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There are some compact lithium-ion powered devices on the market that claim that they can start cars and trucks. Youtube is full of videos with reviews and people actually starting cars with them. And thousands of positive reviews on Amazon... Normally I'd buy one immediately, but given what I know about rechargeable lithium batteries, I just can't believe they are able to start cars.

Someone did a teardown of one here: https://www.youtube.com/watch?v=VlRqu-Iwd0g

They advertise it as a "1000A" jump starter. Some are even rated for 4000A!

In the video you can clearly see the battery is a rechargeable pouch lithium-ion 12Wh battery. Probably 3 cells in series with 2Ah capacity. How can a 3S lithium-ion battery do even a pulse discharge of 100A, let alone 1000A ? The highest discharge cell I've seen on the market gives like 40A. No car will start with 40A. It just doesn't sound possible. But somehow thousands of people can start cars with it. Am I missing something here? Did they patent some miracle technology? How is this possible?


Another video, where somebody did a amp draw test: https://www.youtube.com/watch?v=2mwkIRDCfLU

You can see on the clamp meter that the device puts out 350A for 10 seconds, and it stays above 8V during this time, so I guess it is able to start a car if the video is real. Around 40 times on a single charge. But I'm still puzzled about how can they do this with current li-ion technology?! How can such a small battery output 350A for 10 secs? The 1000A-4000A ratings I figured are part of the usual marketing bs, but 350A is still a lot for such a small li-ion battery.

Another thing I noticed here: https://www.youtube.com/watch?v=_LurYulow0Y The internal battery has printed on it 11.V, 120C, 8000 mAh. 120C?? That's just crazy. Almost 1000A. I'm not aware of any LiPo with anything close to those discharge rates. If anyone knows one please post a link to the official datasheet with such specs.

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  • Very, very large reaction areas. – Ignacio Vazquez-Abrams Feb 18 '18 at 21:14
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    Mike says yes: youtube.com/watch?v=AsTJKSH7mCU – PlasmaHH Feb 18 '18 at 21:15
  • or a large number of cells connected in parallel – John Dvorak Feb 18 '18 at 21:15
  • What I'm curious about is if such a large number of cells is viable economically and from a safety standpoint. – John Dvorak Feb 18 '18 at 21:16
  • How large can be that number if the battery is the size of a phone? Are you saying there are tiny cells out there that are able to discharge large amounts of current? Then how come 18650s can only put out 40A currently? – Alex Feb 18 '18 at 21:23
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Yes, Lithium Ion batteries are capable of delivering the power required to turn over an engine. The thing I am most experienced with is the kind of batteries you put in remote controlled devices, so I will use those for reference. For all intents and purposes they are basically a consumer-friendly version of the bare cells you will find in the devices you are talking about.

How much power (simplify this down to current, since current is a function of the power, voltage and resistance, and for all intents and purposes the voltage and resistance is relatively constant) a lithium ion battery can deliver is a function of how much surface area the electrodes within the battery have. How much voltage the battery has is a function of how many cells it has.

Because more surface area = effectively higher current capabilities, a larger battery pack generally means a pack that can deliver higher current. The batteries discharge rate is measured as a function of its capacity as a results of this. A battery that is 10000mAh (10Ah) and has a 15C constant discharge rating - C standing for the capacity of the pack - means that the battery can be discharged constantly at 15 x 10A = 150A. In other words, you can buy a battery pack capable of delivery 150A at 14.8v off the shelf today from hobbyking (I can't suggest hooking one of the directly up to you car's battery, though).

To add to this, the devices you are talking about aren't just a battery pack on it's own, they also include all sorts of charge circuitry and whatever else the manufacturer has put in them... as a result of this I would say it's safe to hazard a guess they aren't just battery packs like what I have described here (Even though we have come to the conclusion a battery pack on its own can deliver the power required for to turn over a reasonably sized modern engine), I would say they probably have a small capacitor bank alongside the battery. Basically, capacitors can deliver significantly higher sustained current, but do not have much capacity. Think of a capacitor as a bucket, the cable from the battery to the capacitor as a hose, and the battery as a 50 gallon drum. You can only fill up the bucket as fast as the water moves through the hose, but once the bucket is full you can dump the entire bucket at once on whatever it is you need. Because a starter only needs it's peak current for a very short time, you could use a capacitor bank as a buffer from the battery to the starter to turn over the motor with very high currents.

In short, a battery alone could turn over an engine relatively easily, but a battery alongside a bank of capacitors would do it without breaking a sweat. Throw in some circuitry for over-current protection and you can make the whole thing work perfectly with little worry of it going wrong and blowing up in your face.

