I was tempted to put this is the sound engineer section, but thought it might be useful for a wider range of people than that - after all, almost all of us will have had to use radio mics at some stage.
When we moved churches a couple of years back our new church was using alkaline 9V batteries to power radio mics, and these were often failing mid-meeting. The obvious thing was to switch to rechargeables, so I bought a bunch of the highest capacity I could find (280mAh). They were OK, in that the batteries (mostly) didn't run out, but for professional quality mics (SM58s) they seemed to be poor with feedback rejection and sounded a bit muddy.
Fast forward 2 years, and I've just replaced the batteries again as they were coming to the end of their lives. This time I bought a lower capacity but higher voltage: 9.6V 220mAh vs the old 8.4V 280mAh.
The difference is really quite surprising. Mics have much more clarity, feedback rejection is vastly better and it all sounds much crisper. A lot of electronic equipment requires a certain minimum voltage to work well, and the lower voltage batteries weren't really delivering enough, especially as they aged.
So my suggestion is that if you need 9v batteries for radio mics, look for a higher voltage rather than higher capacity.
That is very interesting. I have always hated wireless mics because of poor reception and part about batteries dying so soon...and the cost of replacements...(that is why we don't have any wireless device). Perhaps that was my problem.
I can't give specifics, but I know that many years ago, I discovered that for guitar effects, the "cheap" 9v batteries actually worked better (gave a better, clearer sound) than the alkalines... for a day or so. And my understanding of that was that the cheap batteries were charged up to 10v or so, so that they would last a while before falling below the "useless" voltage. And since, even back then, I would constantly forget to unplug the guitar from the effect box, causing the battery to run down that same day anyway, I decided that the cheap batteries were the way to go.
Currently, for our wireless mics, we have found that we have to use the Duracell brand, none of the other alkaline batteries would work as well. I wonder if that's voltage related. The system we've switched to since the 700 mhz ban (which runs in the 180 mhz range or something close) has not been working very well - a lot of static and such. I wonder what would happen if we used a mighty Ray-O-Vac non alkaline and just replaced it every week (or every service!). We've been having a lot of static on the wireless lately, especially when the pastor is standing on the opposite side of the mostly-metal communion table from the receiver... hmm, I feel an experiment coming on!
My own past experience with rechargable batteries has not been happy, but quite likely we were using the lower voltage ones... so if it seems like a higher voltage will help, maybe it'll be time to look at what's available again...
I did a quick google search for "9.6V rechargeable pp3" and got a lot of hits on different brands in the UK. I'd suggest you do the same locally, since these days it's harder to google for information in other countries. I'm paying about the same as for an 'ordinary' 9 volt rechargeable, but a couple of vendors were charging about double, so watch out for that.
Please can you say what make and model your radio mics are.
In general, most radio mic manufacturers don't recommend using rechargables, (1) because their capacity is lower - an alkaline PP3 is typically 550-600 mAH, 2-3 times the figures you quoted and (2) as you say the voltage isn't good - historically, most rechargeable PP3s had 7 x 1.2V cells giving 8.4v nominal output as opposed to 8 giving 9.6V.
Most radio mics that take PP3s quote an 8 hour battery life with a decent alkaline battery. I've found this to be about right. If yours were failing in mid-service, something is badly wrong.
In my experience, I've found that the slightly lower voltage 9V miAh (no lower than 8.4v) with higher capacity -(325 mAh) worked longer than a 9.6 with lower capacity. As far as clarity, I've never paid too much attention since the quality of the system in our church is such where some of those subtle difference wouldn't be noticeable.