Finding and fixing what may be causing your car’s battery to drain overnight can sometimes be as simple as making sure the car key is turned to the OFF position or, that the car door or trunk is firmly closed so that the interior lights remain off and do not drain the battery.
Other times, it may not be as obvious for example, in the suspected case of the AC being the cause of battery drain.
You may even be wondering if the car AC can drain the battery?
And if so, how it comes about? Or how you can confirm it?
How can you check for this?
So, in this post, I’ll explain how the car’s AC can drain the battery, and how you can check this and fix it too.
Quick answer. It is possible for the AC to drain a car’s battery flat.
This can happen when the AC relay, responsible for engaging the car engine to the AC compressor stays stuck in the closed providing a continuous path for current to flow through the relay coil even when the ignition key is in the off position.
Does running the AC drain the Car Battery?
Not all the time. The AC draws most of its power from the car engine. The AC compressor is driven by the car engine when the AC clutch is engaged.
As long as the car engine is running and the alternator is in good condition and able to meet the energy needs of the car’s electrical systems – sound system, charging the battery, lights, and others then the AC will not draw power from the car battery.
If the alternator is faulty and not able to meet the power requirements of the car or it’s working well but the demand for electrical powerfrom the car’s systems exceeds what it can supply then power to engage the clutch is drawn from the battery and it can be drained flat.
Additionally, should there be a fault with the ignition switch or if the clutch relay remains engaged even when the ignition switch is in the OFF position then the AC can drain the car’s battery.
Will the Car battery Die if the AC is left Running?
No, provided the car engine is running and the alternator can meet all the power requirements of the car including keeping the battery topped up.
If on the other hand, the power requirements exceed what the alternator can supply or if the AC clutch relay is stuck then the car’s electrical system will use the battery to make up for the deficit and can drain the battery flat depending on the duration of use of the AC system.
As an example, when the AC clutch relay is engaged, current draw through the coil can be as much as 3A! Over an 8 hour period, 8 x 3A = 24 Ahr of a battery capacity can be wasted through the relay coil windings.
How Long a Car Battery lasts with the AC on?
There are many variables at play such as the capacity of the battery, its capacity to hold charge, and the combined power draw from other connected accessories but as an example, a fully charged battery with a capacity of 55Ah can be drained to half of its available capacity about 7- 9 hours.
This assumes the the current draw from the battery with the relay engaged is about 3-4 amps.
A smaller capacity battery or older battery will last a shorter duration compared to a large capacity, newer battery with fewer accessories connected to it.
How to Test if the AC is Draining the Battery?
If the car battery is in good condition yet is drainng overnight then there is a good chance that there is a parasitic load that is running it flat.
To check for a parasitic load, measure the current flow from the battery by placing the black (negative terminal) probe of the meter on the positive battery post and the red (positive) probe on the positive battery cable clamp using a good quality multimeter.
Currents in excess of an estimated 50 mA point to the presence of a parasitic appliance.
If the parasitic draw is caused by the AC circuit, removing the AC compressor fuse from the fuse box usually next to the car battery while monitoring the current flow (with the ignition off) will result in the current draw reducing to about 50mA.
If there is no reduction then the parasitic draw is from another circuit.
Can a Bad Battery Stop the AC from working?
If the battery voltage is low (battery is dead) and the alternator is not working then the AC clutch will not be able to engage the AC compressor and get the AC working.
For the AC to work, the car engine mutst be running and the car alternator working or battery has sufficiently high enough voltage to engage the AC clutch.
So, the AC can still work even though the battery is dead provided the alternator is working and the car engine is running.
Besides the clutch, the blower fan which improves the cooling efficiency is powered by the battery or alternator.
If the battery is down and the alternator is not running then the blower will not be able to run.
Related questions
How Much Fuel a Car Uses when Idling with the AC On?
A 2 liter sedan consumes more that the estimated 0.16 gallons/hr of fuel when idling with the AC on¹.
This is based on the published estimates of fuel consumption for selected gasoline and diesel vehicles of 0.16 gallons/ hour for 2 liter engines by The office of energy efficiency and renewable energy efficiency, 2015.
The actual consumption will depend on several variables such as the capacity of the car engine (liters), the temperature setting it is set too and the surrounding temperature.
Final Word
Yes, the AC can drain the battery flat particularly if the AC clutch relay is stuck in the closed position allowing the power to flow from the battery to the relay coil.
If not detected in time, this can drain the car battery flat. You can check for this by measuring th parasitic current drain with the car’s ignition switched off. If you have substantial current draw in excess of 50 mA which drops town to 50mA or below when the AC fuse is removed then there’s a problem with the AC circuit that is causing battery drain.
¹Fact #861 February 23, 2015 Idle Fuel Consumption for Selected Gasoline and Diesel Vehicles
