The Truth About Charging Your Car Battery by Idling

Charging Your Car Battery By Idling Takes Much Longer Than You Think

So you’ve jump-started your car, and intend to recharge your car battery by idling or driving around the block a few times. I’ve got news for you; it won’t work. It can take up to 4 hours to charge your car battery by idling.

Here’s why it takes so long; there’s simply not enough extra power from the alternator to run all the electrical systems and still have enough left over for your battery. It takes 30-50 amps just to run the computers, fuel pump, ignition and fuel injection systems in your late model car.

First, At idle, the alternator only outputs less than 1/3 of its capacity

Maximum alternator output is rated at 2,000 RPM, which runs your alternator pulley at about 6,000 RPM. But your car probably idles at 600-RPM, which means your alternator can only output about 1/3 of its maximum capacity. If you have a 120-amp alternator, that’s only 30-amps. If you have a 160-amp alternator, that’s only 53-amps.

Second, Modern Vehicles Use a LOT of Power, Even At Idle Speed

Late model cars and trucks can use 30-50-amps just to run the computers, fuel, ignition, and ADAS systems (which are always running when the engine is on). In many cases, that’s more than the alternator outputs at idle. In other words, the computers, fuel injection and ADAS uses more power than you’re producing when idling.

Here’s the breakdown on how much power each vehicle system needs to keep it running

ABS computer current draw— 5 amps: Constant when engine is running
In a driving state with no ABS activation, the computer draws about 5-amps. During ABS operation it can draw up to 50-amps.

Airbag control module current draw— around 10 amps: Constant when engine is running
In a monitoring state, the airbag control module draws less than 10-amps

BCM Body Control Module current draw in amps— 5 amps: Constant when engine is running
In idling mode (no brake lights, headlights in use) computer draw only 5-amps

ECM computer amperage— around 10 amps: Constant when engine is running
A typical ECM draws slightly less than 10-amps

Fuel Injectors current draw— around 4 amps at idle
Gasoline Direct Injectors draw approximately 10-amps
Port Fuel Injectors – 4 to 6 amps peak, 1 amp hold

Headlight current draw in amps— around 8-10 amps when in use
Halogen Headlights low beam- 8 to 9 amps per pair
Halogen Headlights high beam – 9 to 10 amps per pair
Halogen Headlights high and low beams combined) – 17 to 19 amps
Xenon HID Headlights – 12 to 14 amps during initial startup, then 7 to 8 amps to maintain arc
LED Style Headlights – 0.6 to 1 amps per bulb

Heated seats current draw— 3-4 amps when in use
Heated Seats – 3 to 4 amps per seat

High-pressure fuel pump (for direct injection engines— 5 amps: Constant when engine is running
A high pressure fuel pump for direct injection engines doesn’t actually use an electric motor to produce pressure. It’s a mechanical pump that rides off the camshaft to produce up to 2,000-psi for direct injection. But it does use power to control the inlet/outlet port solenoid. The solenoid opens the suction port to allow fuel to enter and then closes to allow the pump to compress the fuel.

High-end sound system current draw in amps— 13-30 amps when in use
A 500 Watt Sound System – 13 to 30 amps

Ignition system need in amps— 6 amps: Constant when engine is running
Old single coil ignition systems – 3 to 4 amps.
Distributorless Ignition Systems – 5 to 6 amps at peak output.
Coil-on-plug ignition systems – 6 amps per coil at peak output.
Ignition System (primary circuit) – 6 to 20 amps.

Instrument cluster amps usage— 7 amps: Constant when engine is running
A typical instrument cluster consumers approximately 7 amps

In-tank fuel pump current usage in amps— 4-5 amps: Constant when engine is running
In general, in-tank fuel pumps draw is approximately 1 amp for every 10-psi of fuel pressure. So a port injection system that requires 45-psi. will consumer approximately 4.5 amps.  But consumption varies based on the fuel needs of the vehicle speed and load. So figure 4-6-amps for a port injection engine.

Oxygen sensor heater amps usage— 5-8 amps: Constant when engine is running
Oxygen sensor heater current draw depends on the type of sensor, whether it’s a wide-band or narrow-band sensor. But in general, oxygen sensor current usage ranges from .2 to 8-amps per sensor.

Radiator fans current draw in amps— 15-28 amps when running
A single radiator fan can draw from 15-28 amps. A dual fan setup can run up to 35-amps.

Rear window electric defogger current draw— 10-20 amps when running
Electric Rear Window Defroster – 10 to 20 amps

TCM Transmission Control Module— 5 amps: Constant when engine is running
Shift solenoids are pulse width modulated so current draw varies based on driving conditions.

That’s why it takes so long to charge a car battery by idling

Think about it; your car needs 30-50-amps to keep it running at idle. Your alternator puts out a total of 30-53 amps at idle. So after covering the basic systems, there’s little power left over to charge your battery. your car uses when running. Your alternator can barely generate enough power at idle to power that, let alone come up with more to recharge your battery. If you want to recharge your battery, you must use a battery charger or get it out on the highway for 20-30 minutes.

Charge a car battery by highway driving or with a battery charger

Idling is simply the worst way to charge your car battery. It takes too long. It wastes gas. Idling for that long degrades your oil. Instead, drive your car at highway speeds for 20-30 minutes after a jump start. Or slap on a digital battery charger. See this post for more information on the best way to charge a car battery

©, 2023 Rick Muscoplat