Can a Heater Drain the Car Battery?

Keeping your car warm by switching on the heater during the cold winter months is necessary for both your safety and comfort.

But where does the car heater draw its energy from? Is it the battery?

And if so, are you at risk of draining the car starter battery flat?

In this post, you’ll get an idea about how much power the car heater can draw from the heater, and how long the car heater can run on battery tips that can help you keep the car nice and warm during the cold winter months without draining the car battery.

Does Leaving the Heater On Drain the Car Battery?

If the car uses an internal combustion engine (ICE) that requires gas to run then there’ll be a drain on the car battery by the blower fan to the extent that the car alternator is not able to meet all the car’s power needs and relies on the car battery to supply the deficit.

In an ICE, the heat that keeps the cabin warm is generated as a by-product of the engine running and not from the car battery.

If the car has (aftermarket) heated seats or steering wheel then power will be drawn from the battery if the alternator is not able to meet the energy needs.

There is no drain on the battery if the car alternator is able to meet all the requirements of the car’s electrical system.

If it is an EV then the car heater draws its power from the high voltage battery of the car and drains the battery so long as it is switched on.

Does the Heater Work when the Ignition is Off?

In an Internal combustion engine, the ignition heater does not work when the engine is off. Recall that the heat is a byproduct of the engine running.

Should the car have an aftermarket heater, heated seats of the steering wheel that are wired (wrongly) to a circuit that is always on or in case of an fault then the heating can work even when the ignition is off.

In an EV the heater is normally switched off when the ignition is off unless in case of a fault in the electrical system.

Why Your Car Battery May Be Drained by the Heater when the Ignition is OFF

Here are a few tips on where you can check if the car battery is drained by the heater even when the ignition is off.

#1. Do the (aftermarket) seats have a manual on/off switch that needs to be switched off?

If they do and they are wired on a circuit that is always on then there is a risk of battery drain if you forget to switch them off.

#2. You might have a faulty relay that is shorting and supplying power to the heater and draining the battery.

It helps to have the car manual on hand or to contact a qualified EV technician to check this out.

You may also be interested in this post: 6 Reasons why your car’s battery may be drained and how to fix it

Should You Leave the Car Heater on all Night?

Unfortunately for cars with that use gas, leaving the car engine running depending on its location and condition carries a risk of the deadly carbon monoxide (CO) fumes potentially escaping into the car.

CO has no smell and is a silent killer. I wouldn’t risk it .

Consider driving further to find a warm place to stay as an alternative for your safety and peace of mind.

For an EV, the heater relies on the battery’s power to keep the cabin warm.

Depending on the battery’s state of charge, condition and duration for which the heat runs, it can be drained flat.

How Much Power Does the Car Heater Draw?

While there are several variables that need to be taken into account, as a rough guide, the car heater in an EV can draw anywhere from 500 – 5,500 watts of power.

How much power the heater actually draws from the high voltage battery depends on:

  • the cabin and outside temperatures
  • whether you use one of the general, seat, steering wheel heaters or a combination.

How long car battery lasts with heater on?

Turning on the heater in an EV uses some of the battery’s capacity and reduces the mileage you can get from the car.

Actual mileage depends on several factors, battery state of charge, battery capacity and the heater’s power consumption.

Tips Prevent Battery Drain when Using the Car Heater

If your car is an EV, you can minimise battery drain when using the heater by:

#1. Preheating you car when the car is still plugged into and charging from the grid. This way the heater does not have to work so hard to attain and maintain the set cabin temperature.

#2. Use the seat and steering wheel heaters and general heating only a you need them. Switching all of them on draws the maximum power from the battery.

Final Word

Turning on the heater in an EV drains the battery. In an EV, the heater is powered from the high voltage battery.

The more power is drawn from the battery, the lower the mileage available from the battery.

In an internal combustion engine, the heat is from the car engine and the power draw for the blower fan can be from the alternator or battery if the alternator is not able to meet the car’s electrical power requirements.

Leave a Comment