Do Hazard Lights Drain the Battery?

You may be concerned about hazard lights draining your battery, especially while the engine is off. I will cover whether hazard lights can drain the battery in this article. Then we’ll see how much they drain while the engine is turned off.

Key Takeaway: Hazard lights can drain the battery like any other vehicle accessory or feature that consumes electrical power. You shouldn’t allow the battery to drain so much that it cannot restart the car. The safe limit for keeping the hazard lights on is 3 to 5 hours. The exact time depends on the bulb’s wattage, the amount of initial charge, and the battery’s condition.

You can keep the hazard lights on for a bit longer, at the risk of possibly being unable to start the car, up to 6 to 10 hours. I don’t recommend deep or full discharging unless it’s an emergency, but a full drain on the battery may allow an 80 Ah battery to operate the hazard lights for up to 20 hours.

Use the table below to estimate how long you should or can keep the hazard lights on.

Safe Limit for Keeping the Hazard Lights On (25% capacity)	Medium Discharge (A Chance the Car Might Not Start) (50% capacity)	Deep Discharge (75% capacity)	Full Discharge (100% capacity)
Small (30-50 Ah)	2-3 hours	3.5-6 hours	7.5-12.5 hours
Medium (50-80 Ah)	3-5 hours	6-10 hours	12.5-20 hours
Large (80-110 Ah)	5-7 hours	10-12 hours	20-27.5 hours

Here is a list of the size of batteries for the most common cars in America to cross reference with.

Car Model	Typical Battery Size (Ah)
Ford F-150	65-75 Ah
Chevrolet Silverado	70-80 Ah
Toyota Camry	45-60 Ah
Honda Accord	45-60 Ah
Honda CR-V	51-60 Ah
Toyota Corolla	45-60 Ah
Ford Explorer	65-75 Ah
Chevrolet Equinox	60-70 Ah
Ram Pickup	70-80 Ah
Nissan Rogue	55-60 Ah

Vehicle Battery Drain

All electrical devices in a vehicle consume the electrical power they get from the battery.

This includes the headlights, radios, and air conditioners. Usually, it’s not a problem while driving because the vehicle’s battery recharges simultaneously. So, the concern is when you use these accessories and features while the engine is off. You might be interested to know how long they can continue operating.

In a recent article, I presented my findings on how long car radios can last with the engine off. You can usually run them for 4 to 6 hours before the battery drains too much.

Here, we will explore how long you can keep your hazard lights on when parked somewhere with the engine off. You might be stranded somewhere, having to turn the hazard lights on. Continue reading if this applies to you.

Power Consumption of Hazard Lights

Turning the hazard lights on will consume the battery like any other electrical device, including the headlights and other indicator lights, because they operate on electricity.

So, what we’re really interested in is how much power hazard lights consume and how long they can remain on while the engine is turned off.

Hazard lights are designed to consume minimal power because they are often used when the car is turned off. But they will still slowly drain the battery. How long that will take depends on:

• How much the battery is charged initially, i.e., just before turning the engine off
• The battery’s type, age, and condition
• The wattage of the hazard light bulbs

Generally, you can keep your hazard lights on (with the car off) for around 3 to 5 hours if the battery is in reasonable condition. It can remain on longer, but I’ve given this estimate keeping in mind that you probably need to maintain sufficient charge to start the car later.

This compares more favorably with headlights, which may only last 2 to 3 hours if kept on continuously. Having them on together (hazard + low-beam headlights) might only give you an hour to an hour and a half.

Exact Power Consumption

You can calculate the exact power consumption, but vehicles use light bulbs with different ratings, which also depend on other factors.

Generally, each hazard light bulb is rated for around 2 amps. Let’s assume you turn on 4 hazard lights that draw 8 amps of current together. Since they blink occasionally, we can regard the total draw as 4 amps.

We will also assume that your car’s battery is rated for 80Ah and is fully charged, new, and in good condition. To find the expected runtime, divide the battery’s capacity by the total current draw of the hazard lights:

80Ah / 4 A = 20 hours

This is a theoretically expected runtime. In practice, the bulbs might not be efficient, and the battery might not be so new and in good condition. Also, we don’t want to drain the battery completely that it will prevent you from starting the car.

That’s why I mentioned around 3 to 5 hours might be a more reasonable expectation for how long you can allow the hazard lights to drain the battery.

Using Hazard Lights

Is It Safe to Leave Hazard Lights on For Longer?

The risk of keeping hazard lights on longer than this estimated safe limit of 3-5 hours is that the battery might drain too much and become unable to start the car.

Usually, keeping them on for up to a couple or three hours should pose no problem unless the battery is in poor condition. You should be able to restart your car. As long as the battery is reasonably healthy or has a higher amp-hour rating than 80Ah, it’s possible to keep them on for up to 3 to 5 hours.

If you keep the hazard lights on longer than this, the battery will continue to drain further. This is risky unless you can monitor the exact remaining charge in real-time and know exactly how much you will need to restart the car so you don’t exceed this critical limit.

If you exceed the limit of having enough charge to restart the car, you will need to jump-start it.

Deep Discharging

Supposing the hazard lights are left on for a very long time, say 8 to 12 hours or more, you may have deeply discharged the battery.

Deep discharging is not good for the battery, whether it happens intentionally or by mistake. You shouldn’t discharge a lead-acid battery more than 50%, which means around 8 to 10 hours should be considered as the upper limit if the battery is to remain healthy. Typical lead-acid batteries are only designed to be starting batteries, not for regularly discharging them deeply.

Deep discharging often will increase sulfation on its plates. Discharging beyond 75% (about 15 hours in the case of an 80Ah battery powering 4A hazard lights) increases sulfation exponentially. It weakens the battery over the long term and will shorten its lifespan.

Using Hazard Lights with a Smaller/Larger Battery

The above example was for a battery with an 80Ah capacity rating.

If you have a battery with a smaller or larger capacity rating, use the chart at the top for a rough idea of how long you can use the hazard lights.

Using Hazard Lights with a Weak Battery

A weak battery can usually still operate the hazard lights.

So, if, for instance, the car won’t start due to insufficient charge, but you must turn the hazard lights on due to an emergency, then you can do that. The hazard lights will work until the battery drains completely.

What to Do After Using the Hazard Lights

What should you do after using the hazard lights for a few hours?

As long as you don’t exceed the 4-5 hours limit and the car restarts easily, you should drive around for about half an hour to one hour to recharge the battery.

References

Tom Denton. Automobile Mechanical and Electrical Systems. Elsevier Ltd. 2011.

Hazard light button. https://commons.wikimedia.org/wiki/File:Hazard_warning_flasher_button.jpg

Video References

DIY Pinto