Can an Alternator Charge a Dead Battery?

An alternator in good working condition can keep your ca’s battery charged for many years. If your battery is drained and unable to start your car, perhaps, you left the dome light on or your car has been parked for several weeks.

Can you rely on the alternator to charge it (after jumpstarting)?

Are there any risks you are exposing your alternator that you may not be aware of?

Quick answer: While the alternator can charge a drained battery, you risk damaging the alternator as a result of the demands to supply a large current over a long period to the battery.

Read on to learn how else you can charge a drained battery without exposing your alternator to the risk of damage, and how long it takes to charge a drained battery by driving.

Dead vs. defective battery

A dead battery in this case is one that has a low charge and has a voltage of about 11-12V which is not enough to start the car. The battery can still be used once its charge has been replenished, i.e. it is not defective – physically damaged – cracked, leaking, or shorting cells.

Why an Alternator May not Charge a Dead Battery?

If the battery is discharged, depending on the charge level, the alternator rotor may not receive a high enough voltage that is necessary for the alternator to create the magnetic field and generate the current to charge the battery.

Note, this voltage level is needed only initially. Once the alternator has starter running and generating the current, it is not needed.

This explains why after jumpstarting a car (and the engine is running and battery voltage still low), you can disconnect the jumper cables and the car engine will keep running and the alternator will charge the abttery.

Will It Charge a Completely Drained Battery?

If the car battery is completely discharged then no because the battery needs to have a high enough voltage to start the car and alternator to charge the battery.

is It Recommended for an Alternator to Charge a Dead or Flat Battery?

Once the car has started (after jumpstarting), the alternator can charge a drained battery but beware, doing do puts enormous strain on the alternator.

Several alternator manufacturers – Denso, DB and Mecc alternators discourage using the alternators to charge discharged batteries. This is because it has to supply a high current for long period which it is not designed to do.

Alternators are designed to provide enough current to keep the battery topped up.

Why Charging a Drained Battery is Bad for The Alternator (risks)?

Deeply discharge batteries require high currents for a longer period to charge the battery which can lead to overheatng in the alternator.

Overheating in the alternator can result in premature failure of the alternator when diodes and windings fail from the excessive temperatures.

Does it matter if the alternator is new?

No it does it matter. New and order alternators face the same risk of premature damage when used to charge discharged batteries.r.

How Long it Takes an Alternator to Charge a Dead Battery?

If the battery is discharged to about 11- 12V and not able to start the car, once the car has been jumpstarted, the battery can build up enough charge to start the car after about 30 min of driving or longer.

Driving at higher RPMs increases the charging rates compared to idling.

The actual time it takes depends on the alternator rating (capacity in amps), battery state of charge, and if there are accessories competition for its power – headlamps, sound system and others.

If the battery is deeply discharged and other accessories are switched on the battery will take longer to charge.

Alternative Ways to Charge a Dead Battery

Jumper cables and jump starters

Besides using a set of jumper cables to start your car of the battery is drained, you may use a battery based jump starter pack to start your car and then run it for sometime to charge the battery.

Trickle Charger and Battery Chargers

You may also connect it to a trickle charger – this will take a loner time or use a battery charger rated 2A, 6A, 8A or 10 A to charge the battery faster.

Some battery chargers are able to charge batteries whose voltages are as low as a few volts.

Using a battery charger protects the alternator from the external stress that comes from charging a deeply drained battery.

Final Word

Once the car has been jumpstarted, the alternator can charge a dead battery but this is not recommended.

Charging drained batteries demands high current draws from the alternator for a long period which can cause the alternator to fail prematurely.

Instead, you can consider using a car battery charger to replenish your drained battery charge.

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