Is your car’s battery drained flat? Are you considering recharging it by letting your car engine run for a while? While it sounds convenient, it is not as effective as you may think!
In this post, I share insights into:
- whether leaving your car running is effective in charging the battery
- whether how long the engine runs (stationary car) has an effect on how well a battery charges
- Alternatives to charging your car’s battery by running a stationary engine
Finally, how you can know whether your battery is fully charged
Does Leaving Your Car Running Charge the Battery?
Yes but not significantly.
Leaving your car engine running while the car is stationary can charge the battery but since the car’s engine is idling (low rpm), the charge rate is low.
Also if you have any appliances switched on such as lights, radio, or other accessories there is not enough power left over to significantly change the battery’s state of charge.
Car batteries do not charge effectively when the engine is idling.
Does how long the car runs affect how well the battery charges?
Not in a significant way if the car is parked or stationary.
You need to get your engine speed at about 2,000 rpm or greater to effectively charge your battery. In fact, the higher the car’s engine speed the faster your battery will charge.
So, if you run your engine for longer while your car is parked or moving slowly (less than 2,000 rpm) your battery will not change significantly.
This can explain why a car can fail to start even after the engine has run for about 30 min following the jumpstarting of the car.
Leaving your car running at a low rpm (<2,000) does not significantly charge the battery
What are the Alternatives to Charge your Car Battery after Jump Starting?
To recharge your car’s battery after jumpstarting you can:
Option #1. Revv the car (if stationary) to increase the engine speed to at least 2,000 rpm or higher several times depending on how heavily drained it is.
This may not be enough to fully charge the battery and may only be good enough to help you start your car successfully a few minutes later.
In addition, implement option # 2 or option # 3 below to fully charge your car’s battery.
Option #2. I find it more effective and easier though to recharge the car’s battery by driving for 30 minutes or longer depending on how deeply discharged your battery was.
Option #3. If you need to charge your car’s battery while it is parked, you can recharge your battery using an external battery charger or use another car’s battery and a set of jumper cables.
Make sure you follow the manufacturer’s instructions for connecting the charger to and from the battery safely.
How long to leave your car running after jump start
You should leave your engine running for about 10-15 minutes after jump-starting your car so that the car’s battery gains some charge.
I consulted several service manuals for Toyota and Honda and there seems to be a consensus around this.
If you run your car’s engine for a shorter period, you risk not being able to start your car because the battery has not picked up enough charge.
As the engine runs, switch off all car accessories, for example, the audio system, air-conditioner, fan, and others for all the energy generated by the car’s alternator to go to the car’s battery.
In fact, your car engine may die again if the appliances are switched on as they are competing for the available power with the battery.
Running your car engine for 10-15mins will quite likely not fully charge your car’s battery but should enable you to start your car in the next 10-30 minutes or so depending on the condition of your battery.
For more information on how you can fully charge your car’s battery, check here.
How Can you Check that the Car’s Battery is Charged?
Once you have charged your car’s battery, here are three quick ways you can use to check whether your car’s battery is charged:
- If you battery is charged, the battery warning light on the dashboard should be off, i.e not lighting when the ignition switch is turned to ACC or ON.
- When you crank the engine, the start should be crisp without any hesistation
- You can also measure the battery voltage using a good quality car battery tester or a digital multi-meter.
If your battery voltage reads about 12.8V DC with a digital multimeter, then it is fully charged. On the other hand, a reading of 12.2 – 12.7VDC shows that the battery needs to be charged.
Should the multi-meter reading be below 12.2VDC, then the car’s battery is completely discharged and needs to be charged immediately!
If the battery voltage is low or the battery does not hold a charge even after charging it considerably, you may have a case of parasitic load or a bad battery.
For more information, check out this guide on What can drain a battery when the ignition is OFF
# 1. How long to charge car battery after jumpstart?
To charge your car’s battery after jumpstarting, drive for about 30 minutes or longer depending on how deeply discharged your battery is.
Do not leave the car stationary or parked idling because the alternator does not generate enough voltage to fully charge your car’s battery.
# 2. Does leaving your car running drain the battery?
Leaving your car running does not drain the battery provided your alternator is working well and can supply the power requirements of the car accessories – radio, lights, heater, and others.
Your car’s battery can be drained if there is a problem with the alternator and it is not able to provide power for the electrical accessories that are switched on in the car.
In this case, the car’s accessories will draw their power from the battery and drain it.
If the battery is old, poor quality there is a risk that it can fail if it is deeply discharged.