Myth or fact: Does jumping a totally dead battery kill your alternator?

Battery is dead.
You jump start the car.
You let it run for an hour.

Does this kill the alternator?
How exactly does the alternator work harder?
It’s spinning at the same rate.

Connecting a battery to a dead battery does not add much of a charge to it, so regardless of whether the alternator is spinning the same speed, the voltage regulator is commanding full charging trying to bring the battery back up to a normal level. The regulator will determine how much to charge to maintain the battery level. That’s why its best to use a battery charger overnight on a dead battery.

Duplicate post.

I third the answer.

Jump starting can damage the alternator, either in the jumped car, or the one providing the jump. I had this happen to my Corolla, jump starting someone’s car. I can think of two possible explanations. One is that the voltage regulators of the two cars fight with each other. One might be trying to keep the voltage at 14 volts, while the other is trying to keep it at 13 volts. And that might cause a larger than normal current draw. Also the process of connecting up and removing the wires while doing the jump produces current spikes, and these can damage the alternator too. Usually what happens is that most of the alternator is ok, but a diode or diodes gets burned out. Sometimes the voltage regulator (usually part of the alternator these days) gets damaged. Too much current through a diode, or reverse biasing the diode too much can definitely cause damage.

@GeorgeSanJose: Most cars made since the early 90s do not have the voltage regulator in the alternator–the ECU regulates the charging.

It’s a little confusing, but it sounds like the alternator in the car with the dead battery burned out - although it is possible to burn out the alternator in the car doing the jumping.

Some alternators aren’t designed to charge a battery from a fully discharged condition. So, yes, running a car for an hour to charge up the battery COULD kill the alternator.

As has been said, putting the battery on a charger is less risky - the longer the better.

“How exactly does the alternator work harder?”

A battery is a box with two plates in it, one with an excess of electrons stored on, the other with a relative depletion of electrons, and a “dielectric” to control the flow of electrons between the two. An alternator charges the battery by creating a flow of electrons by using interacting magnetic fields (collapsing magnetic fields “induce” electricity).

The work done in an electrical circuit is done by the moving electrons, called a “current flow”. The more “work” that needs to be done, the higher the current flow has to be to do it. The current amount, the current “flow” is determined for a given voltage by the amount of work being done. This relationship is known as “current draw”. A large demand “draws” a large amount of current.

Charging a totally dead battery “draws” a large amount of current. The problem is that flowing current also creates heat energy. This is an energy loss to the system, but in the case of a lightbulb it is a usable byproduct; the heat creates the light. However, an alternator is not designed to dissipate the amount of heat generated by the amount of current being drawn by a totally dead battery for the amount of time it’ll take to recharge it. The heat can damage some of t he alternator’s internal components.

I have recharged a dead battery after a jump by just driving the car and letting the alternator do the work. But it’s been a couple of decades. I have also killed an alternator on a car after jumping the neighbour’s car, though it may have been about to go anyway.

I think the difference these days is that alternators are smaller and crammed into ever smaller under-hood space, yet outputs are higher than ever–today’s stock alternators put out as much or more than high-output alternators of yesteryear.

Back in the 70s a 60A alternator was pretty much the standard on a V8 vehicle. In the 80s we saw 80A+ on some vehicles. The alternator case was much bigger too. Now a luxury car may have 120A output or more with a much smaller case and buried in a crammed engine compartment. Engines are running ever hotter too for emissions and fuel economy. They just can’t dissipate heat like they used to, and a 60A alternator is somewhat self-limiting–even at full load for a long time it’s not nearly as likely to overheat.