Auxiliary & Back-up Automotive Batteries Explained

Auxiliary Battery Overviewaux-4

Modern vehicles with CO2 reduction technologies, high levels of specification, and new electronic driver aids may feature an auxiliary battery alongside the main vehicle starter battery or high voltage system battery on Hybrid and electric vehicles. Auxiliary batteries vary in size and specification dependent on the demands placed on it by the vehicle electrical system and can be used as a safety back-up to support the main battery when required or to provide voltage for specific vehicle systems all of the time.

IC Dual Battery Systems

The dual battery system isolates all power supply sensitive electrical components which may be affected by low voltage from the primary battery during the engine starting phase.aux-1

Two contact switches are used to change the power supply into two separate circuits when an engine start is required.

Electrical power is then supplied to the sensitive electrical components from the secondary battery when an engine start is in progress.

The primary battery supplies power to the starter motor and maintains essential power to the Engine Management System (EMS) which is essential for engine starting.

Primary and secondary battery voltages are monitored to ensure sufficient voltage is available for the next start event and charge can be supplied to the secondary battery when required.

IC Dual Battery System Components

System components:Yuasa IC Dual Battery System Diagram

1. Starter motor
2. Primary battery
3. Power & EMS loads
4. Field effect transistor
5. Contact switch 1
6. Contact switch 2
7. Secondary battery
8. Sensitive loads
9. Alternator
IC Dual Battery System Operation

System Conditions – Start RequiredYuasa IC Dual Battery System Diagram

• Contact 1 open
• Contact 2 closed

The primary battey supplies the starter motor and the secondary battery supports power supply sensitive components

System Conditions – Engine RunningYuasa IC Dual Battery System Diagram

• Contact 1 closed
• Contact 2 open

The primary battery is being charged by the alternator and the secondary battery is isolated from the circuit.

If the system detects a low auxiliary battery voltage contact 2 will remain closed once the engine is running to enable the auxiliary battery to be fully charged by the alternator.

Once the auxiliary battery is fully charged contact 2 will open to prevent damage to the axuiliary battery and reduce alternator and consequently engine loads to save fuel and reduce emissions.


Hybrid Vehicle Auxiliary Battery

aux-2Most Hybrid vehicles such as the Toyota Prius feature a conventional 12 Volt auxiliary battery in addition to the high voltage hybrid system battery.

The 12 Volt battery is not used for engine starting or to power the traction motors but is used to supply power to:

  • Accessory systems
  • Headlights
  • Audio systems
  • Computer controls


Electric Vehicle Auxiliary Battery

aux-3Electric vehicles such as the Mitsubishi i-miev feature a conventional 12 Volt auxiliary battery in addition to the high voltage traction battery.

The auxiliary battery is not used by the traction motor but is charged by the traction battery and is used to support all of the electrical systems on the vehicle with the exception of:


  • Air conditioning
  • Heating system