What Is The Purpose Of The 12-Volt Battery In An EV?
Whilst the high-voltage battery pack supplies energy to the vehicle’s drive motor(s), a 12V lead-acid battery supplies energy to most other electrical “consumers” in the car – typically many of the same things it powers in a gasoline or diesel car, e.g., door locks, sound systems, fans, etc.
An electric vehicle, also known as an EV, is propelled by one or more electric motors that are powered by a dedicated EV battery pack, whereas a vehicle with an internal combustion engine (ICE) requires gasoline or diesel fuel for propulsion. Still, these two types of vehicles have many features in common – power windows, driver-assist capabilities, cameras, infotainment systems, telematics, etc. — that operate with the assistance of a 12-volt battery.
This is why your EV probably houses a regular 12-volt battery like every other car on the road. But why do EVs, particularly battery electric vehicles (BEVs) and plug-in hybrid electric vehicles (PHEVs), which can drive on electric power alone, still need this 12-volt battery? Why not use the “big” battery that propels the car? Let’s take a closer look.
Why do EVs have a 12V battery, and what does it do?
In an EV, there are two distinct electrical systems:
1. A high-voltage (generally in the 400 to 900-volt range) battery pack, typically made of lithium-ion, that supplies energy to the vehicle’s drive motor(s).
2. A 12V lead-acid battery that supplies energy to most other electrical “consumers” in the car.
The 12-volt battery typically powers many of the same things it powers in a gasoline or diesel car. These may include:
● Door locks
● Remote entry and alarm/immobilizer sensors
● Power liftgates
● Windows
● Power seat and steering column adjusters
● Driver-assist features
● Cameras and lidar
● The electrical control units and “computers”
● Infotainment and telematics
● Sound systems
● Fans
● Electric defrosters in mirrors
However, in an EV, the 12-volt battery also controls all the relays that power the electronics that essentially “start” the high-voltage electrical system, and — perhaps more importantly — disconnect the high-voltage system in the event of a collision. Hybrids and plug-in hybrids require a 12-volt battery for the same reasons.
An EV’s 12-volt battery is typically found somewhere under the hood, rear seat, or trunk. (If you’re poking around your hybrid or EV looking for this battery, consult your Owner’s Manual and avoid the wires — usually orange — and looms that carry the high-voltage panel). A hybrid or EV may also package the low-voltage alongside the high-voltage battery pack for temperature control and/or to keep the weight down low.
The 12-volt battery may be a typical flooded lead-acid design or an absorbed glass mat (AGM) battery, the latter of which is more common in electric vehicles and vehicles with automatic engine start/stop. Alternatively, the 12-volt battery might be made with lithium-ion, which is prized for being lightweight and having high power density, making it a popular option in high-performance road cars.
Why can’t an EV’s high-voltage battery replace the 12V battery?
If EVs have a big, high-voltage battery pack, why can’t that be used for all the 12-volt functions? The short and simple answers are safety and legacy.
The 12-volt electrical system for automobiles may not be perfected, but it’s very close. It’s also extremely well-established. Many electrical functions could indeed be run from energy provided by the high-voltage battery pack, but there is something to be said for the “if it ain’t broke, don’t fix it" approach. Revamping a vehicle’s 12-volt architecture would be a substantial undertaking and, as of right now, those resources are being applied elsewhere.
The other aspect in consideration here is safety. High-voltage batteries can, as their name implies, deliver quite a shock. Using low-voltage electronics to control high-voltage systems provides an extra layer of safety. Imagine, for example, needing to jump-start an 800-volt battery in the rain, or finding the seat control switch has lost some insulation, and taking a 400-volt shock versus a 12-volt wakeup. For additional context, in cars that use high-voltage hybrids or EVs, the driver must be told when it is safe to exit the car when something goes wrong, and the response crews must also be careful when they approach and work on the vehicle.
How does an EV charge the 12V battery?
