Intelligent battery sensor and battery

Brief component description

The following components for voltage supply are described:
  • Battery
  • Intelligent battery sensor

Battery

The capacity of the built-in battery depends on the engine used and the vehicle equipment. Selection criteria for the required capacity are:

  • the cold-start behaviour of the engine
  • the standby current consumption of the vehicle
  • the energy requirement of the auxiliary consumer units (auxiliary heater, telephone, etc.)

Usually one AGM battery is installed (identifiable on the black housing for batteries installed ex works). In other cases (vehicles without intelligent alternator control and automatic engine start-stop function in some countries), a normal lead-acid battery is fitted.

The AGM battery primarily offers the advantage of greater rechargeability.

IBS: Intelligent battery sensor

The IBS is a mechatronic, intelligent battery sensor with its own microcontroller. The microcontroller is an element of the electronics module. The electronics module records the voltage, the current flow and the temperature of the battery. The following components are fitted in the electronics module:

  • a precision resistor (resistor for current measurement)
  • a temperature sensor
  • evaluation electronics on a printed circuit board

The IBS continuously measures the following values on the battery:

  • Terminal voltage
  • Charge current
  • Discharge current
  • Battery temperature

For data transfer, the IBS is connected to the DME (Digital Engine Electronics) or DDE (Digital Diesel Electronics) via the BSD (bit-serial data interface).

Item Explanation Item Explanation
1 Measurement of the battery voltage between the positive battery terminal and negative battery terminal 2 Temperature measurement of the battery (T)
3 Microcontroller (μC) in the intelligent battery sensor (IBS) 4 Digital Engine Electronics (DME) or Digital Diesel Electronics (DDE)
5 Current measurement (A) [indirect, via the proportionally voltage drop (V) at the precision resistor (shunt)] 6 Negative battery terminal
7 Positive battery terminal
BSD Bit-serial data interface (BSD) for transfer of the values to the DME or DDE

System functions

The following system functions are described for power management (”Advanced Power Management”):
  • Determining the state of charge
  • Determining the startability limit
  • Determining the battery condition

Battery state of charge

The APM with the intelligent battery sensor determines the state of charge during driving and when the vehicle is at a standstill on the basis of measured data:

  • Driving:

    • Balancing the charge and discharge current of the battery.
    • Calculation of the current characteristics on engine start to ascertain the battery condition.

    While the vehicle is being driven, the IBS transfers the data across the bit-serial data interface (BSD) to the engine control unit (DME/DDE). The software in the IBS controls the communication with the higher-level engine control unit (DME/DDE).

  • Vehicle standstill

    When the vehicle is at a standstill, the measured values (open-circuit voltage measurement) are queried in cycles to detect energy losses. The measured values are entered in the IBS in the memory and transferred to the DME/DDE after restarting the engine.

For the history of the state of charge, the following values are stored in the DME/DDE:

  • Battery state of charge of the last 5 days.
  • Charging status histogram showing periods in the ranges 0 - 20 %, 20 - 40 %, 40 - 60 %, 60 - 80 % and 80 - 100 %. The charging status histogram is reset in the following cases: programming the DME/DDE or registering a battery replacement.

Notice! Evaluation of the state of charge

The battery voltage measured by the IBS approaches the actual open-circuit voltage slowly after the vehicle has "gone asleep". The accuracy of the measured value increases with the duration of the rest phase: The measured state of charge is reliable after a rest phase of at least 3 hours. However, an insufficient rest phase or standby current violation may result in the state of charge not being properly determined: the state of charge is implausible.

Startability limit

The APM calculates a lower and an upper startability limit for the battery:

  • The lower startability limit corresponds to the minimum charge state of the battery so that the vehicle can still be started.
  • To counteract discharge down to the lower startability limit, a certain charge volume is kept as a reserve. To achieve this, the upper startability limit is calculated.

The startability limit is calculated by evaluating the following measured variables:

  • Average ambient temperature with vehicle parked.
  • Ambient temperature of the last journey.
  • Current state of charge.
  • Voltage dip of the last engine start (trend for ageing of the battery).

The startability limit is therefore dependent on the ambient temperature: The colder the ambient temperature, the more energy required for the engine start. Therefore the startability limit is higher for cold ambient temperatures:

  • Startability limit is approx. 30% state of charge at 15 °C.
  • Startability limit is approx. 50% state of charge at -15 °C.

Battery condition

The battery condition cannot be determined based on the battery state of charge alone. All batteries are subject to natural wear due to the natural ageing process. The chemical reactions in the battery, consisting of the charging cycles with battery charging and discharging, mean that deposits form in the battery, preventing the battery from reaching full capacity.

Item Explanation Item Explanation
1 Charge battery. 2 Discharge battery.
3 Ageing / deposits 4 Self-discharging.

Each total discharge results in a loss of battery capacity: The longer the battery remains completely discharged, the greater the loss of battery capacity. The batteries installed at BMW and MINI can withstand several short total discharges or up to two long total discharges, however, when they are fully recharged with a constant charging voltage of 14.8 V after the total discharge.

The diagnosis system determines the battery condition using the following criteria:

  • Ageing: The energy flow rate (cumulative discharge) and the voltage dip evaluation from recent engine starts are evaluated as the measured variable.
  • Damage due to total discharges or operation at a low state of charge: fault code entry and the time in a state of charge below 20 % are evaluated as the measured variable.

Notice! Fault entry: Battery very aged or defective

If monitoring of the battery condition identifies an aged or defective battery, a fault entry is registered in the engine control unit. The fault entry can only be deleted once the batterie is replaced and the battery replacement was registered.

Notes for Service department

General notes

Notice! Charging and trickle charging of the battery

The battery may only be recharged with a battery charger that has been approved by BMW and that has a constant charging voltage of 14.8V.

If possible, the battery temperature should be between 15 °C and 25 °C during charging. Under these preconditions, the battery is adequately charged when the charge current drops below 2.5 amps.

If the battery is recharged at low temperatures, the charging procedure should not be ended until the charge current drops below 1.5 A.

On vehicles with IBS, recharging the battery directly at the battery terminals could lead to a misinterpretation of the battery condition and even unwanted Check Control messages or fault entries.

If the battery is recharged while it is installed, it must be recharged using the jump start terminal points, whenever jump start terminal points are provided in the engine compartment. Only then can you be sure that charging is correctly recognised by the vehicle electronics on vehicles with an intelligent battery sensor (IBS).

Except for MINI from R55 model: These vehicles do not have any jump start terminal points in the engine compartment. On these vehicles, the positive terminal of the battery charger must be directly connected to the battery. On vehicles with a petrol engine, the negative terminal can be connected to the lifting eye (suspension lug) on the transmission and on vehicles with diesel engines to a flange on the engine bearing block on the engine.

Notice! Trickle charging of the battery for stored or stationary vehicles

In the case of stored or stationary vehicles the battery must be recharged at regular intervals in order to avoid a total discharge and thus damage. See the following document: Battery recharge interval for booth vehicles.

Notice! Determining the state of charge after replacement of the IBS or the battery

After replacement of the battery and registration of the battery replacement, or after replacement of the intelligent battery sensor (IBS), the vehicle must be in rest state for at least 3 hours: Only then can the new state of charge be determined by open-circuit voltage measurement.

Diagnosis instructions

Notice! Register battery replacement

After installing a new battery, the service function 'Register battery replacement' should be run. The battery replacement has to be registered in order to tell the power management system that a new battery has been installed in the vehicle. If the battery replacement is not registered, the power management will not function properly, with the result that Check Control messages may be displayed and functions limited by individual electrical consumers being switched off or having their power consumption reduced, for example.

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