Rick's Free Auto Repair Advice

Can you charge your car battery while parked

You can charge your car battery while parked, but it takes a LOT longer than you think

You can charge your car battery while parked, but but plan on idling it for almost 4 hours to fully recharge a dead battery.

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 your battery while parked?

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.

It takes a lot more power than you think to run your engine

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 on a late model vehicle still 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 while parked, even 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 just 15 amps left to recharge the battery.

If your vehicle has a typical 60Ah battery that’s 80% discharged and charge it from your alternator using the 15 amps available, well, here’s the math:

60Ah x 80% discharged = 48Ah needed to bring it back to full charge. 48Ah÷15 charging amps = 3.2 hours to fully recharge.

It makes no sense to charge a car battery while parked

Clore PL23220 battery charger

Buy a 10 amp Clore 2310 battery charger for $85 or a 20-amp Clore battery charger for $81 at amazon

In my opinion, it’s just crazy to run your engine to recharge a dead battery when you can do a much better job using a battery charger. They just not that expensive.

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 pair
Parking/marker lamps 3.2 amps
Electric Rear Window Defroster – 10 to 20 amps
Windshield wipers – 2 to 10 amps depending on load
Heated Seats – 3 to 4 amps per seat
Heater A/C blower motor 30 amps
Air Conditioner Compressor Clutch – 2.5 to 5 amps
500 Watt Sound System – 13 to 30 amps
Electric 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 peak
Electric Fuel Pump – 4 to 12 amps
Electric Cooling Fan – 6 to 30 amps

©, 2022 Rick Muscoplat

Posted on by Rick Muscoplat

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