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A friend who drives a Leaf recently came very close to running out of charge, which prompted another friend to ask if it was possible to "jumpstart" a Leaf from another car. Obviously you couldn't do that in the usual way, but in principle you could use another car as a generator to charge the battery. In practice, is there a convenient way to do this?

  • Even if you could, why would you? – Captain Kenpachi Oct 20 '15 at 9:30
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    Because if you did run out of charge, it would be relatively easy to get another car to show up and give you enough charge to get home. – ShadSterling Oct 20 '15 at 15:53
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    But gas-powered cars are icky and stupid. – Captain Kenpachi Oct 20 '15 at 16:01
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    Perhaps, but for now they're much easier to find than charging stations, and much more portable than power outlets. – ShadSterling Oct 20 '15 at 16:16
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No, you cannot.

Well, you might be able to, but not in a feasible way in any likely manner.

Electric Vehicles commonly have two separate electric circuits.

One running at the normal 12V, which ties to all the common electronics that all other types of cars have. Light bulbs, radios, in many cases also a starter motor for the gasoline engine if it in fact has one those (which the Leaf doesn't if I'm not mistaken).

The other runs at a voltage ranging between 96V and nearer 300V (depending on brand and such), which drives the motors.

For why? You may ask.

Well, if the electric motor is 30kW, which is very modest for where the Electric cars are headed, but I'd imagine a Leaf being somewhere near that, that would be:

  • 30000W / 12V = 2500A at 12V
  • 30000W / 48V = 625A at 48V
  • 30000W / 96V = 312.5A at 96V
  • 30000W / 150V = 200A at 150V
  • 30000W / 300V = 100A at 300V

As you can see, getting that power to the motors takes quite an insane current at only 12V and relaistically it only starts becoming really feasible at 150V. Some cars then have a 96V battery, I believe, and drive the engines such that the final wiring, for the longer part towards the motor effectively runs at hundreds of volts.

But even having the controller doing that right next to the batteries, 2500A for 12V input would mean adding extra support beams, if you look at the cross section of metal needed to sustain that somewhat loss free.

So, if you want to do that you need:

  1. A step-up converter from 12V to whatever is needed (which may differ between brands, unless you use the 230VAC input)
  2. Run your engine at 3000rpm+ to get maximum alternator output (wasting a lot of fuel)
  3. Thick Cables
  4. a HUGE amount of patience (and fuel), since your alternator usually can only supply 1.5 to 5kW of power, depending on your car's size and type, of which some is always wasted by the car itself. (And those batteries usually range from 10kWh to 80kWh, AFAIK)

EDIT/Addition based on your comment:

To clarify, from memory a plug-in Prius has 4kWh spare power, with an actual range of about 15km flat roads (here in the Netherlands is a very good place to get those numbers), which is about 10miles, give or take. In some situations it may be 15miles, and I believe they blue-sky report 18-ish miles themselves. Regardless, the mile charge requirement for such a car is likely between 0.3 and 0.8kWh depending on the trip. Maybe the leaf gets 0.25kWh per mile average, because it has no fuel system to lug around, but I know only people with Plug-In Priusses and Plug-In Outlanders, and factory data is not to be trusted.

It is unlikely the car charging can actually supply 1.5kW to the outside, since the alternators get designed for about ({everything the car needs} + {what could possibly be added})*1.3; so that usually leaves no more than 50% of the actual alternator power, usually less, to be gotten from the car, while running at the engine speed at which the alternator is at optimum.

Note how I say "alternator at optimum" this speed is nearly never the best unloaded operating point of the engine, so your fuel consumption will be very much sub-optimal.

If I'd make a real-world estimation, you could possibly take out 600W (=50A already!!) from any medium-size car, maybe 1kW from a large one, a small/efficient car will not enjoy giving you more than 400W at most. So, let's blue-sky this, knowing it will never work out this positively:

You have a source of 1kW at 12V, or you know what, blue-sky: 15V.

That means: 1000W / 15V =~ 66A

Let's say you have 10mm^2 cables (quite thick for jumpers already) running to the converter that turns it into 300VDC (again, blue-sky, you take the highest voltage that's feasible, to allow a lower current, which allows lower losses, but we'll see that soon enough), these cables are a measly 3meters in total (so 1.5meter each) and connected at the alternator, so no losses inside the car itself (again very blue sky).

The cable then has about 2 mili Ohm per meter, gives a reduction of 132 millivolt per meter, is a total reduction of 0.39V (unfairly rounded down for blue-sky) in the cables. Peanuts, right? Does mean, however that your power has already dropped by 26W:

Power at converter: ~66A * (15V - 0.39V) =~ 974W

And that's not even considering the contact resistance of 5 to 35 milli Ohm per clamp, which would take away a minimum of another 44W. But, we will ignore that as well.

Now, up-converting that to a high voltage is not lossless. Technically at these scales the best you can hope for on any realistic budget is 85% efficiency. So, we'll happily round that up to 90%.

Output Power at the Converter at 300V: 0.9 * 974W =~ 877W.

At 300V that is only: 877W / 300V =~ 2.9A , which you can easily transport over 5meter in a pair of 3mm^2 cables, as they will be about 6 to 7 milli Ohm per meter, making for a loss over 10meters of complete path of only 0.7W, and since at this point we've already imagined away close to 80W in losses, we can easily ignore that. Same goes for connector losses. Also assumed to be zero.

So, at the car we are allowed in this blue-sky world to imagine it being a nice constant stream of 877W at 300V.

It is highly unlikely the car itself has no electronics, since it will have an input range (e.g. 250V to 350V). So, there is the conversion loss again, but probably going the other way, from 300V to 180-ish volt maybe? Either way, if it's only drop or boost, it may be assumed to be around the same 85% efficiency. Again, we'll blue-sky that up to 90%.

