Calculation Of The Secondary Battery's Charge Status

The Central electronic module (CEM) monitors the secondary battery's voltage and controls the secondary battery's charge. When the secondary battery requires charging, the Central electronic module (CEM) sends a request to the Engine control module (ECM) stating that the secondary battery must be activated and connected to the vehicle's electrical system. When the alternator charges and the secondary battery is connected, the secondary battery is charged. The Central electronic module (CEM) calculates how long the secondary battery needs to be connected for to be charged.

Every time the ignition is switched on, the Central electronic module (CEM) reads off the voltage from the secondary battery and stores the value. If the vehicle has been parked with the ignition off for longer than 10 hours, the Central electronic module (CEM) uses the new voltage value to calculate how long the secondary battery needs to be charged for. If the vehicle has been parked with the ignition off for less than 10 hours, the Central electronic module (CEM) uses the previously stored voltage value to calculate how the secondary battery needs to be charged for.

The time the secondary battery can be charged varies between 3-3600 seconds. A secondary battery voltage of 12.9 V gives a charging time of about 5 seconds after each auto stop throughout the entire current operating cycle. If the secondary battery has a lower voltage, the load time increases for each auto stop.

Every time autostart is activated, the Central electronic module (CEM) continuously reads the secondary battery's voltage (whilst the secondary battery powers the vehicle). The lowest value that is registered is stored in the Central electronic module (CEM). At the next autostart this sequence is repeated and the Central electronic module (CEM) then calculates the average value of the previously registered voltage value and the last read value. The average value is calculated at each autostart. This average value is the basis for the measurement for SoC and capacity. If the calculated average voltage falls below 10.4 V the automatic start/stop of the engine function is not permitted in the current driving cycle, this is to prevent the secondary battery from discharging too much.

As the next operating cycle starts, the Central electronic module (CEM) counts up the stored value by 0.1 V so that an autostop is permitted. This is to enable the secondary battery's SoC and capacity to be recalculated. If the average voltage drops to 10.4 V again, the automatic start/stop of the engine function is not permitted. If the average voltage, on the other hand, is above 10.4 V, autostop is permitted until further notice. Causes of the average voltage being too low at the end may be due to too high energy consumption in the vehicle during autostart. This may be caused by the vehicle having extra connected accessories that draw too much energy.

HINT: If the vehicle's power is cut, the average value is reset and the Central electronic module (CEM) starts the calculation again with a value of 13.0 V. 

When the engine is on (alternator charging) and the secondary battery is connected to the vehicle's electrical system, the secondary battery's voltage will be the same as the rest of the electrical system's voltage. If the secondary battery is disconnected, the voltage will be lower. When the secondary battery is disconnected, the voltage will slowly drop.

For readable parameters in the Central electronic module (CEM) and Engine control module (ECM), see the relevant description of parameters.

Each time the ignition is turned on the central electronic module (CEM) reads off the voltage of the secondary battery. If it is high enough key start is allowed to use the same start sequence as at auto start.

At each key start, the Central electronic module (CEM) continuously reads the secondary battery's voltage (whilst the secondary battery powers the vehicle). The lowest value that is registered is stored in the Central electronic module (CEM). At the next key start this sequence is repeated and the Central electronic module (CEM) then calculates the average value of the previously registered voltage value and the last read value. The average value is calculated in this way at each key start. This average value is the basis for the measurement for SoC and capacity. If the calculated average voltage falls below 8.0 V at key start the secondary battery is not permitted to be used to key start the engine in future operating cycles. This is to prevent the secondary battery from discharging too much.

When the next driving cycle begins, the central electronic module (CEM) adds up the stored value by 0.1 V so that use of the secondary battery at key start is permitted again. This is to once again be able to calculate the secondary battery state of charge (SOC) and capacity. If the average voltage then drops below 8.0 V again, use of the secondary battery at key start of the engine is not permitted. If the average voltage is above 8.0 V, use of the secondary battery at key start of the engine is permitted until further notice.

Reasons for the average voltage becoming too low after a time is a high energy consumption in the vehicle during key start, repeated starts in cold weather or the car is driven short distances between key starts. This can be caused by the vehicle having additional connected accessories that consume power, or that the secondary battery is not charging enough between the key starts. This especially applies in cold climates where longer charge times are required for the secondary battery to be recharged after each start key. Another reason for low average voltage may be that secondary battery charging is not working as intended e.g. in the event of a relay fault.