Skip to content

What Is An Auxiliary Battery Used For In Hybrid Electric Vehicles and Electric Vehicles?

What Is An Auxiliary Battery Used For In Hybrid Electric Vehicles (HEVs) and Electric Vehicles (EVs)

Hybrid Electric Vehicles (HEVs) and Electric Vehicles (EVs) use auxiliary batteries to support the operation of the 12-volt electrical system. Instead of utilizing the High Voltage (HV) battery to power these accessories, the auxiliary battery allows systems that require a constant voltage supply to remain powered up. Let’s take a look at the often-overlooked auxiliary battery used by many of these vehicles.

Why The Need?

The big difference between HEV and EV auxiliary battery systems from conventional vehicle systems is the method used to recharge the battery. Instead of using an alternator to charge the auxiliary battery, HEVs and EVs are recharged by the HV battery using the inverter/converter.

Although there are exceptions, many HEVs do not use the 12-volt auxiliary battery for starting the Internal Combustion Engine (ICE). In addition, the auxiliary battery is not used by the traction motor(s) but is charged by the traction battery.

Generally, the auxiliary battery is used to support all 12-volt electrical systems on the vehicle. The exceptions are the air conditioning and heating systems. In most cases, the auxiliary battery supplies power to:

  • Accessory systems
  • Headlights
  • Audio systems
  • Computer controls

An auxiliary battery can be used as a safety backup to support the main battery when required or to provide constant voltage for specific vehicle systems. Many vehicles with Start/Stop and ADAS (Advanced Driver-Assistance Systems) systems may also utilize an auxiliary battery alongside the main vehicle starter battery.

12-Volt Auxiliary Battery Types

Most auxiliary 12-volt systems use an Absorbent Glass Mat (AGM) or a Valve Regulated Lead Acid (VRLA) battery. The auxiliary battery can be installed almost anywhere in the vehicle. Common locations include:

  • In the trunk
  • Under the rear hatch floor
  • Under a seat
  • In the engine compartment
  • Under the floorboard

Because the 12-volt battery is not used to crank the engine and typically only provides 12 volts to run electronics and accessories, it will have a very low energy capacity. In fact, some auxiliary batteries look more like a motorcycle battery than an automotive battery. Because of this low capacity, the battery will discharge quickly if there is a fault associated with the battery’s recharging function.

The first generation Toyota Prius 12-volt system uses an AGM battery located next to the power capacitors in the left side of the trunk.
The first generation Toyota Prius 12-volt system uses an AGM battery located next to the power capacitors in the left side of the trunk.

There Are Always Exceptions

As stated earlier most HEVs do not use the 12-volt auxiliary battery for starting the ICE. But this is not a hard-fast rule. For example, some Honda HEVs start the ICE using the Integrated Motor Assist (IMA) system; however, if the IMA has a fault, the engine starts with a 12-volt starting system utilizing a conventional starter. Cold climates, or low HV battery State Of Charge (SOC), will automatically activate the 12-volt cranking system.

If the ICE is started using the 12-volt auxiliary starting system, the IMA fault may prevent the HV system from recharging the 12-volt auxiliary battery and will lose its SOC very rapidly. The same is true if the ICE is started or cranked by the 12-volt starting system with the HV service disconnect turned off. In either case, a diagnostic trouble code (DTC) will be set and will need to be cleared to reactivate IMA operation.

On Honda HEVs, if the service disconnect is switched to the OFF position, the ICE will still start.
On Honda HEVs, if the service disconnect is switched to the OFF position, the ICE will still start.

Dual Battery Systems

As mentioned, HEVs and EVs are not the only vehicles that may use an auxiliary battery. Some conventional vehicles may use a dual-battery system where the primary battery supplies current to the starter motor while maintaining essential power to the Engine Management System (EMS) necessary for engine starting. The auxiliary battery is used to power additional electrical accessories. The dual-battery system is used by many vehicles that have the Start/Stop function.

Since the accessory electrical components may be sensitive to low-voltage conditions that can cause them to malfunction, the electrical accessory circuit is separated from the starting battery circuit. With the most significant load placed on a battery during the engine starting phase, the voltage of a single battery system may drop low enough to cause electronic devices to power down. This may result in electrical systems shutting down during Start/Stop operation. In addition, the loss of battery voltage may result in the loss of certain computer memory functions. During the engine starting phase, the dual-battery system isolates all power supply to the sensitive electrical components from the primary battery. This is accomplished by opening the Starter Circuit Contact Switch and closing the Accessory Circuit Contact Switch to separate the circuits. During the engine start phase, electrical power is supplied to the sensitive electrical components from the auxiliary battery.

The Mercedes-Benz auxiliary battery is located under the cabin air filter housing while the main battery is in the trunk.
The Mercedes-Benz auxiliary battery is located under the cabin air filter housing while the main battery is in the trunk.

When the engine is started, the alternator supplies voltage to the electrical accessories and the EMS circuits. Primary and auxiliary battery voltages are monitored to ensure the primary battery has sufficient voltage for the next engine starting event and that the auxiliary battery maintains the proper SOC. The opening and closing of the contact switches direct alternator output to the batteries.

