We did a lot of testing with rechargeable batteries when I was in College. We tested a bunch of different brands and chargers. We put them in mics in different noise
level environments to test how long they lasted, and all that. The conclusions that we came to were basically like this:
9V rechargeables were not reliable enough to use in wireless mics. They don't come in high enough capacities. We did find that they were reliable enough for the older wireless
Clear-Com that we used, so they went to that, and all the wireless mics that used 9V batters still got alkaline.
For mics using AA batteries we found that with a good brand, high capacity rechargeable we got great performance. We certainly had enough usage time to do a
sound check and then deploy the mics at half hour. If I remember correctly we got an average of at least 4 hours on the 2700mAh batteries.
The real kicker was figuring out how to deploy rechargeable batteries. We knew we needed the batteries and chargers, and all that jazz. We settled on needing to have a minimum of three sets of batteries. This meant we could have one set in the mics, one set as a backup and one set on charge. It is a little
bit of an organization hassle for the techs, but after a while it becomes routine. It is imperative to have everything clearly labeled. You need to have good chargers, they don't have to be super fancy, but it is good to have chargers that condition the batteries every so often. Then of course you have to have space to set up all the chargers and store the batteries. Also, make sure you have some regular alkaline batteries sitting
standby for when something goes wrong.
It is a great way to go, and in the end it will save you money, but deploying the
system costs as much as a years worth of batteries.