I have a 12 year old 5 Series BMW. It is giving me a warning to replace the key fob battery. This is the second time in the last two months and strangely both mine and my wife's fobs have the same warning within a few days. The original battery from when I bought the car new lasted 12 years, this problem has only just happened in the last couple of months. The fob battery is a CR2450 lithium and is rated at 3 volts. I checked the battery which is in the fob with my multimeter and it is giving 3.03 volts the one in my wife's fob is showing 3.04 volts. I checked a new battery straight out of the packet and it gives 3.32 volts (notwithstanding it says 3v on the packaging). The fob itself functions perfectly. It opens and locks the doors without hesitation and it starts the car fine. I've looked on Google and various BMW forums but mostly they are full of the usual amateur rubbish and none of them offers a convincing proper explanation.
Does anyone on here have any insight into what's going on? Why should it originally last 12 years without a peep and now it is warning of a battery shortfall within weeks of a fitting a new battery which is still showing full voltage on a volt meter?