So, towards the battery we get: 877W * 0.9 =~ 789W

It's easy to now assume any kind of battery just absorbs that and then delivers that right to the motor. Very forward looking cars have some form of conditioned Lithium based cell, which would offer a base absorption of up to 97% in practice when charged at 1/10th their capacity. Luckily at 18kWh this is 1/10th or less, so that's fine. As a note, there are quite a few brand at the time of writing that still use NiCd, which have a much lower charging efficiency. It would be fairer to say, in a finished storage product featuring Lithium based cells it's likely to hang around 92%, due to the required conditioning and margin over life time. (Over 10 years this margin is still greatly optimistic, by the way!).

But, I'll just use the 97% as the final number: Battery Stored Energy per unit time: 0.97*789 = 765W.

Miles per hour charged, if I'm allowed to switch back to slightly more realistic than perfect blue-sky, with 382.5Wh per mile, would be 2 miles per hour.

Say you ran out only 4 miles away from a place where you'd be comfortable staying until it's charged enough to continue, you'd need at least 2 hours, but then knowing that if it's a bit colder than "specification temperature" for the parts, you may end up running out half a mile before you get there if you're too tight on the time.

And to then completely answer your comment: Bear in mind, that whether you're waiting for a friend to tow you, or a friend to charge you, you're waiting for that friend, regardless. So you're effectively adding 2 hours to that wait time. And it'll have to be a friend with a car that supplies 1kW at a jumper-capable point, so you're already cutting out a group of friends just on that requirement making your chances even slimmer. Although, I do find, that people with smaller cars in certain cultures tend to be happier to wait for 4 hours than people with larger cars to wait for 2, but I'm not a sociologist, so I'll leave that out of consideration.

Oh, and also spending at least 20 times (gut feeling it's more like 100 times) the amount of fuel towing someone with an electric vehicle in "release"/"unclutched" mode over 4 miles would cost.

  • Also, I imagine that 2500 A would generate an insane amount of heat vs 100 A – Zaid Oct 20 '15 at 12:10
  • @Zaid That is a consequence of the high losses. If you have a 10cm cross secotion cable at 2500A the losses may be limited and heat as well, but a cable like that will not be feasible in any way. A normal car-type cable (up to 10mm^2 usually - Though 40mm^2 does occur) would definitely generate a good amount of loss and heat. – Asmyldof Oct 20 '15 at 12:34
  • I don't want to drive the car on a 12v battery, I just want to charge its battery. A typical wall outlet at 110V is limited to 15A, which is 1.65kW, and can give a full charge overnight. If another car can provide 1.50kW and an appropriate adapter were available, I would imagine it would be possible to get a few miles worth of charge in under an hour. I don't see in your answer why it wouldn't be possible, only why it wouldn't be easy. If you've got a friend nearby and will have to wait hours for a tow, it might be better to be able to charge from the friends car than to wait. – ShadSterling Oct 20 '15 at 16:15
  • @Polyergic The point being, that most cars do not have 1.5kW available to the outside world, even at 3000~6000rpm. And if they did, you'd need to convert that and loose an approximate 20% or more in that, added to that that even 1.5kW constant power at 12V is much more than a jumper cable should be made to endure. And that 1.5kWh for most E-cars is about 0.5mile, assuming charge efficiency of 90% (it's lower) that still requires 2 hours of very wasteful operation of the charging car to get less than a mile. – Asmyldof Oct 20 '15 at 16:28
  • Are we sure that a converter would even be necessary? I know that electric cars use power from the main battery to keep the 12V battery charged. Does that work in reverse? I doubt it because of the needs of the main battery, but you would be able to charge the main battery by "jumping" the 12V battery, if that were the case. – Poisson Fish Oct 20 '15 at 17:55
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Old post but I want to leave my experience since I did this in real life. I have a fiat 500e (well, my wife does). Same, some day she was so close to run out of battery. Nowadays there is an app called chargepoint or others that will show you the charging stations and the "open outlets" where you can charge your car, so, there is so little change that you are running out of charge without making it to any station or outlet. BUT, i wanted to do it anyway, "just in case" and because I live in florida and hurricanes happens and I always want to have a plan B. So I bought an inverter, 3000W 12v to 110v. Simple as that. I have a Jeep Grand Cherokee Limited 1999, with stock alternator of 120a, but I did a common upgrade for the alternator (direct fit) of the dodge, that will be 160a, for less than 90U$. The standard 110v charger is 12a or 1350w. So an efficient inverter will need a little more that 100a at 12v to generate that 12a at 110v. I connected the inverter with the shortest and bigger cables that I could, and with the jeep on, I connected the fiat, and HUALA, it charges. Its 110v charge so will charge around 6 miles each hour. Not best scenario.

Another thing that I thought is to pull the car. If the car is on, will generate energy itself when moving, to recharge the battery, in this case the regenerator of the fiat could even regenerate up to 36KWh (the screen says that...) so, pulling hard from the car will full charge the car in less than 40 minutes... hahaha. Anyway, I was thinking if I could use 2 inverters, to generate 240v output in 2 fases, to charge quicker the car, adding a secondary alternator.

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I am not an expert on electric vehicles, but I believe that they are only equipped to charge of mains (110v, 220v) household electric. Since non-electric cars usually only have 12v electric circuits, this would not be possible without an 'inverter' to step the voltage up to mains voltages. Even with this, you would probably have to sit with 'jump leads' on for several hours to get a charge to get you home. You would probably be better towing the electric vehicle to a charge point.

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