Can I assume that it was pretty dark outside? If so, then it's possible that your flash was firing at close to full power on each shot and possibly also pre-flash to assist focusing - great way to eat batteries. When used primarily as fill flash you should get nearly 3 to 4 times the number of fires on a fully charged set of batteries. The batteries may have been part of the issue too - not all NiMH rechargeables are created equal (were they new?) - check out the link below for a great demonstration on the differences between different batteries and some of the technical factors to consider.


http://photography-on-the.net/forum/showthread.php?t=677074


I've gone with the Powerex 2700 mAh from Thomas Distributing after reading this article. Although Eneloops are great too (I may have gone with them, but the only place that I knew carried them was Amazon.com - which won't ship to Canada, and Amazon.ca doesn't carry them).