A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

Do nickel-cadmium batteries develop "charge memory" if not fully discharged?

March 12, 1999

Dear Cecil:

For years I've heard that nickel-cadmium batteries which are not fully discharged will "develop a memory" upon recharging, and--after a time--if they are only partially used each time before recharging, eventually the full capacity of the battery will not be available. Is this true?

Cecil replies:

It's a myth, bud, though one with a basis in fact. Many years ago in the space program it was discovered that a computer-monitored nicad battery repeatedly discharged to exactly 25 percent capacity did develop "memory"--eventually a quarter of its charge became permanently unavailable. But this kind of thing seldom if ever occurs in earthbound applications. More commonly one sees a condition that mimics true memory, called voltage depression. In some overcharged batteries the available voltage may drop partway through the discharge cycle, spoofing a low-battery monitor (on a laptop computer, say) into indicating that the battery is low. But there's still plenty of good juice left.

Fully discharging a single nicad cell (e.g., a flashlight battery) is usually harmless and may sometimes be desirable. But fully discharging a true nicad battery--that is, a series of cells wired together, which you typically find in your higher-voltage devices--is definitely a bad idea. One cell may become fully discharged, but the remaining cells continue to force current through it, causing permanent damage. So the best advice is, don't fully discharge your nicad batteries, and don't overcharge them either.

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