How long a car battery lasts with ignition on?

How long a car battery lasts with the ignition set to the ON position mainly depends on:

  • the car’s battery capacity
  • the type of vehicle,
  • the accessories switched on in the car at that time
  • the battery’s condition.

For example, a 2015 model 2 liter car’s 50 AH battery lasts for an estimated 5 hrs if the ignition is set to the ON position.

This assumes an estimated current draw by the ignition of about 4.5A and no accessories such as radios, lights drawing power from the car’s battery system.

Effect of battery capacity on how long a battery lasts

The larger the battery capacity, the longer a battery can last with the ignition set to the ON position.

A 65AH battery, for example, lasts longer before running flat compared to an 80Ah battery.

Effect of car accessories on how long a battery lasts

The more the accessories are connected and drawing power from the car’s battery the shorter the duration the battery lasts for.

If you want the battery to last as long as possible, switch off the radio, dome lights, phone charging, laptop, or any other connected accessories.

Effect of battery condition

A weak, aged, or partially charged battery will last for a shorter time compared to a good quality fully charged battery.

The combined effect of battery capacity, condition, and connected accessories can therefore explain why a car’s battery may last for a short time.

For example, why a battery may not be able to start or may crank slowly if the car’s ignition has been in the ON position for a while.

How to avoid accidentally draining the battery

It is relatively easy to forget to switch off the car’s ignition.

If you plan to sleep and the radio is playing for example keep the car’s engine running to avoid the battery running flat.

Switch off any unnecessary accessories if the car’s ignition is on.

You might be surprised by the power draw of the combined accessories – radio, dome lights, heated seats, or others that you may not be aware of.

If you have to make a stop, switch the ignition off to prevent unnecessary battery drain.

Better still keep the engine running.

Emergency equipment to use in case of a flat battery

keep the following with you in the car in the event that your battery has been drained flat:

  • A set of good quality jumper cables
  • Portable battery jumper starter. At the time of writing this post, prices range from USD 60 – 110 for once capable of delivering about 1,000A.

Will a battery die if the ignition is left on?

Good quality batteries can survive several deep discharges and recover after a full charge.

However, if you have a weak, aged, or poor quality battery, draining it flat can permanently damage it.

Note though that frequent deep discharges shorten the life of the battery. To avoid draining your battery flat if you want it to last.

Will the battery be okay after jumping?

This really depends on the condition of the battery after being draining flat. Good quality and newer batteries can survive occasional deep discharges.

If the battery has been damaged after being drained then whereas it may attain full voltage (12.6V), it will not be able to hold the charge.

Instead, the battery voltage will drop quickly once a load such as the radio or rights are switched on or if it is left standing overnight.

How to confirm battery condition after jumping

To confirm that your battery is still in good condition after jumping, work through the following steps

  • Inspect the battery condition – make sure that the case is intact and there is no damage
  • Check the battery cables and terminals to make sure they are intact, clean and firmly hold the battery terminals
  • Using a digital multi-meter, measure the open circuit voltage of the battery. It should measure 12.6V and above after charging.

Lastly, the battery should be subjected to a load test (usually available at the Auto store).

Here the battery is subjected to a load at high discharge rates to observe the voltage at those loads.

If the voltage holds stable then the battery is still in good condition

Related questions

Does leaving the car switched to accessory mode drain the battery?

Switching the car to accessory mode also drains the battery too although the current draw is lower compared to a car whose ignition is set to ignition mode.

This assumes that no accessories such as radio, cigarette lighter socket are switched off in both scenarios.

How long switching a car’s ignition to ACC mode takes to drain the battery

The duration depends on battery capacity, the type of vehicle and the accessories switched on in the car at that time, and the battery’s condition.

A 2015 sedan draws approximately xx mA when its ignition is switched to ACC (accessories mode).

This assumes no accessories are switched on. A 65Ah AGM battery will last an estimated 5 hours before draining the battery. Note: This assumes a current draw of about 0.37A from the battery when the ignition is switched to ACC mode

It is best therefore to switch the ignition to off or if on ACC or ON mode, keep the car engine running if the ignition will set to those modes for an extended period.

All in all, how long a battery lasts when the car’s ignition is switched to ON or ACC mode depends mainly on:

  • the car’s battery capacity
  • the type of vehicle
  • the accessories switched on in the car at that time
  • the battery’s condition.

To avoid running the battery flat, keep the engine running.

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