they are different voltages to what they are designed for.
Its always been a pain fitting rechargeables especially if you have a 9V battery required.
If you stick a rechargeable battery in even though it says 9V its actually a 8.4V and it does work for about 4-6 hours and then dies compared to a 9v Duracell at 25-30 hours. A rechargeable has about 200mAh and a lithium about 500mAh
Also I suspect but don't know that if you have them fitted and a charging circuit incorporated the authorities will very quickly require some form of testing to ensure it won't over heat while charging and go one fire aka 787.
Just by them in bulk.
I get mine from
Battery Force. Selling brand name batteries online at cheap discounted prices. Duracell Energizer Ansmann Panasonic Uniross Camcorder Camera Phone Watch Charger Tester Batteries
you can get a bulk pack of Duracell procells AA's for 45p each.
Duracell Procell AA/LR6 Battery Box of 10 at Battery Force
Duracell Procell AA/LR6 Battery Pack of 30 at Battery Force
for 40p
After your first order they give you a discount code and you get 5% off that price as well.
oh and get one of these as well so they don't short circuit in your bag.
Ansmann Battery Box 8 Plus at Battery Force
Never had any problem with security having a battery box with 4 9v in it. Which is about 100 hours flying for me. Saves have yet another gadget to haul around as well for charging.
Don't muck about with getting cheap ones either I always just go for Duracells or JCB, check the packaging as well as some of the industrial pack the cells come with protection caps fitted or individually shrinked wraped which will save the day if your bag gets soaked but the box does that.