Generally the solution to these problems is don't disconnect the battery from the car. In order to change you battery you need to connect a small 12 volt battery to the terminals before you disconnect the car battery. It need not be a large battery since the drain will be minimal and the time short. A set of flashlight batteries works fine. Mercedes are particularily difficult because you loose the radio among other things if you disconnect the battery. You must get a dealer to reset it. This negates anything you save by installing your own battery. Note a battery charger also works.
Battery powered remote control cars are more efficient and economical.
A used keyless entry remote for a Kia might be available at a scrap yard. Cars that are crushed have extra parts that a scrap yard is going to save to sell.
Remote control cars use battery and some cars can run on just battery so, yeah.
you can't change it... you have to bring it to the dealer to get recharged. The above is wrong. One of two systems exist in current M3s. If the car has the Comfort Access option, the battery needs changing when it dies. In cars w/o CA, the "key" fob needs to be inserted into the ignition and trickle charges a different style battery. If if no longer holds a charge, change it out as well.
Yes you can recharge batteries for remote control cars. Some remote control cars come with a rechargeable battery pack, while some you have to buy rechargeable AA's for them.
You will not get one remote to work for both of those cars. If you had two cars that use the same style of keyless entry remotes(also called Key FOBs) you could program the two cars to use the same remote.
Gas operated remote control cars are faster, but require much more maintenance and care.
There are many cars with keyless entry, mine is included (a 2002 ford focus). Keyless entry has been used since around the 90s and have been used ever since. There are still cars without keyless entry but they are all older models.
There are many items that can be charged with a battery. Examples of things that can be charged with a battery include radios, remote control cars, and Sony Walkman devices.
A 7.2 volt battery is used in most remote control cars. They are also called RC cars and some places sell RC batteries for these specific cars. There are most likely other batteries that can also be used.
The battery that starts the engine and runs all the electronics is a rechargeable battery. The only disposable battery may be the one in the remote for the car.
Personally I don't believe its your car's remote that is causing your battery to die out, you should probably try having your battery tested, I've owned 5 cars which all came with the remote entry and I've never had that problem, even with children constantly playing with it.