What it does
The IBS measures battery current, voltage and terminal temperature at the negative post and reports to the engine management over the BSD line (LIN on some models). From that data the power management calculates state of charge and start capability, sets the charging strategy and watches what the car draws while it is parked.
How it fails, and what gets replaced first by mistake
A corroded or dead sensor feeds nonsense into the strategy — the car throws battery-discharge warnings, undercharges or overcharges, and owners chase an overnight drain that is actually bad data. Replacing the battery without registering it makes the picture worse: the management keeps charging for the old battery. The sensor is cheap next to the battery people buy first.
BMW sets the limit for the parked draw at 80 mA: above it the engine computer stores a fault and the car can warn of increased battery discharge. A healthy car reads far lower once asleep — BMW quotes 40 mA for an E90 after 30 minutes, 60 to 70 minutes with a telematics unit, and 7 to 21 mA for an F01 or F10 after 30 minutes — so the energy history is the first record to read in a battery-drain case.
Generations this applies to
From a documented source: BMW training, Power Management.
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