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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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    In a pinch, you usually don't have enough cars or jumper cables to do this, but I presume you want to know if this is technically feasible?
    – Zaid
    Dec 22, 2016 at 15:33
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    It's usually a factor of not having enough jumper cable (too thin of cable will not pass enough amperage), not enough amperage output from the donor car.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Dec 22, 2016 at 15:48
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    @Pᴀᴜʟsᴛᴇʀ2 is correct: I actually had exactly this issue recently. Friend called for a jump, I forgot my 2AWG jumper cables (I could jump start an aircraft carrier with those suckers), his wimpy little cables couldn't draw enough current to crank the starter. We had to let the cars sit and charge the dead battery for several minutes before it would start.
    – user4896
    Dec 22, 2016 at 16:30
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    @Snowman: that's really the proper jumping procedure. Even with thick cables, often the alligator clips are usually the limiting factor for how much power you can deliver.
    – whatsisname
    Dec 22, 2016 at 20:05
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    Incidentally, assuming that the non-starting car has a good battery, then all you need is more time. The combination of the donor car and cables may not have enough juice to start the car, but likely has enough juice to charge the other car's battery. Just ensure that the donor car is running at a high enough rpm to produce 13+ volts and give it time (sometimes up to 15-20 minutes). Eventually the battery will be charged enough that the combination of the two will start the car.
    – SteveJ
    Dec 22, 2016 at 20:44

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To answer your question, it will not cause any issues as long as the cables are connected correctly. If both donor cars are connected in parallel (negative to negative/positive to positive) to the recipient vehicle, there should be no issue.

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    While we're here, you might want to explain that they want to be careful about the order of the connections, not just the topology...
    – user541686
    Dec 23, 2016 at 2:41
  • @mehrdad, why? I don't see an order that matters
    – Sam
    Dec 23, 2016 at 17:29
  • @Sam: You want to avoid sparks near the battery.
    – Lightness Races in Orbit
    Dec 23, 2016 at 19:17
  • Proper connection order also reduces the chances of a dropped clamp resulting in a dead short circuit and fire/injury/etc.
    – nobody
    Jan 12, 2017 at 23:36
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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:

enter image description here

Source

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.

Two cars in parallel

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.

TL;DR:

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.

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  • @dlu, thanks for the edit... that was embarrassing. :)
    – anonymous2
    Dec 22, 2016 at 16:37
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    Well defined answer. Very well done.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Dec 22, 2016 at 16:37
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    @anonymous2, but not nearly as embarrassing than my "please read the FAQ for me" block heater question yesterday… I only caught your's on a second reading – not quite sure what made it catch my eye.
    – dlu
    Dec 22, 2016 at 16:50
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No problem.

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.

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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)

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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.

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  • What jf the goal is to get the dead car cranking and started as quickly as possible, rather than letting the vehicles sit and charge eachother for a while? You'd still get a roughly combined initial boost when you're trying to pull a few hundred amps, even though the charging situation doesn't end up ideal, right? Like two batteries with 500 CCA capacity each trying to start a car/truck that needs 800 amps to start?
    – Jason C
    Dec 22, 2016 at 22:01
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This depends on the cars

Why you shouldn't jumpstart a modern car

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.

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    That article sounds fishy. They're basically saying, “don't try to fix anything yourself, you're not to be trusted; leave it to us instead”. Now, I wouldn't bet on it that all modern cars receive well to traditional jump-starting, but just saying “you can never do this anymore, because computers” is nonsense. There is no reason the electronics in a modern car couldn't be as robust as those from 30 years ago, in fact advances in solid-state technology have made it possible to decouple each component. If a car completely breaks as a result of a jump start, it's a design failure, nothing else.
    – leftaroundabout
    Dec 23, 2016 at 12:44
  • @leftaroundabout, you say that, but there are also questions here like this where the answer is "jumpstarting can damage the electrics"
    – Separatrix
    Dec 23, 2016 at 12:48
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    Yeah. Again, I don't say that this isn't a concern in certain cars, I'm just saying that we should demand something better. If a car's electronics don't stand a jump-start, then it should be the manufacturer's obligation to fix the broken design they've messed up, not the customer to blame for executing a perfectly sensible DIY action.
    – leftaroundabout
    Dec 23, 2016 at 12:55
  • @leftaroundabout, however it is a risk and it's important that we notify people that the risk exists not just say "go for it, it'll be fine, no worries" then blame the manufacturer if it goes wrong.
    – Separatrix
    Dec 23, 2016 at 13:01
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    Perhaps, except the more people do it the less manufacturers will be able to get away with this rubbish.
    – leftaroundabout
    Dec 23, 2016 at 13:11

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