If your car engine is acting up and you’re forced to turn on the hazard lights, the last thing you’re going to want is a dead battery if you leave them on for too long.
Knowing how long you have is the difference between having a bad day and an even worse day.
I took the liberty of using a clamp-on multimeter to test how much amperage is drawn for the hazard lights by themselves and also if you have the headlights on as well.
I’ve got a chart below that you can check out but if you’re just here for the quick and easy answer I won’t waste anymore of your time.
You can leave the hazard lights on for about 4 to 5 hours before the battery is too weak to start the vehicle. If you have the headlights on as well you will only have 60 to 90 minutes until you won’t be able to start your car.
How Long a Car Battery Will Power Hazard Lights?
Like I mentioned above, you’ll have about 4 to 5 hours of hazard light use until your battery probably won’t turn over the engine any longer. If you want to be conservative or if you know your battery is a couple years old already, you might not want to go much longer than 3 hours. If you have the hazard lights on as well as the headlights, like if you broke down at night, then you’re only going to have about 60 to 90 minutes.
It’s really just a function of math to figure out how much time you have, but it gets a little tricky because all car batteries are different ages, in different states of “health”, have different ratings as far as their total amp-hours, and all vehicles will all have a different amperage being drawn when the hazard and headlights are turned on (vehicles have different types and numbers of bulbs).
I used a clamp-on multimeter to measure the amperage of five different vehicles (different makes and models) and found that when the hazard lights flash they use an average of 11 amps. In between the flash there is still a draw on the battery of about 0.5 amps.
If we take 11 amps and subtract 0.5, we have 10.5. Now if we consider that the hazards are only blinking 50% of the time and are off the other 50%, then our average amps per hour is 5.25.
Car batteries are a whole different animal though, and I have a full article here where I show the calculations on how I figured these ballpark amp hour ratings.
Long story short, batteries in vehicles are rarely rated in amp hours and we have only Reserve capacity or cold-cranking amps with which to measure them. These ratings are not useful when it comes to measuring how long a battery could power something when you know the amperage of what is being drawn.
Basically all I did was expanded upon a battery manufacturers chart of charging times by reverse engineering the formula they used and I applied this to all of the different sizes of car batteries to get an estimate of how many amp hours there are. It’s not perfect, but it’s going to be close.
In the chart below you’ll see in the left-hand column the different battery sizes and then I put how much time you’ll have until the battery is at 50% of charge when you have only the hazard lights on and also when you have the hazard lights on with the headlights.
50% charge is probably the lowest you can get until your battery won’t turn over the engine, but your battery may be different and might not turn over the engine when it gets to 60% state of charge. Take these numbers and be conservative with them so that you don’t leave yourself stranded.
How Long Can You Leave the Hazard Lights On?
|Hazard Lights Only, Until Battery at 50%||Hazard Lights + Low Beam Headlights, Until 50%|
|Small Car Battery|
|4 Hours||75 Minutes|
|Mid-Sized Car Battery|
|5 Hours||90 Minutes|
|Large Vehicle Battery|
|7.5 Hours||2 Hours and 15 minutes|
**In 5 vehicles tested, hazard lights alone averaged a high of 11amps and a low of 0.5amps for an average time of 5.25amps ((11-0.5)/2) since the hazards are only engaged 50% of the time.
***In 5 vehicles tested, hazard lights plus the low beam headlights had a high of 23 and a low of 14, for an average of 18.5 since the hazards are only on 50% of the time.
Will Leaving the Hazard Lights on Damage the Car Battery?
Leaving your hazard lights on for extended periods of time while the engine is not running will start to damage your car’s battery by allowing the plates to sulfate which reduces their ability to accept the charge and also to give off electricity.
The length of time that you leave your hazard lights on is the factor you have to look at the true answer this question. If you leave your hazard lights on for an hour or less and then start your engine and drive around for 30 minutes then you really haven’t done much, if anything, to your batteries state of health.
If, however, you leave your hazard lights on for 4 hours or more and maybe you need a jump to get your car started, then you have just successfully done a deep discharge of your car battery.
The problem with doing a deep discharge of your car battery, is that car batteries are engineered to be starting batteries and will have a dramatically shortened lifespan if you discharge them to 50% or below.
Marine deep cycle batteries, or golf cart batteries, are designed internally to experience deep discharges and be able to be recharged over and over again hundreds of times.
A car battery, on the other hand, will only give you a dozen or less full deep discharges. You’ll probably get a couple dozen if you only go to about 50%.
When a car battery state of charge starts getting below even 75%, the condition starts increasing exponentially for sulfation (lead sulfate) to set in. This is basically a white crystalline coating that sticks on top of the lead plates inside the battery and prohibits them from accepting a charge and also from giving off that high burst of current when your vehicle is starting.
If you know that you’ve left your hazard lights on for an extended period of time, then it is best to either run your vehicle and drive it around for 30 to 60 minutes, or hook it up to a dedicated battery charger when you get home in order to break up any of a lead sulfate crystals on the plates before they harden and become impossible to remove.
I personally recommend this charger here Amazon, that will apply a full but gentle charged your car battery and we’ll have it ready overnight. You can even leave this hooked up to your car battery indefinitely if you happen to be going on vacation and don’t want to come home to a dead battery after a month or two or even more.
I’ve used it daily for 6 years straight on my vehicles and home battery bank and I couldn’t be happier with its performance.
What to do if the Hazard Lights are Left on too Long and the Car Won’t Start?
If your car battery won’t start your engine after your hazard lights have been on for too long, then you’re left with the option of getting a jump from another vehicle, or providing that jump yourself. There are plenty of awesome compact jump starter devices that you can carry in your vehicles and there’s really no reason not to have one.
This one, seen on Amazon, packs a good punch to get your car started and is extremely small when you consider how big these Jump Start packs used to be even a decade ago. These are truly lifesavers and are worth every dollar in an emergency. If you need a jump, this pays for itself the first time you use it.