If I'm in a pinch, and I have a small car that can't put out enough cranking amps to start a bigger one, could I connect two cars in parallel to start the bigger one?
Will it mess up the alternators or something?
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When you jump start a car, you are always putting the cars in a parallel circuit. Here's an illustration of parallel vs. series:
Think of the bigger car's battery as the one on the left and the smaller car's battery as the one on the right. When you connect jumper cables, you connect the positive terminal on the bigger car to the positive terminal on the smaller car and the negative terminal on the bigger car to the negative terminal on the smaller car.
Thus, you can think of the two positive terminals as being one connection point and the two negative terminals as being one connection point. From that connection point (in the case of a car, directly on the bigger car's battery; in the diagram, halfway between the two batteries), the electricity runs to the car's starter.
In order to hook up the car's batteries in series, you would have to disconnect one of the starter's terminals from the bigger car's battery and connect it to the smaller car's battery and run a single cable between the two batteries, connecting + with - or vice versa.
When you need to jump the car, what is lacking is not voltage, what is lacking is amperage. When you connect the jumper cables in parallel, you are effictively adding the amperages of both batteries together, giving a greater potential energy force to turn the starter. So if the dead car's CCAs (Cold Cranking Amps) is around 100 and the live car is around 250 and it takes a minimum of 300 CCA to start the car,
100 CCA + 250 CCA = 350 CCA > 300 CCA
... which means that you have enough power available to start the car. Extra power simply won't be drawn into the circuit.
Now lets add in the possibility of two cars jumping a single car. Now we have three amperages to deal with: Bigger car, smaller car 1, and smaller car 2 (BC, SC1, SC2). For arguments sake, let's say the BC has 100 CCA, the SC1 has 150 and SC2 has 150. If we connect them all together, we end up with:
100 CCA + 150 CCA + 150 CCA = 400 CCA > 300 CCA
Same principle. Batteries connected in parallel add available amperage, batteries connected in series add voltage. Having extra amperage available won't hurt a thing – in fact it is solving the exact problem you have.
Essentially, connecting batteries in parallel is exactly what you do when you jump the car. You will not blow anything by having additional amperage (cranking capacity) available to your starter.
You just get more amperage flowing to your battery which will charge it up faster and send more energy to the starter motor which will make it easier to start because the motor will turn faster because it has more amps available.
As said in comments, make sure you have large enough cables to send the amperage over, but you should be able to attach both cars to your battery at the same time if you hook them up carefully.
Getting a second car lined up within a cables' length (and still being able to get in and clip on) wouldn't be instant. That time would probably be enough to top up the flat battery unless it's completely dead.
If you do decide to do this, the cables should be in parallel as well -- i.e. the donor cars should have separate cables to the recipient. Otherwise you're trying to pull all the current through the one set of cables that's probably the limiting factor in the first place. Getting a second set of jumper cables onto the recipient battery may be a bit of a squeeze if you've got good solid clips.
If one of your donor cars has a really modern charging system (the sort that is programmed with the battery details) I would avoid doing this at all, to avoid the risk of confusing the charging system if it tries to put out one voltage and sees a higher voltage (before involving one of these cars ina jumpstart at all it's probably worth checking the manual)
I suspect it won't catch fire (batteries, regulators and alternators are quite robust)
but it won't do you much good.
One of your donor cars will have a less charged battery / higher terminal resistance / smaller alternator and as a consequence, deliver less (on load) voltage than the other => current flows from high voltage to low, so the effect is that the strongest donor car will, as well as trying to start the dead vehicle, be trying to charge the weaker donor car.
EDIT: Nah, I'm (mostly) wrong. I think it depends on the ratio of the voltages - so with two reasonably good batteries, the current from both will flow to the dud one.
Jaguar warned us that you could damage the whole wiring harness if you try to jump-start one of their cars, and quite frankly that warning is probably applicable to most vehicles built within the past five years.
I was also informed by an AA (UK) patrol that it can damage the ECU in the 'donor' car as well (in this case he specifically referred to a Ford Mondeo). With modern cars in a case like this you actually risk writing off all three vehicles.
A five-year-old Audi was recently taken to an NRMA Approved Repairer with an electrical system so badly damaged by an attempted jumpstart that, in the end, it was actually cheaper to write the car off rather than repairing the damage.