As you may expect, how long a car battery lasts with headlights switched on will depend on several variables – the battery capacity, state of charge of the battery, and others.
With that said, however, it is helpful to know indicatively what you can expect if the car headlamps are left on as it can help you catch underlying problems earlier than would otherwise have taken you by surprise, for example, a permanently damaged battery.
Even when the battery is recharged, for your peace of mind, you may want to know the long-lasting effects that draining the battery may have had on the lifespan of the battery, i.e. whether it has lost some of its capacity or not.
So, read on to learn how long you can expect the car headlights to last when on battery and if the car battery can be recharged once drained by leaving the headlights on.
Additionally, you’ll find what to look out for should the headlamps not last as long as expected and suggestions on the course of action to take if the battery charge is running down faster than usual with the headlights on
How Long a Car Battery Lasts with Headlights On
The car headlights can last for between 2 – 3 hours before they drain the car battery to 50 % of its capacity.
This estimate assumes a fully charged, 55 Ah battery still in good condition (able to hold charge) and that each of the two headlamps is rated 55 watts.
When the headlights last for a short time (Is it normal)
If they last for a considerably shorter time, for example, an hour or less, then there is likely an underlying issue that you need to investigate.
You may also be interested in the post: How long the car battery lasts with the radio on
Why the Battery maybe Lasting for a short time?
If the car battery is lasting for a short time with the headlights on you can check the following:
# 1. Confirm that there are no additional accessories drawing power from the car battery. Power consumption from the running blower, interior lights, or car sound system as an example can contribute to the car battery lasting a shorter time.
You may also be interested in this post: 6 Reasons why the car battery power may be drained and how to fix them
#2. Is the car battery fully charged or partially charged? Confirm if the battery was fully charged or partially charged.
You can measure the battery voltage. A fully charged battery measures about 12.6 volts while at 12.3V, the battery has lost about 35% of its capacity.
#3. Aging or reduced battery capacity to hold charge. If the battery is aging and has lost its ability to hold charge or is damaged with shorting cells then it will last for a considerably shorter time.
You can have the car battery tested at an auto car shop to confirm if it is still in good condition or is degraded and should be replaced.
#4. Cold winter weather. Battery capacity reduces during the cold winter weather and some batteries are not able to hold up as well as others.
That being the case, the runtime on batteries with the headlamps can be considerably lower during winter.
All in all, If the battery capacity is lower, the car battery is aging and has lost some of its ability to hold charge or it is winter then expect the lights to last for a shorter time.
How Many amps Car Headlights Draw?
Two car headlights each with a 55-watt bulb draw between 9 to 10 amps when lighting. If the ignition is switched on, the current draw can be higher.
The math
The current draw can be obtained from the formula: current (amps) = (combined power consumption of the headlamps)/car battery voltage
(2 x 55 watts)/12 = 9.16 amps
Tips to Prevent the Interior Lights from Draining the Car Battery Flat
Many cars have a warning chime to alert you if the ignition key is removed while the headlights are still switched On.
Additionally, here are some tips that can minimize the likelihood of the car battery being drained flat by the interior light(s).
#1. Are the headlights set to ON mode or auto? If the light selector switch position is set to ON, then the headlights can stay on draining the battery even when the ignition key is removed.
#2. You can also consider installing a battery saver or low voltage disconnect switch that protects the car battery from damaging deep discharges.
Will the Car Battery Die if Headlights are Left On?
Not necessarily. Depending on the state of charge of the battery and the age of the battery, the car battery may still be able to work once it is charged fully.
An older battery is less likely to hold charge and may be more susceptible to permanent damage if it is drained flat.
If you have concerns about the condition of the battery and whether its capacity has been affected, you can have it tested at a car repair facility.
Regardless, you should charge it fully. If not going on a long drive, definitely consider using an external charger to charge it fully.
Final Word
The car headlights can last for between 2 – 3 hours before they drain the car battery to 50 % of its capacity.
All in all the actual time it takes depends on whether the battery is fully charged or not, its ability to hold charge, its capacity (Ah), and how many other accessories are switched on and drawing power from the battery as well as the headlights.
