Close this search box.

Yesterday, Now, Tomorrow of Hybrid Auxiliary Battery

Table of Contents

Auxiliary battery for Hybrid Electric Vehicles (HEVs) and Electric Vehicles (EVs)  supports 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.


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
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.

S34B20R hybrid auxiliary battery
S34B20R hybrid auxiliary battery

What Is an Auxiliary Battery?

The auxiliary battery is typically a secondary battery that keeps certain electrical systems running even when the engine is off. Vans and pickups built with a camper package will have an auxiliary battery to power the lights in the camper.

Likewise, bobtail box trucks equipped with 12 volt lights and accessories in the box part of the truck that are routinely used for extended periods of time will have an auxiliary battery to power those units. The auxiliary battery on these vehicles is isolated from the vehicle’s main electrical system to prevent the primary battery from being drained. However, when the engine is started, a solenoid will close, and the auxiliary battery will be charged as the vehicle is driven.

Some utility vans will have auxiliary batteries to power active anti-theft systems.

All hybrid and electric vehicles (EVs) have a high-voltage battery to provide drive power to the vehicle, but they also have a 12 volt battery to power everything else. The 12 volt battery can be referred to as an auxiliary battery, but it’s just as important as the high voltage battery that powers the motors that drive the vehicle down the highway.

How Does an Auxiliary Battery Work?

Most light vehicle diesels will have two main batteries wired in parallel. Neither of these is considered an auxiliary battery. Some diesels even have 2 alternators that work together to keep the batteries up.

In a conventional system (non-hybrid, non-EV) with a primary and auxiliary battery, the auxiliary battery supplies power to certain electrical components that would kill the primary battery and cause a no-start situation if used for an extended period.

The auxiliary battery on non-EV or hybrid vehicles usually operates in an isolated circuit upon engine startup.

But on hybrids and EVs, the 12 volt battery is charged when the main battery’s contactors are engaged through the DC-to-DC converter. If the 12 volt battery on a hybrid or EV is dead, the contactors that energize the main high voltage battery won’t close.

If the engine computer detects a low voltage output from this battery, one of the contact switches will remain closed while the engine is running. This lets the alternator recharge the auxiliary battery.

Once the auxiliary battery is recharged, a contact switch will open to reduce engine load. This reduces exhaust emissions and optimizes fuel economy.

Yesterday, Now and Tomorrow

Before Model 3, There is no auxiliary battery in CAR.

Model 3/Model Y  uses lead-acid Battery for hybrid auxiliary battery.

Model S/Model X use an all-new lithium-ion 12 V auxiliary battery.

Maybe, for next one, auxiliary and power battery ALL IN ONE.


Brava Battery specialized in VRLA batteries for Solar storage, Golf cart, Caravan etc.

Our excellent sales and technical team help your business find the best ROI rate.


Welcome importers, wholesalers and distributors require free sample

Contact Form Demo (#3)


Contact Form Demo (#3)