What are HV and LV batteries in electric cars and what are their roles?

When most people think of an electric car, they imagine one big battery pack under the floor. That is true, but there is more to the story. Every EV actually has two different batteries: a high-voltage (HV) battery and a low-voltage (LV) battery, usually 12V. Both have unique roles, and the car cannot function without either of them.

Think of the EV as a city. The HV battery is the massive power plant that runs trains and industries. The LV battery is the local supply that powers homes, traffic lights, and shops. Both are needed for the city to function smoothly.

What is the HV battery?

The HV battery is the heart of the electric vehicle.

• It typically operates between 200 and 800 volts.
• It supplies energy to the motor for propulsion.
• It handles fast charging and supports high-demand systems like cabin cooling or heating.
• It is a large lithium-ion pack, often ranging from 30 kWh to over 100 kWh depending on the car model. Want to know which EV battery chemistry is best? Dive into the differences between LFP, Lithium-ion, and Sodium-ion.

Without the HV battery, the car cannot move at all. It is the part that gets all the attention because it defines range, charging speed, and performance.

What is the LV battery?

The LV battery, usually 12V, looks small but plays a vital role in keeping the car alive.

• It powers electronics such as lights, wipers, infotainment, door locks, windows, and airbags.
• It also keeps the control computers running, which communicate across the car.
• In short, it is the nervous system that makes the EV usable.

Most EVs still use a 12V lead-acid battery, similar to the one in petrol or diesel cars. Some newer models are switching to lithium-ion 12V packs that are lighter and longer-lasting.

Why not run electronics directly from the HV battery?

This is a common question. If the car already has a huge battery pack, why not use it to run everything? The reasons are safety, cost, and reliability.

• Safety: HV systems are dangerous to handle. Every small part would need high-voltage insulation, which would be bulky and risky.
• Standardisation: The auto industry is built on 12V systems. Reusing them in EVs is easier and cheaper.
• Reliability: The 12V system can stay alive even if the HV system shuts down, which allows critical systems like airbags and hazard lights to work in emergencies.

The two batteries are connected through a DC-DC converter, which steps the high voltage down to 12V to keep the LV battery charged. This is the EV equivalent of an alternator in an ICE car. Know the difference between AC & DC chargers.

What happens if the LV battery fails?

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Here is the surprising part. Even if your HV battery is full, the car may not boot up if the LV battery is dead. The electronics that wake the car will not switch on, and the relays that connect the HV battery will not close.

Many owners of EVs have faced this issue. The fix is often simple: a jump-start of the 12V battery or a quick replacement brings the car back to life.

Early signs of a weak 12V battery include:

• Dim lights or slow power windows
• Door locks that feel sluggish
• Random error messages
• Repeated “12V battery” warnings on the display

The future of LV batteries in EVs

Manufacturers are improving this system. Some cars now use lithium-ion 12V batteries, which are smaller and more reliable. Premium EVs are also testing 48V systems, but the 12V setup will remain common for many years because of cost and compatibility.

Know the difference between charging speed and battery capacity

Conclusion

Electric cars depend on both HV and LV batteries. The HV battery drives the wheels, while the LV battery keeps everything else alive. One supplies the muscles, the other powers the brain and nerves. Together, they make sure the EV runs safely and smoothly.

So the next time you think of your EV’s battery, remember there are actually two — and sometimes the little one under the hood is just as important as the big one under the floor.