Message to all in this thread. If your wireless mics uses AA or AAA batteries get Energizer Ultimate lithium batteries for them. They cost more than twice every other batteries but they are worth it. They will last a long time and will keep their energy a long time when bought new and stored.
Although it may seem wise to buy the most expensive, newest batteries you can find, I buy the store-brand Costco or BJ's Wholesale Club batteries - at around $8 for 48 of them, there's no excuse for not putting fresh batteries in my microphone before every event. I don't need them to last a long time - just until the end of the event.
Recycle them!