With regards to asking for a similar battery online... its possible the manufacturer of the device is sourcing directly from a battery manufacturer to their desired specifications. This is a pretty heavy duty pack that I can't imagine has much mass-market appeal due to what would be incredibly rapid discharge times running at its rated load... You can find 2S 8000mAh 60C lipos online, put two of these parallel and you effectively have a 4S 8000mAh 120C lipo... this is 1S over the one in the video but you get the idea. You could maybe team up a 1S and a 2S but i'd be worried of getting cells that are too different.. a least with 2 identical pack's you have more chance of them being near identical. It's also possible that the 120C rating is the pack 'burst' rating, not its continuous current rating. this is more likely as it's very easy to find 3S 60C 8000mAh packs with a burst rating of up to 120C, the manufacturer might be boasting the burst rating for this specific product as that is the bit that will matter with this, since the peak load with be short lived.

If you want to see how high continuous load you can go this route, there's also products like this 5600mAh, 90C pack, thats 504A continuous current. At that rate you should be able to deliver 504A for ~30 seconds before the pack is discharged. that's a lot better than the 350A for 10 seconds you are talking about in your question and again, this is just from hobbyist gear off the shelf. The manufacturer of these devices probably has a much wider range of suppliers to choose from, I can't tell if they are the producer of the battery pack in the device (since it's branded the same as the actual device) or if they are re-branding the packs, but it only strengthens the argument that these are packs produced specifically for this purpose.

  • wow that answers it. I didn't know those things existed. Tho I still don't understand how can a 300g battery give 500A continuous, but 300g worth of the latest 18650 cells paired in any combination can't deliver anything like that.. – Alex Feb 19 '18 at 14:34
  • 18650s are a different target than prismatic pouches. The pouches are designed for high discharge ratings at the expense of capacity. – Bryan Boettcher Feb 19 '18 at 16:18
  • @Alex like I say, how much current a pack can deliver is a function of the surface area of the electrode. In cylindrical cells like the 18650 as far as I am aware they "coil" the electrodes to fit in the package, this obviously limits how much area they can squeeze in, especially since there are defined dimensions to stay within. With a pouch they can just make the electrodes lay out flat to the surface area of the pack, and make the pack as big or as small as they desire. That's why these packs can deliver better than your typical cylindrical cell. – Trotski94 Feb 20 '18 at 9:03
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I also doubt the numbers they claim on the package but the real question is ... can you start a truck with it? Yes you can!

What you seem to forget is that the car or truck you connect this battery to has its own battery which can handle much higher current. As soon as you connect the jump starter pack it will start charging the trucks battery. If the charge current is 10A you you might already have transferred enough energy after 30 seconds (there is enough to give 100A for 3 seconds from the trucks battery now, add to that another 20-30A from the jump starter and there you go).

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    in one the video I saw they disconnected the vehicle battery. – Jasen Feb 19 '18 at 4:04
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    Yes, a lot of videos show the car starting withut the vehicle battery connected, so this doesn't answer the question (example: youtube.com/watch?v=m-mg8RfCNCY) – Alex Feb 19 '18 at 10:51
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Firstly: Of course the batteries can supply current far beyond the datasheet recommended levels. However, it may be dangerous (risk of fire or explosion) if the condition exists for too long. Reference to for example an 18650 battery shows it's capable of 35+ A for a considerable period of time. Reduce the period of time and tolerate a terminal voltage drop and you can obviously draw more current as shown below: From this web site.

enter image description here

Secondly: The starter motor in a car does not draw hundreds of amps continuously.
Like any motor with a load, it initially draws enough energy to get the engine rotating, but as it overcomes the inertia, the current will drop. It may take only a few seconds (especially with fuel injection) to start a modern car engine and the critical parameter is that the terminal voltage should not drop below that required to run the electrics.

Clearly drawing very large pulses of current for longer period of time will shorten the life of the Li-Ion cells. You also need good mechanical connecting pressure and wire sizes to carry a large current. It's doubtful that you'd need more than 200-300 A peak to start most modern vehicles, so decent size Li-Ion batteries should be viable.

  • So are you saying that the device short-circuits the batteries to put out 200A ? – Alex Feb 19 '18 at 10:50
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    Pouch cells typically deliver significantly more current than cylindricals. – Someone Somewhere Feb 19 '18 at 10:56
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    I may be wrong, but i think the short circuit current is irrelevant to this discussion and is a bit of a red herring. That is just used for calculating the internal resistance of the battery pack for it use in applications, hence why it is listed on the datasheet. – Trotski94 Feb 19 '18 at 11:28
  • @JamesTrotter ..in fact most datasheets do not show short circuit current levels. The manufacturers do not recommend short circuiting cells due to safety concerns. As pointed out in the comment above, the pouch cells often used in Car Starter product applications have much greater pulse current capability due to a much higher anode area. – Jack Creasey Feb 19 '18 at 16:57

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