If the system detects a low auxiliary battery SOC, the Accessory Circuit Contact will close to enable the auxiliary battery to be charged by the alternator. When the SOC of the auxiliary battery is restored, the contact will open to prevent damage to the auxiliary battery and reduce alternator loads.

In a dual-battery system, two contact switches are used to isolate the power supply into two separate circuits while the engine is being started.
In a dual-battery system, two contact switches are used to isolate the power supply into two separate circuits while the engine is being started.
When the engine is running, the alternator can charge either the primary battery, the auxiliary battery, or both batteries.
When the engine is running, the alternator can charge either the primary battery, the auxiliary battery, or both batteries.

Servicing The Auxiliary Battery

An auxiliary battery malfunction can cause the customer to experience performance issues with the vehicle’s electrical systems. These include the entertainment system, GPS, Blind Spot Assist, Lane Keeping Assist, and Start/Stop functionality. If the control system identifies a fault in the auxiliary battery system, an error message (such as “Auxiliary Battery Malfunction”) may be displayed in the instrument cluster or vehicle information center. Another indicator of a malfunction is the illumination of a small battery symbol. In some cases the message, “Stop Vehicle Leave Engine Running” may also be displayed. This indicates that the auxiliary battery SOC is below the threshold, and the HV system will be used in an attempt to restore it. Any time a fault is detected, a Diagnostic Trouble Code (DTC) is recorded. System diagnostics are DTC driven; however, testing of the battery is very similar to any other 12-volt battery. This includes load testing with a conductance tester.

An issue you are likely to encounter involves accessing the auxiliary battery after its SOC is too low to activate the electrical accessory system. As mentioned, a common location for the auxiliary battery is in the trunk or hatch. If the 12-volt battery discharges, the latch to access the battery cannot be opened. Battery jump studs are provided under the engine hood to connect auxiliary power (a charger or jumper cables). Some vehicles may have a release to open the hatch located in the glove box, by the kick panel, or near the rear seat.

On HEVs with 12-volt starting capabilities, take the opportunity to confirm proper starter operation by using a scan tool to activate the starter a few times. Since the starter is typically not used, it may not be functioning correctly, and the owner will not be aware of the issue.

Like the batteries used in conventional vehicles, auxiliary battery size and capacity selection are determined by the demands placed on the battery by the vehicle’s electrical system. Replacement auxiliary batteries must be of approved Original Equipment Manufacturer (OEM) design and construction.

Replacing the auxiliary battery is usually a straightforward process. When removing the auxiliary battery in a dualbattery system, it is recommended that you disconnect the main battery first. Like any battery, when removing the battery disconnect the ground terminal first. When installing the battery, the ground terminal is the last to be connected.

Charging AGM Batteries

Most auxiliary batteries are AGM batteries. These batteries are used due to their reduced gassing. If the AGM auxiliary battery requires recharging, you need to follow any special charging procedures outlined in the service information.

AGM batteries require a low and clean voltage during charging; thus, they must be charged with voltage regulated chargers. AGM batteries cannot tolerate the quick speed charging that standard high-voltage chargers deliver. AGM batteries need to be supplied with a slow and steady charge over several hours rather than a quick speed charge. Charging voltage must be maintained within 14.1- and 14.4-volts DC. Avoid overcharging the battery. If the AGM battery is overcharged, there is a distinct possibility that it will no longer hold the same amount of a charge and it will have a tendency to discharge quickly.

Tech Tips

In many HEVs and EVs, if the 12-volt auxiliary battery is discharged the vehicle will not start.

  • Auxiliary batteries are also used in emergency and special-use vehicles where additional electrical loads are installed.
  • There is a movement away from lead-acid auxiliary batteries to lithium-ion. Tesla has announced it will make the changeover in its future versions of their S and X models.
  • Failure of the 12-volt auxiliary battery in an HEV or EV can lead to failure of the inverter unit.
  • Honda IMA systems may indicate a low charge rate after the 12-volt battery is disconnected or has a low SOC.
  • Failure to completely latch the HV service disconnect may result in the 12-volt auxiliary battery not charging.
  • The replacement auxiliary battery must be of approved OEM design and construction.
  • A conductance tester must be used to test an AGM battery. Accurate test results can only be obtained using lead adapters since steel bolts will disrupt the conductance frequencies.

Check out all of the batteries available on NAPAOnline or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on auxiliary batteries, chat with a knowledgeable expert at your local NAPA Auto Parts store.

NAPA Auto Parts View All

More than 90 years ago, the National Automotive Parts Association ("NAPA") was created to meet America’s growing need for an effective auto parts distribution system. Today, 91% of do-it-yourself customers recognize the NAPA brand name. We have over 6,000 NAPA AUTO PARTS Stores nationwide serving all 50 states with a unique inventory control system that helps you find the exact part that you need.

Leave a Reply

Your email address will not be published. Required fields are marked *