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EV and Hybrid 12 V Auxiliary Battery Guide

The role of the 12 V auxiliary battery in BEV, HEV and PHEV vehicles, the technologies used, venting for in-cabin and boot installations, and what to check when replacing one.

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Battery electric, hybrid and plug-in hybrid vehicles carry a high-voltage traction battery, but they also still carry a conventional 12 V battery. A weak 12 V battery can leave a hybrid or electric vehicle unable to start, and replacement batteries for these vehicles often have requirements that a standard starter battery does not meet. This guide explains what the auxiliary battery does, which technologies are used and what to check before quoting a replacement.

What the 12 V battery does

Varta (Clarios) describes electrified vehicles as using a dual-battery architecture: the high-voltage lithium-ion battery powers the drivetrain, and the 12 V battery supplies the low-voltage systems and activates essential vehicle electronics. According to Varta, the 12 V battery:

  • when the vehicle is off, starts the car by engaging the connectors for the high-voltage battery, keeps functions such as the alarm system running, powers connected-vehicle functions such as over-the-air updates and manages the charging process;
  • when the vehicle is driving, supplies peak loads that exceed the capability of the DC/DC converter, which converts high-voltage energy into low-voltage power for the 12 V system;
  • supports modern vehicles with more than 50 electronic control units, as well as functions such as lighting, door locking, driver assistance, wipers, power steering and brakes.

Vehicle manufacturers describe the same dependency. Nissan's owner's manual for the 2022 LEAF states that the 12 V battery powers systems such as the audio system, supplemental restraint systems, headlights and windscreen wipers, and that the lithium-ion traction battery also charges the 12 V battery. It adds that if the 12 V battery is discharged, the power switch cannot be moved from the OFF position. Toyota's manual for the current Prius hybrid similarly warns that a discharged auxiliary battery can prevent the hybrid system from starting and may prevent the vehicle from being shifted out of P.

The key difference from a conventional vehicle is how the 12 V battery is charged: in the architecture Varta describes, low-voltage power comes from the high-voltage battery through the DC/DC converter rather than from an engine-driven alternator.

Technologies in use

There is no single auxiliary battery technology. The type is set by the vehicle manufacturer and must be matched on replacement.

Technology Where it is found Source
AGM (valve-regulated) Varta describes AGM as commonly used in hybrid and electric vehicles and markets its Dynamic AGM range for them Varta
VRLA with JIS codes prefixed S Auxiliary batteries in Toyota-type hybrid vehicles, often installed in the cabin or luggage area; the S stands for "sealed" GS Yuasa
EFB Varta notes that EFB batteries are increasingly used as 12 V auxiliary batteries in electric vehicles Varta
Vented flooded The 2022 Nissan LEAF manual describes a 12 V battery with vent caps, the same as in petrol vehicles Nissan
Vented calcium battery with central exhaust Toyota specifies an LN1 European-size calcium battery with a central exhaust for the current Prius hybrid Toyota
Lithium-ion 12 V Used in some models; InsideEVs reported in 2021, from a teardown, that the Tesla Model S Plaid uses a lithium-ion low-voltage battery in place of a lead-acid AGM battery InsideEVs (press report)

Two consequences for distributors:

  • The case size does not identify the technology. An S55B24R and a 55B24R share a case, but GS Yuasa states that the VRLA type cannot be replaced with a conventional battery because the construction differs.
  • A lithium-ion 12 V battery is a vehicle-specific part integrated with the vehicle's electronics. It is not replaced with a lead-acid battery of similar size, and it falls under lithium battery transport rules.

JIS S-prefix codes are explained in JIS battery size codes explained.

Venting for in-cabin and boot installations

Lead-acid batteries can release hydrogen, particularly during charging. When the auxiliary battery sits in the cabin, under a seat or in the luggage area, the gas must not accumulate inside the vehicle.

  • VRLA batteries are closed under normal conditions and release gas through a valve only when internal pressure exceeds a predetermined value, as the European battery standard defines them. GS Yuasa explains that Toyota-type hybrids installed in the cabin or boot widely use VRLA batteries with a structure designed not to leak internally generated gas.
  • Vented batteries with central exhaust route gas through a single outlet to a hose. Toyota's Prius manual instructs that after replacement the exhaust hose and vent plug must be fitted securely to the battery's vent hole, and warns that if they are not, flammable gas can enter the cabin or ignite and explode.

For an order, that means confirming whether the original battery has a vent-hose connection and which side it is on, and supplying a battery with a compatible degassing arrangement.

Replacement considerations

Check Why
Exact technology (AGM, VRLA, EFB, flooded, lithium-ion) Varta states that an installed AGM battery must always be replaced with another AGM battery; GS Yuasa states that S-type VRLA batteries cannot be replaced with conventional batteries
Case size and hold-down Toyota specifies the same case size (LN1 for the current Prius) because a different size will not be secured correctly
Minimum 20-hour capacity and CCA Toyota warns that a lower 20-hour capacity can let the battery run flat even after short periods of non-use, preventing the hybrid system from starting
Venting arrangement Vent hose and plug must connect as on the original
Terminal layout and post type As for any starter battery; see the battery terminal layout guide
Battery registration Varta notes that BMW, Mercedes and Volkswagen group energy management systems require battery registration
Installer competence Varta states that special training is mandatory for 12 V battery replacement in xEVs

Jump-starting and service information

Electrified vehicles often have specific jump-start points and restrictions that installers should know about:

  • Toyota's current Prius has a dedicated jump-start terminal in the fuse box for charging its own auxiliary battery from another vehicle. Toyota states that this terminal cannot be used to jump-start another car, and that connecting booster cables anywhere other than the specified points can harm electronic equipment.
  • Nissan instructs that the negative jumper cable on the LEAF is connected to body ground, not to the 12 V battery, and states that the LEAF cannot be used as a booster vehicle.

Always follow the vehicle manufacturer's procedure for the model concerned.

Common questions

Can a standard flooded battery replace an AGM auxiliary battery if the size matches?

No. Varta's guidance is that AGM must be replaced with AGM, and on in-cabin installations a vented battery without the correct degassing arrangement is also a safety issue.

Why do auxiliary batteries in hybrids fail when the car is parked?

The 12 V battery supports loads such as alarms, locking and connected services while the vehicle is off. Toyota specifically links under-sized replacement batteries to discharge during short periods of non-use.

Do EV auxiliary batteries have their own size codes?

Generally no. They use JIS (including S-prefixed VRLA codes), EN LN sizes or manufacturer part numbers. The technology and venting requirements are what set them apart.

Sourcing auxiliary batteries

Browse EV and hybrid auxiliary batteries and AGM batteries, or use the battery vehicle finder as a starting point. On a request, give vehicle make, model, powertrain (BEV, HEV or PHEV), year, the code on the existing battery, its technology and whether it has a vent hose, then send it through the battery requirement form.

References

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