Rick's Free Auto Repair Advice

Can a jump start damage a car?

Can a jump start damage your vehicle or the other vehicle?

The simple answer is yes; jump start damage is real. But it’s a bit more complicated because the likelihood of damage is related to the age of the vehicles and how discharged the “dead” battery is. We’re not talking about the kind of damage you cause by incorrectly connecting jumper cables, that type of damage can run from something as simple as a blown fuse to fried computers.

No, I’m talking about the kind of damage you can cause even if you follow proper jump starting procedures. I’ll get to how that damage occurs in a minute. But first a warning; it doesn’t matter if you’ve given or received jump starts for the last 20-years without damaging a thing. What you have done in the past is irrelevant. The electronics in late model vehicles are completely different than earlier vehicles.

Understanding voltage drop

Let’s start with the concept of voltage drop on a perfectly good car with a fully charged battery. If you connect a scope lead to the positive battery terminal and ground and crank the engine, the scope trace shows that battery voltage drops to around 3-volts for about 15-ms before climbing back to 6-volts and then ramps up to 12-volts. See the chart below.

cold cranking battery voltage drop chart

This cranking voltage drop is a really important concept to consider in a jump starting situation, especially if you try to start the dead vehicle while the donor vehicle is running. If the donor vehicle is running and the cables are attached to the dead vehicle while you  crank the dead vehicle, the voltage in both batteries will drop like shown in the above graph. Since the donor vehicle is running, that vehicle’s voltage regulator will respond to the voltage drop by boosting field coil current to the maximum. When you stop cranking the dead vehicle (or it starts), the alternator in the running vehicle will sense the higher voltage and command a load dump within the next 85-ms. It’s that load dump and accompanying voltage spike from the collapsing field coil that can damage the solid state electronics in the donor vehicle.

All newer vehicles are vulnerable to damage from voltage transients

Newer vehicles contain multiple computerized engine, powertrain, transmission computers as well as computerized heater controls, ABS, Stability control, Air bag, entertainment system, memory seats, mirrors, pedals, etc. Just like your home electronics, these solid state devices can be damaged by voltage spikes, RFI, and back EMF.

So let’s start with an examination of how you get voltage spikes when you jump start a vehicle.

Connecting jumper cables with donor engine off

In this case you’re connecting the jumper cables between a donor car that’s NOT running and a dead car (obviously not running). We’ll also assume that you’ll be connecting the cables correctly–positive to positive, negative to good ground point.

As you move the jumper cable clamp closer to make the final connection, the voltage potential between the fully charged battery in the donor vehicle and the lower voltage in the dead battery results in a spark. This is due to breakdown voltage, where the ions in the air at the connection point become conductive. The spark is hot enough to ignite any hydrogen gas under the hood–which is why the last connection should be made far from the battery.

In addition to the possibility of igniting hydrogen, the spark itself causes a voltage spike on the dead vehicle, along with electrical noise on the electrical systems of both vehicles.

Here’s a common scenario with a discharged battery. Once the battery in the dead vehicle  drops below 9.6 volts, the computers and modules often shut down. But the instant you connect the jumper cables, the dead vehicle’s electrical systems “wake up” and draw current.

As the “hot at all times” ECM/PCM, body control module, remote keyless entry and security system modules draw current they activate or deactivate relays. Although this current draw is minor in comparison to current drawn by the dead battery, the quick relay activation/activation can cause transient power surges in the dead vehicle’s electrical systems, as well as RFI interference and some degree of back EMF.

The voltage surges aren’t limited to just the dead vehicle’s electronics. Depending on the state of charge of the dead battery, a large voltage drop in the donor vehicle can cause that vehicle’s computers and relays to shut down and then re-awaken, causing the same types of voltage spikes.

How serious is the risk when neither engine is running? It’s pretty low if the dead battery is just slightly discharged but still above the 9.6-volt threshold, but it’s not zero. In that case, there’s a low voltage differential between the two vehicles. However, the chance of voltage spikes and damage increases as the voltage differential gap widens.

The possibility of jump start damage is much higher if one or both of the engines are running when the jumper cables are connected or disconnected.

Jump start damage with donor engine running.

Now consider the possible jump start damage when automotive voltage transientsthe donor vehicle is running. The instant you make the final connection to the inductive load (dead battery), the donor vehicle experiences a voltage drop that’s directly related to the dead battery’s state of charge.

In response to the voltage drop, the voltage regulator in the running alternator commands full field excitation, which produces a short (320µS) transient voltage spike of 80 to 300-volts, more than enough voltage to damage the electronics in both vehicles. The voltage regulator recovers from the over-voltage situation by lowering field current to reach a safer charging voltage of around 15-volts or less.

