Battery (Main Battery) And Secondary Battery

Courtesy of VOLVO CARS CORPORATION

In addition to the regular battery (main battery), the vehicle also has a secondary battery. The task of the secondary battery is to supply the main part of the vehicle's power consumers during autostart when the starter motor is running.

Courtesy of VOLVO CARS CORPORATION

Both the battery (main battery) (6) and secondary battery (1) have their own master switch relay: a battery relay (main battery) (4) and a secondary battery relay (2). During autostart, the electrical system reconnects via these two relays so that the battery (main battery) only powers the starter motor's circuit and the secondary battery powers the rest of the vehicle's power consumers. The Engine Control Module (ECM) controls both the relays.

During the start sequence when the starter motor starts and cranks the engine, the starter motor consumes a high current. This quickly (in about 300 ms) creates a great drop in voltage in the electrical system. This voltage drop can cause, for example, control modules to restart (reset) and lamps to flash. To prevent the low voltage travelling to the vehicle's other electrical system, the secondary battery engages during the start sequence. This is the only occasion when the secondary battery is used in the vehicle. The image below gives an example of this short sequence.

Courtesy of VOLVO CARS CORPORATION

1. The vehicle's voltage has battery voltage. The starter motor starts and the voltage in the electrical system drops dramatically.
2. The starter motor has started and is working. Short voltage pulses occur, one for each compression stroke. The engine starts and the starter motor's power demand reduces thereby reducing the voltage drop.
3. Engine started.
4. Displays the period when the secondary battery is connected and powers the vehicle.

Both batteries are of the AGM type. See appropriate Design and Function information for the Battery.

Courtesy of VOLVO CARS CORPORATION

The secondary battery is fused by two 70 A fuses; one in the integrated relay/fusebox (1) and one by the secondary battery (2). As the cable between the secondary battery and the integrated relay/fusebox can be supplied from the battery (main battery) (connected secondary battery relay) and from the secondary battery, there are two fuses to prevent damage or fire in the event of a collision.