fake batteries

Ive bought off brand batteries all the time for my 40D and T2i. Never a problem. Just don't go the the very cheapest. Use a known brand like sterling trek or powerex. Amazon sells many for a good price, usually less than half of canon's and the have more power thus last longer.

It is illegal for canon to void a warranty just because you used another battery.
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http://www.bonedoc1.com