What does it mean?
Vehicles that carry more than one battery, such as stop-start systems, mild hybrids and dual battery installations, monitor each battery with a sensor that measures voltage, current and temperature and calculates a state of charge from them. This code is stored when battery B does not reach or hold the state of charge the module expects even though the charging system has had the chance to replenish it. It is a judgement made over time rather than an instant voltage reading, so it usually means the battery is failing, the charging path to it is weak, or the calculation itself is being fed bad data. While the code is active many vehicles disable stop-start and reduce non essential electrical loads.
Symptoms
- Stop-start no longer working, often with a message on the display
- Slow cranking, or a click without cranking, after the vehicle has stood
- Battery or charging warning lamp alongside the MIL (check engine lamp)
- Accessories resetting, dim lighting at idle and a system voltage that swings more than usual
- Auxiliary systems such as heating or the audio unit shutting down early
- The problem being worst in cold weather or after several short journeys
Likely causes
- Battery B aged, sulphated or with a failing cell so it will not accept or hold charge
- High resistance in the cables, fuses or ground connections feeding battery B, including corroded terminals
- Faulty battery current sensor or a data fault in its communication, so state of charge is calculated wrongly
- Charging fault such as a weak alternator, a failed converter or a battery isolation relay that does not close
- Excessive parasitic drain discharging the battery while the vehicle is parked
- A replacement battery that was never registered to the vehicle, so the charging strategy does not match its type or capacity
- A usage pattern of very short trips that never allows a full recharge
How to fix it
- Read the state of charge, current and temperature values reported for battery B on a scan tool, then compare them with a measured open circuit voltage
- Fully recharge battery B and perform a proper load or conductance test rather than judging it by voltage alone
- Inspect and clean all terminals, ground straps and the fuses or fusible links in the battery B circuit, then measure voltage drop under load
- Measure charging voltage at battery B with the engine running and loads switched on, and confirm any isolation relay or converter operates
- Check parasitic drain after the modules go to sleep and trace any circuit that stays awake
- Register or code a replacement battery where the system requires it, matching capacity and technology
- Replace the battery sensor only after the battery, cables and charging path have all been proven good