Yes, gently revving the donor car's engine (to about 2,000-2,500 RPM) helps by increasing the alternator's output, providing more electrical power to help start the dead car, but avoid aggressive revving, which can stress electronics or potentially flood older engines. It's a helpful technique, but the primary charge comes from the battery connection, and turning off accessories is crucial.
The alternator charges the battery, and its charging rate is generally determined by the engine's idle or running state, not by revving. Revving might actually put unnecessary stress on the engine and components.
Let it run for 1-2 minutes and then rev the engine slightly by pressing on the gas pedal lightly. Start the engine of the car with the dead battery (it may require a few attempts). Caution: Running an engine with a dead battery may damage the cars' electrical and computer systems of some cars.
The short answer is no, you cannot take jump leads off with the engine running. This is because jump leads transfer electricity from one car battery to another, and they must remain connected while the process is taking place.
While it is technically possible to charge your car battery by just idling, it's not very efficient. You'd likely have to leave your car running for hours to even come close to a full charge. This is due to the amount of charge that is initially lost when cranking the engine.
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Revving the engine will only increase the voltage by a few hundredths, which won't be enough to revive a dead battery. Most automotive batteries last anywhere between three and seven years, but some practices can reduce their lifespan.
The 20/80 battery rule suggests keeping lithium-ion batteries (in phones, EVs, etc.) charged between 20% and 80% for daily use to slow degradation and extend overall lifespan, avoiding the stress and heat from frequent full charges (100%) or deep discharges (near 0%). While not strictly necessary for short-term use, following this guideline minimizes battery strain, leading to better long-term health, though modern devices have built-in optimization features.
The biggest danger is spinning a bearing. There are metal shells that sit in the block and line all the contact points, and the shaft is suspended on a film of oil. If you spin it too fast while the oil is cold and thick, it has extra drag and the shells can get stripped out of place and spin with the shaft.
If your battery is completely dead, jump-starting the vehicle and letting it idle for a few minutes won't fully recharge it. Instead, after jump-starting it, drive for at least 30 minutes or until you reach your destination to partially charge the battery, then connect a battery charger to complete the recharge.
If the battery seems reasonably clean, stick it to your tongue. You will feel a slight jolt and metallic taste. You'll probably need to calibrate your tongue by testing a fresh battery and a dead battery to get an idea of what to expect.
Signs Your Car Battery Is Dying and When to Replace It
A car battery can drain when the car is off due to parasitic draws (interior lights, radio, faulty relays, aftermarket accessories like dash cams), a failing alternator preventing recharge, loose/corroded connections, or simply an old battery reaching the end of its life, with normal drains from the clock and alarm systems becoming problematic when combined with other issues.
Conclusion: Safety First When You Jump Start Car Battery
By avoiding these five common mistakes—incorrect cable order, insufficient running time, vehicle contact, attempting to charge damaged batteries, and neglecting safety equipment—you can safely get your vehicle running again without causing injury or damage.
When connecting a car battery or jumper cables, always connect the red (positive) cable first, then the black (negative) cable to a ground point away from the battery to prevent dangerous sparks from flammable battery gases. When disconnecting, do the reverse: black (negative) first, then red (positive).
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Idling can charge a car battery, but it's slow and not nearly as effective as driving. The alternator produces more power at higher RPM, so you only get a partial and gradual recharge at idle. Factors like battery age, weather, and accessory usage affect how much charge you recover.