[Starscream]

correct,
It has a built in "microchip" (oooo buzzword) according to BMW that monitors the voltage and maintains and if needed charges your battery through the cigarette lighter and keeps your cars very expensive (for us M guys) battery charged,

The price of a new battery from BMW is staggering, and I don't drive the car a lot and I love this little thing,

Also the car felt peppier after a charge too,
I know I have drained the battery recently just letting the car sit, so the alternator was on double time!

I would recommend this to anyone who stores the car or doesn't drive in bad weather (like me)