With no engine-driven alternator, an EV uses an onboard inverter or converter to take energy from the high-voltage propulsion battery pack and change it to 12 volts, charging the low-voltage battery. This process is generally done using “trickle charging”— very low amperage charging for longer 12-volt battery life, much like a battery “tender” or “maintainer” you may see on a boat, snowmobile, or RV battery. On most EVs, when they are plugged into a charger (of any level) the 12-volt battery is automatically topped up as well.
Hybrids and plug-ins may use a similar approach, or they may use a component —often called a starter/motor/generator — that serves three functions at different times:
- Starting the gas engine when needed
- Serving as a motor to provide added or sole propulsion
- Generating electricity as the vehicle coasts, slows, or while it’s cruising, and the gas engine is making more power than needed just to drive the wheels
All the above are controlled automatically by the car and its low-voltage electronics, though some PHEVs allow the driver to choose different operating modes.
What happens if a 12V battery dies in an electric car?
No battery will last forever — and EVs require maintenance, just like any other vehicle. But what do you do if your 12V battery dies at home or while you’re out on the road? Unfortunately, you can’t just keep driving. Namely, you can’t start your EV’s propulsion system without a working 12V battery because the inverter responsible for converting the battery’s direct current (DC) power to alternating current (AC) power relies on the 12-volt system.
When this happens, there are a few options available to you, although as always, we recommend consulting your Owner’s Manual for best practices for your vehicle.
If you happened to be using your EV when your 12V battery died, you can try to jump-start it using a portable jump starter, another vehicle, or a different 12V power source. Just remember to be careful and take all necessary safety precautions. Even though you’re working with the 12V battery, the high-voltage battery pack introduces a new level of risk.
Some EVs have a driver-activated switch that allows you to “reset” the low-voltage battery by “jump-starting” it from the high-voltage battery pack using the power electronics. This is often enough to get you moving to a service facility.
If your 12V battery dies and you can’t jump-start or recharge it, you’ll need to get a replacement battery. In most cases, this means getting your vehicle to a repair shop or service center and letting a professional handle the rest. However, if you’re very familiar with batteries and have the appropriate tools, you may be able to replace the vehicle’s 12V battery on your own.
What happens if a 12V battery dies in a hybrid?
Many hybrid vehicles rely on 12V batteries to start the engine, and some use the 12-volt system for electric steering and brake assists. Even if your high-voltage battery pack is completely charged, your vehicle won’t operate when the 12V battery is dead.
On some EVs and hybrids, the charging connector is secured with an electric signal. If this is the case for your vehicle and your low-voltage battery isn’t charged during vehicle charging, check the Owner’s Manual to see if there are instructions on how to manually release the cable. In some cases, when you have to jump-start an EV or a hybrid, the car will begin a countdown clock, allowing only so many restarts before you have to replace the 12-volt battery and reset the car.
Explore the Mazda CX-70 and CX-90 PHEV
The Mazda Plug-in Hybrid vehicles (PHEVs) offer the best of both worlds. Instead of relying solely on electric power or fuel, PHEVs run on batteries to power the electric motor and fuel to power the internal combustion engine. These vehicles are capable of charging the high-voltage EV battery pack via a plug-in charger, automatically during driving via regenerative braking, or with the engine acting as an onboard generator.
The Mazda CX-70 PHEV and CX-90 PHEV are PHEVs with a powerful electric motor and gasoline engine, allowing for uninterrupted power wherever the road takes you. Not only are they crafted with the finest materials and with comfort in mind, but they’re also equipped with advanced safety features and the latest driver assistance technology to enhance your driving experience.
To experience what Mazda vehicles have to offer for yourself, visit your local dealer, browse our guide to buying an EV, or simply build your ultimate Mazda vehicle online today.
This article is intended for general informational purposes only and is based on the latest competitive information available at the time of posting. Information herein is subject to change without notice and without Mazda incurring any obligations. Please review a variety of resources prior to making a purchasing decision. Visit Resource Center for more articles.