Canon Speedlite 90EX - Manual (Page 8)

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 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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