If you use wireless mics, then you probably go through a lot of batteries. Here are some tips to both save money and be sure you never have a mic die in the middle of a performance / service.
Before every performance / service every mic should have NEW batteries! A new battery should last 4 to 6 hours, so that should be good for the whole day if there is multiple performances / services. Make sure you use Alkaline batteries. I have found that the Lowes brand Utilitech AA batteries are every bit as good as the name brands and they cost $22.97 for 100!
As for that pile of half used batteries, they can be used for rehearsals where failure is an option, or they are also great for flashlights, kids toys, remote controls etc. Wondering how much life is left in those batteries? I use the ZTS Mini Multi-Battery Tester. It costs about $45.
Dont forget to recycle the old batteries!
For More: http://www.infocomm.org/cps/rde/xchg/SID-FCE9BA57-603B5A6D/infocomm/hs.xsl/31931.htm