Canon Speedlite 90EX Instruction Manual page 8

Be sure to use a new set of two batteries of the same brand. When replacing the

batteries, replace both batteries at once.
Do not fire more than 20 continuous flashes in a short time.

If you change the batteries after firing flashes continuously, be aware that the batteries

might be hot.
Using size-AAA/LR03 batteries other than the alkaline type may cause improper battery

contact due to the irregular shape of the battery contacts.
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