> > Can you recharge your car battery by idling
Your car battery is dead and you’re planning on recharging it by letting your engine idle. How long do you have to idle your engine to recharge the battery? Let’s look into that.
You can recharge your car battery by idling, but it takes a LOT longer than you think
Some people idle their engines for 20 minutes and think that’s long enough to recharge a battery. Then they wonder why it won’t start the next day. Why? Because it takes almost 4-hours of idling to fully recharge a dead battery.
How much does the alternator recharge at idle speeds?
Disregard the alternator’s maximum output rating
Older alternators have a maximum output of around 100 amps. But, that maximum output rating is based off of an alternator pulley speed of 6,000-RPM and an engine speed of 2,000 RPM. (Most engines have a 1:3 pulley ratio, so 2,000 RPM at the crank pulley equals 6,000 RPM at the alternator.)
However, at idle, an alternator outputs less than 1/3 of its maximum rated value
At an engine idle speed of 600 RPM, the alternator output is less than 1/3 of its maximum output. In theory, a 100-amp alternator should be able to charge the battery at slightly less than 33-amps. But that assumes there are no other electrical loads at the time; which is a false assumption.
Any time an engine runs the engine systems draw power
Let’s assume you’ve turned off all your power consuming devices like headlights, blower motor, defogger and heated seats. Your running engine is still drawing power from the alternator.
An idling engine needs approximately 35-50 amps to power the electric fuel pump., ignition system, fuel injectors, heated oxygen sensors, ECM, BCM, TCM and all other modules. A 100-amp alternator running at idle speed has barely enough left over power to recharge your battery.
How long does it take to recharge a battery at idle with a larger alternator?
Let’s look at a 140-amp alternator running at idle speed. The alternator pulley speed is just 1,800 at idle, so a 140-amp alternator can output approximately 42 amps. If the running engine consumes as little as 27-amps, you’d have 15 amps left to recharge the battery.
Let’s take a typical 60Ah battery that’s 80% discharged and charge it from your alternator using a left-over 15-amps that’s not being used by the engine systems.
60Ah x 80% discharged = 48Ah needed to bring it back to full charge. 48Ah÷15 charging amps = 3.2 hours to fully recharge.
How long does it take to recharge a battery when driving at highway speeds?
Time needed to recharge a dead battery at highway speeds with a 100-amp alternator running at 2,000 engine RPM/6,000 alternator pulley speed
We’ve got a maximum of 100 charging amps minus approximately 50-amp current draw at 2,000 RPM (the fuel pump, ignition, O2 heater, transmission solenoids and ECM, TCM, ABS systems current draw more power at higher speeds). That leaves approximately 50 amps to charge battery
The same 60Ah battery that’s 80% discharged = 48Ah needed to bring it back to full charge. 48Ah÷50 charging amps = slightly less than 1-hr of highway driving to fully recharge the battery.
Time needed to recharge a dead battery at highway speeds with a 140-amp alternator running at 2,000 engine RPM/6,000 alternator pulley speed
After deducting engine and systems current draw of 50 amps, we’d have approximately 90-amps to recharge the battery
The same 60Ah battery that’s 80% discharged = 48Ah needed to bring it back to full charge. 48Ah÷15 charging amps = slightly less than 1/2-hr of highway driving to fully recharge the battery.
CVT and 6, 8, 9 speed transmissions increase charging time at highway speeds
The newer energy saving transmissions shift into overdrive much sooner than traditional transmissions and run at lower RPMs at highway speeds. So it’s typical to see a highway RPM of around 1,500-RPM when driving 60-MPH. That’s a 30% drop in alternator RPM compared to a traditional transmission. If you have a 140-amp alternator, that leaves 98-amps to run accessories and recharge the battery
At highway speeds, figure on approximately 50-amps to run engine systems (chart shown below). Then, deduct the power need to run accessories
Headlights (halogen low beam) – 18 amps per pairParking/marker lamps 3.2 ampsElectric Rear Window Defroster – 10 to 20 ampsWindshield wipers – 2 to 10 amps depending on loadHeated Seats – 3 to 4 amps per seatHeater A/C blower motor 30 ampsAir Conditioner Compressor Clutch – 2.5 to 5 amps500 Watt Sound System – 13 to 30 ampsElectric Power Steering – 2 to 40 amps depending on load
At highway speeds, the ignition, fuel, transmission and cooling system use more power
Ignition Coil (coil-on-plug) – 6 amps per coil at peak output.Ignition System (primary circuit) – 6 to 20 amps.Fuel Injectors – 4 to 6 amps peakElectric Fuel Pump – 4 to 12 ampsElectric Cooling Fan – 6 to 30 amps
