a more powerful 9v battery. Portable radios are worth running on a rechargeable 9v if you use them often. I'm a radio guy, I need news talk on if I am near the house, but I've moved to radios with AA batteries and use eneloops. Same deal with 9v rechargeable, some have more capacity. I keep one old charger around that can charge a 9v battery. The 9v regular and rechargeable are relatively low capacity. you'll find them from 250mah to maybe 800mah capacity. A regular disposable 9v alkaline is probably only 500mah. There are 6 or more tiny cells in both the rechargeable and regular 9v batteries. The voltages of those little cells add, but not the mah capacity of the cells. What is inside compares to low quality AAA cells. Not great batteries, but we are stuck with them. Much more power in AA.</blockquote>