Let’s assume the donor and dead vehicle’s electronics survive the extremely short voltage spike created when connecting the jumper cables and you leave the jumper cables connected to finish charging the dead vehicle. Here are the next three options:

1) Try starting the dead vehicles with the donor engine running and cables connected

2) Disconnect the jumper cables from the dead vehicle while the donor engine is running

3) Shut off the donor engine and disconnect the jumper cables before trying to start the dead vehicle.

In scenario #1, cranking the dead vehicle with the donor engine running causes the same cranking voltage drop and voltage regulator surge response (80-300-volt surge) as noted in the voltage drop chart above. That surge would more likely damage the donor vehicle’s electronics.

In scenario #2, disconnecting the jumper cables would cause the donor vehicle’s voltage regulator to load dump the field coil current, causing a voltage spike around 120-volts or less in that vehicle’s electrical system, an electronics damaging voltage spike.

Scenario #3 is the safest way to avoid generating a voltage spike when using jumper cables.

Is jump start damage a given?

No. I’m not saying that every jump start will damage your car or the dead vehicle. There are too many other factors that can eliminate electronics damage, like the degree of surge protection built into the vehicle’s electronics, the condition of the surge protection components, and the degree of voltage difference between the jumping and dead battery.

But as you can see from these examples, the safest way to jump start a dead vehicle is to connect the cables with both engines off. Then start the donor vehicle and let it run to charge up the dead battery and shut off the donor vehicle before trying to start the dead vehicle. Also, shut down both vehicles before removing the cables.

So, in effect, you’re using the good battery and alternator in the donor vehicle as a “jumper pack” to boost the dead battery.

However, even if you follow this safe procedure, you still have one issue to worry about: overheating the donor vehicle alternator. Alternator pulley speed is approximately three times crankshaft RPM, so it’s turning at 1,800 RPM at engine idle speed and 6,000 RPM as cruising speed. But an alternator can only output a maximum of 35% of its rated capacity at 1,800 RPM (600-700 engine idle RPM).

Worse yet, since many of these jump starts are conducted in winter conditions, it’s not uncommon for the donor vehicle to be using headlights or the blower motor. Here’s the current draw for these electrical accessories:

Headlights (low) 15-amps/20-amps high, Blower motor on high 20-amps, Ignition system 6-amps, Rear window defogger 25-amps, Heated seats 5-amps per seat.

So you commonly have a situation where the donor vehicle has its headlights on and possibly the blower motor while jumping the dead vehicle. In that case, the donor vehicle alternator is running at maximum field at idle speed. That causes the field coil windings and rectifier diodes to overheat.  The longer you run in this condition, the greater the chances of damaging the donor vehicle’s alternator.

Avoid jump start damage with a jumper pack

Using a jumper pack to start the dead vehicle eliminates most of the damaging conditions noted above. Connect the jumper pack cables just like you would regular jumper cables.

jumper pack, jump start a car, jump dead batterySome jumper packs have an ON/OFF switch. In that case just  secure the positive clamp to the positive battery terminal and the negative clamp to a good metal ground.

If your jumper pack doesn’t have a switch, connect the positive clamp to the positive battery terminal and the negative clamp to a good ground.

Since the jumper pack contains a lead/acid battery, the jumping voltage is limited to 12.7 volts. So you won’t have a huge voltage spike like you might when jumping with a running donor car. That’s dramatically reduce the chance of blowing a computer.

Click HERE to learn how to buy a jumper pack

jumper pack for jump starting engines

Jumper Pack Care and Feeding

Consumer grade jumper packs are build with off-the-shelf sealed lead acid batteries. The specs show they can provide the peak amperage. But they really weren’t designed to provide that kind of power for very long. So the plates inside the battery heat up, warp, and short out. Expect short life out of those inexpensive consumer grade models.

Professional grade jumper packs are made with specially designed sealed lead acid batteries that can put out the amps for long enough to get the dead car started. Yes, they’re more money. But you’re getting more for your money.

But know this: All jumper packs, consumer and pro, must be maintained. The very nature of lead acid batteries is that they lose a certain percentage of their charge every day, even if they’re not in use. If you let a lead acid battery drain down to nothing, you’ll permanently damage it. Kiss the warranty goodbye.

So you must charge it at least once per month. You can do that by plugging it in to AC power in your house or garage until its fully charged (don’t leave it on the charger full time). Or, some models provide a charging cable to use with your cigarette light or power port in your car. So you can leave the jumper pack in your car over the winter and just plug it into the lighter occasionally while you’re driving to keep it at full charge. Then, when your work buddy asks for a jump, grab it and it’ll be ready to go.

©, 2013 Rick Muscoplat








Posted on by Rick Muscoplat


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