CarTalk.com Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Battery

can you jump a car off.if the alternator is bad.

You can but it won’t go very far, the engine needs some electricity to keep running and if neither the battery nor the alternator is supplying it, the engine will die.

If you fully charge the battery, you can drive for awhile but car batteries are not designed for deep cycling so it will shorten the life of the battery. You can do this if you just need to get the car home or to a garage for repair.

Many modern cars have sophisticated electronics that can be ruined by jump starting. A fully charged battery may run the car for a while, or may not at all if the computer management system requires a working alternator. Some high-end cars such as BMW and Jaguar need a software re-program when you replace the battery. The charging circuit computer needs to know it’s a new battery and re-map the charge profile.

Just another way for dealerships to make more money.

It will only hurt the electronics if the jumper cables are hooked up backwards, even then it is more hazardous to the vehicle doing the jumping rather than the one getting the jump. I always hook up the cables with both engines off, safer for the jumper that way. I connect the +cables first, then the black cable to a ground point on the engine of the dead vehicle, then to a ground point on the jump vehicle. Then I start the engine on the jumper.

I always use the engine for the ground in case there is a spark. All the sensitive electronics are grounded to the chassis so if there is a surge, I want it to be as isolated as possible.

Since I rarely need to use jumper cables, like four to five years between each use, I now carry those little 8’ long 10ga cables with the small clamps on them. This has a couple of advantages, they don’t take up very much room and they limit the amount of current that can surge from one vehicle to the other. You have to run the jump vehicle for at least a minute to transfer enough charge to the dead battery so that it can start the car with the dead battery, but to me that is better for these modern cars anyway.

It can take as much as three minutes to transfer enough juice for some larger engines, but since I don’t make a living jumping dead batteries for people, its not a big deal.

Actually, power surges can damage the computer electronics during jump starts. There are jump start systems available that prevent this. Michelin offers smart cables that prevent power surges and even determines what pole the cables are connected to. Connect the cables to the good battery first, then connect one cable to the positive pole of the bade battery. The remaining cable is connected to clean metal on the engine.

Your question in the form of a text message didn’t make it clear whether you meant the bad alternator was on the car doing the jumping or being jumped.

-If you’re jumping a car with a dead alternator, you will be able to drive it for a while as long as you put enough charge in the battery before trying to drive it to last a while. Obviously the more accessories --lights, heater fan, etc. that you use, the faster the battery will die with nothing replenishing it. On old cars you could often drive until the battery was so dead it would barely light up the headlights. On newer cars with computers and fuel injection (anything after about 1984), these systems need a steady supply of power to run the electric fuel pump, injectors, and computers. Plus the lower the voltage drops, the more confused and erratic the system will behave. The computer needs a steady ‘reference’ voltage to make sense of the inputs from the engine’s sensors. A modern vehicle will not run very well or nearly as long with a discharging battery as an old car with a mechanical fuel pump, carburetor, ignition coil, and points or simple electronic ignition. Very early cars (turn of the 20th century) sometimes had a magneto ignition like your lawnmower and would run with a totally dead battery or none installed. The battery was just for lighting, etc.

-If the car you’re using to jump a dead vehicle has a bad alternator, it might work, but the battery voltage needs to be higher than the discharged vehicle that needs a boost. There has to be a difference in “pressure” for current to flow to the discharged vehicle. Obviously this is going to deplete the battery on the vehicle with the bad alternator if it is doing the jumping.

It appears that my last post got deleted here, I didn’t break any rules with it so I am going to repost with as much as I remember.

The procedures that jt lists are the accepted way of connecting jumper cables. These procedures have been around long before cars went electronic and no one has questioned them.

Well I am questioning them. I think they are wrong, they were wrong to begin with and even more wrong now with the sensitive electronics in todays cars. When you hook the cables to the charged battery first, there is maximum difference of potential on the cables, the cables are at their maximum charge.

The discharged battery, if it is any good will try to charge itself chemically. That reaction will generate hydrogen and hydrogen sulfide gases, more so than a fully charged battery. I would make the last connection as far away from the dead battery as possible, and that means the last connection should be on the vehicle with the donor battery.

I start with the positive terminals first. Then I double check the that the terminals are connected correctly before connecting the negative terminals. I connect the negative terminal of the dead car first at the engine block. I make the last connection to the donor vehicle at its engine block with the engine off.

It is the last connection that has the most potential to do harm. Depending on the state of charge of the dead battery, usually they are not completely dead, just too low to start the vehicle, there will be a spark. Some think this is the source of the surge, but if the donor engine is off, there will be no surge significant enough to do any damage.

With all the electronics grounded to the chassis, connecting at the engine block will absorb any surges before they travel very far. Remember, the surge is limited to the 12.6 volts of the donor battery.

The spark however does generate a burst of EMI centered around 300 MHZ, which is the frequency most damaging to microcircuits. The electronics is well shielded from EMI that is broadcast. The only issue left is the EMI that tries to travel through the wiring and ground. Again, the spark is on the ground and the mass of the engine block will quickly absorb this signal. It will dampen out within a few millimeters of the source of the spark.

After the last connection, I start the engine in the donor car first, let it put a small charge on the dead battery 1-3 minutes. Then we start the dead vehicle.

I like the smaller jumper cables as well. but as I stated in an earlier post, I don’t use jumpers very often. But so far, I have not damaged any vehicles.

@jtsanders
The electronics are not “sensitive” as you claim. The devices that provide power to the computer are rock solid and fail in normal service at less than 1 in 25000 for the life time of the car. Proper jump starting does zero damage. Reprogramming on some cars is only due to volitile memory being used where it should not be used. Stupid design. What “POWER SURGE”? What are you talking about? The power moved and used is exactly the same as when the battery and alternator are brand new. Grounding the cable to the cleanest engine contact is a really good idea but the battery works just as well from an electronic point of veiw. A few ohms(2-200) of rust resistance means nothing at 12 volts, in terms of extra current.

If you need to jump the car, the battery is dead. As soon as you diconnect the jump vehicle, the engine will die again. With a dead battery and a bad alternator, it has no source of electricity.

We might continue with the technical arguments, but the car ain’t goin’ anywhere. Why take the risk?

“The electronics are not “sensitive” as you claim.”

http://www.motorweek.org/features/goss_garage/jump_starting

The first post by Keith, tells Dripper all he needs to know…

For protection, use a $59 jumper battery that has neither the surplus power to do Much damage or cost to bear the loss of itself. It’s pretty much SOP for shops around here to use them. I refuse to carry jumper cables. No one is using my car or boat to jump their vehicle but are welcome to to borrow either of my $39 or $59 jumpers. Neither has failed to start anything yet, including cars, my house generator and a plethora of boats. They also make great little emergency trolling motors for up to an hour running time. One is ten years old, the other new last year out of guilt for the first one lasting so long. The keys IMO; store them inside, keep them charged and take them out only as needed and recharge after use. Don’t expect to turn over a one ton truck unless it just needs a little help.

Throw your cables away and keep one of these and a AAA card handy. Been around to see the results of a battery blowing up…it’s not pretty and you might as well be next to a twelve gauge shooting acid every where.

Btw, the very first sign of a “slow” or non strart, get the battery and system checked and parts replaced or mainatined as necessary. Don’t mess with having to jump it any more then getting to a shop or repairing/replacing it yourself. As my dad always said, a battery never gets better on it’s own. For some strange reason, some drivers will drive around with jumper Cables knowing their battery is weak. It’s being a co dependent to help these types …walk away. That felt good. Just a little dead battery venting as winter rolls into high gear. Oops, almost forgot. Wear eye protection and gloves.