E46 Key fob problem

[phatG60]

I have got a problem with my key fob,sometimes i can arm and disarm the car with the fob but most times it doesn't work,i thought the battery might be flat in the fob but was told it doesn't have one and the key should charge whilst it is in the ignition barrel.Is there any way to check the fob or do i need to leave it in the ignition for a while to see if it charges up?