Can You Jump Start a Car With a Bad Alternator?

When the car starting battery is discharged, one can jump-start a car with another car's battery using jump-start cables or using a car jump starter.

However, there are many reasons why the battery is dead in the first place, including parasite load over a longer period of time, a bad or simply old battery, or a bad or faulty alternator.

If the battery is almost new and there are no loads that would discharge the battery, and one nonetheless has to jump-start the car, there is a great probability that there is something wrong with the alternator or charge regulator, leading to the question if the car can be jump-started with a bad alternator...

Updated: January 25, 2023.

The answer is simple - Yes, You can jump-start a car with a bad alternator.

But, can You drive such a car, and for how long, that is another question.

Bad or Faulty Alternator or Charge Controller

When the car's alternator or its charge controller goes bad, the car's battery is not being charged, or it is being insufficiently charged.

Modern cars depend on electric power heavily, not just for powering alarms, GPS, maps, multimedia devices, etc., but also for powering the car's central computer, fuel pump, and a few more critical components which doesn't require much power, at least not as some multimedia devices, but nonetheless, they do require a reliable source of electric power.

What happens when the engine is running, and the car battery voltage starts to drop?

Signs of Alternator Problems

Well, first, the car will warn the user about a voltage drop, but the engine will continue to run - in most cases, this message will be in the form of a warning or stop message.

But, after some time, either the fuel pump will stop causing the engine to stop, or the central computer will stop causing the engine to stop too, or the computer will shut down the engine to prevent any damage and signal "General Electric/Electronic Failure" or will give some similar error message.

Of course, a large "Stop" on the dashboard will signal the user to stop the car before all of this happens, but if You wondered what would happen if You decide to ignore the dashboard's "Stop" sign, now You know. :)

However, if the alternator is providing at least some power to the car, it is possible to jump-start a car and drive it directly to the car's repair shop - just be sure to turn off everything that is not essentially needed.

Note: even before the onboard computer warns the user about upcoming problems, the user can spot typical signs of a bad alternator/discharged battery in the form of slow/lazy starts, dim lights, frequent engine stalling, the smell of burning wires or rubber, whining and other odd noises, malfunctioning electronics, etc. If You spot any of these signs, take your car to the mechanic to run some diagnostics. It can save You plenty of time in the near future.

How to Jump-Start a Car Using Jumper Cables?

To jump-start a car using jumper cables, one needs another car with a good battery and working alternator.

So, park the second car (one with a good battery) next to the first car (one with a discharged battery) and:

• connect a red clamp of the jumper cable to the positive (+) terminal (usually the red one) of the dead battery,
• connect the other red clamp of the jumper cable to the positive (+) terminal (the red one) of the good battery,
• connect a black clamp of the jumper cable to the negative (-) terminal (usually the black one) of the good battery,
• connect the other black clamp of the jumper cable to the negative (-) terminal (the black one) of the bad battery,
• let the second car run for a few minutes - this will provide some "surface charge" to the bad battery and help both batteries crank the engine.

Note: while being connected, the alternator of the second car is under a larger than-normal load, so don't keep the cars connected for a very long time.

After the first car cranks its engine, disconnect the cables in reverse (start with the negative clamp from the bad battery).

If the first attempt was not successful (the electric system of the first car didn't have enough charge/power for the car's starter to kick in), let both cars stay connected for a few more minutes - just be sure to monitor the dashboard of the second car, just in case - and try again to crank the first car's engine.

If it starts, good - disconnect the jumper cables,  turn off everything non-essential and drive to the car repair shop.

If it doesn't start - disconnect the jumper cables, and call the tow service.

Disconnect the cables in reverse, starting with the negative clamp from the dead/bad battery.

How to Jump-Start a Car Using Car Jump Starter?

Car jump starters are small and compact devices that feature lithium rechargeable batteries and control electronics that enable them to provide insanely strong currents, considering their weight and size - and they are relatively cheap as well.

A good car jump starter can fit most glove compartments and backpacks and can be carried around - just be sure not to leave them in the car during freezing cold days or hot summer days.

If the car's engine doesn't want to start, connect the jump starter to the battery (starting with the positive clamp!), turn ON the jump starter, let it analyze the battery, and when the jump starter signals, crank the engine - it is that simple.

Note: lithium jump starters are very easy to use, but their exact starting procedures may differ a little bit, so keep the Owner's Guide with your car jump starter, just in case.

Disconnect the jump starter and if You suspect that the bad alternator and/or charge controller are the reason for heaving dead battery, drive directly to the car's repair shop - just be sure to turn off every non-essential electric load.

Frequently Asked Questions (FAQ)

Here are some of the most common Frequently Asked Questions (FAQ) about cars, alternators, batteries, and similar.

Will a car start with a bad alternator?

If the engine's starter is provided with enough energy, it will start the engine regardless of the alternator's condition.

Now, does that energy come from the onboard battery, or is it provided by jumper cables or a portable car jump starter, that is another question.

Will a car run with a bad alternator?

Maybe, depending on the remaining battery charge - if the alternator is bad, the battery must supply power for various electric and electronic systems, which can drain the battery rather quickly, especially if the battery was already rather discharged.

How can I tell if my alternator is bad?

After starting the engine, the car's voltage is still very low. Also, revving up the engine doesn't increase the line voltage. Of course, You can also run diagnostics.

Would a bad alternator cause no start?

A bad alternator causes discharged battery, which is then unable to crank the engine properly, so yes, one can say that a bad alternator causes the engine not to start.

How do you tell if it's your battery or your alternator?

Run diagnostics, for short.

However, if You manage to crank the engine using jumper cables or a car portable jump starter, and the car's voltage increases, especially when the engine revs faster, then the alternator is (probably) good, and the battery is either discharged (let the engine run for at least 30 minutes, let it rest for 5 minutes and then check the voltage and try to crank the engine) or it simply can't accept a charge - it is dead.

If You manage to crank the engine, and the car's voltage doesn't change, either the alternator or charge controller/regulator are bad and must be repaired/replaced. Also, since the battery is discharged, consider recharging it ASAP to prolong its operating lifetime.

Long Story Short: As one can see, there are several ways of cranking the car's engine with a discharged/bad battery.

However, the question is if that car is safe for driving - after cranking the engine, check the dashboard and if there are no errors or warnings, drive to the repair shop.

Also, if You are using car jumper cables, having two cars connected for a longer period of time can overload the alternator and/or charge controller of the car with a good battery - again, keep your eyes on its dashboard.

Whatever You do in such a situation, stay safe - after all, it is your own responsibility.