Battery sensor / battery management (IBS, BEM)
Definition
On vehicles with start-stop and smart charging, a battery sensor clamped on the negative terminal measures current, voltage and temperature and reports state of charge and state of health to the energy management module over LIN. The system decides when start-stop is allowed, how hard the alternator charges (including recuperation on deceleration), which comfort loads to shed when the battery is low, and it tracks the battery's ageing so charging voltage can be raised as it gets older. When a new battery is fitted it must be registered (BMW, Mercedes) or coded with its BEM number and capacity (VAG), otherwise the module keeps using the old ageing data and under- or over-charges the new battery, start-stop stays disabled and battery life is shortened. The sensor itself fails from corrosion, a damaged LIN wire or a bad clamp, causing a dead battery, loss of start-stop and warnings about electrical consumers being switched off. Codes: manufacturer-specific such as BMW 2E8x/A6CF (IBS), VAG 01325/B2001/P0A1F, Mercedes B21xx, and generic U0111 (lost communication with battery energy control module), P0560/P0562.