For all of you, but for event photographers in particular, here’s your tip of the day: get one of these:
For about $10, you can get a battery tester like this. It is not a regular voltmeter; rather, it is a meter that tests batteries on load, with a load appropriate to the battery type.
And before each shoot, test your batteries: the ones in the flashes, and the ones in ancillary equipment like pocketwizards, light meter, and so on.
This way you avoid unnecessary changes while ensuring that you never run out mid shoot. There are few things as embarrassing. Ask me.