AA Batteries

I've done a little research on AA batteries . . .

First of all, for small rechargeable batteries, it's best to stay away from NiCad.
I'll ignore both NiCad and the newer Alkaline high current batteries.

NiMH rechargeable and standard Alkaline batteries have not changed much since they were introduced.

The NiMH batteries are best for high current applications where you go through a bunch of batteries quickly (hours or days, not weeks or months), like a digital camera or a motor driven toy. The standard alkalines are best for low current, low use applications like a radio or a flashlight that doesn't get used every day.

One reason for the above is self-discharge. A fully charged unused NiMH battery will self discharge to 75-80% in 30 days at 68 degrees. To 20-40% in 30 days at 113 degrees!! An alkaline battery will still be at 90% in 5 years. A NiCad self-discharges 10% in the first 24 hours, then about 10% per month thereafter.

The other reason is that an alkaline battery will supply more power at lower current rates. They are good for about 2000 milliamp-hours when you are drawing 100mA or less. But only good for 1250 mAh when drawing 500mA. The NiMH batteries will supply roughly the same power no matter what the current, within reason of course. You can get 1300mAh or 2300mAh NiMH batteries.

Another interesting bit of information: the voltage from an alkaline battery goes down as it gets discharged. They start at about 1.5 volts and end up at about 1 volt. With 1.25 volts being ROUGHLY the half-discharged point. So it looks like a voltmeter is a good test for the usability of alkaline batteries. At least that's what I've found on the Internet.

Energizer's battery tester checks alkaline batteries as follows...
D, C, AA, AAA 1.5V for general use: 1.1 Volt min with a 10 ohm load.
9V for general use: 6.6 volts min with a 250 ohm load.

NiMH battery voltage stays pretty much constant at 1.2 volts until it gets almost discharged, then drops rapidly.

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