Digital Battery Checker

I've seen these for years but this iteration is the first I've come across that works with coin cells too.

If I were a kid I'd be so making the rounds of my 'hood with mine, offering to test people's batteries for a nickel a pop — cheap at twice the price in misery when the power goes out and your flashlight doesn't work.

From the website:

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Quickly and easily test your batteries... works with AAA, AA, C, D, and 1.5V coin cells, too.

Our compact battery checker uses a color-coded liquid crystal display to accurately display your battery's status as "good," "weak," or "poor."

Stop guessing if your batteries are good... check them. 

Compatible with NiCad and NiMH rechargeable batteries.

Requires 1 AAA battery (included). 

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$11.49.