Why do batteries have a limited number of charges?

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Ellingson, Kenneth L

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Ellingson, Kenneth L  asked the Naked Scientists:

Chris

Why do batteries have a limited number of charges?

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Why does batteries performance decline over time?

Knute Ellingson, Chicago

What do you think?

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Offline Pumblechook

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Why do batteries have a limited number of charges?
« Reply #1 on: 13/10/2008 23:26:33 »
All to the do with the chemistry.

http://www.batteryuniversity.com/

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Offline techmind

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Why do batteries have a limited number of charges?
« Reply #2 on: 14/10/2008 22:59:50 »
The most common way of sending a rechargeable battery to an early grave is by overcharging it. In the case of NiCd and NiMH the battery vents hydrogen gas when overcharged, so it's loosing a chemical with which it stores energy. Modern "fast" chargers (anything which charges in 3-4 hours or less) sense an abrupt rise in temperature and/or a change in voltage on the cell to detect end-of-charge. Occasionally this sensing glitches which can result in overcharge.

I believe serious overcharging of Li-Ion/Li-polymer can lead to fire, but I'm less familiar with the mechanism (yeild Li metal?).
"It has been said that the primary function of schools is to impart enough facts to make children stop asking questions. Some, with whom the schools do not succeed, become scientists." - Schmidt-Nielsen "Memoirs of a curious scientist"