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Building the actual circuits is beyond me at this point, but I'm wondering if it's technically possible to jump start a car from the power contained in a large lithium-ion laptop battery.

For example, I have this laptop battery with the following specs:

11.1V 8.4Ah 94Wh

And I see a car jump-starter product here with the following specs:

Battery Capacity: 12Ah
Gas powered vehicles - 4.0L and lower displacements
Jumpstart your car battery 20 times on a full charge

Would it be technically possible to MacGyver a connection that could jump start a small car (let's say 4-cylinder gasoline for the sake of example) from my laptop battery?

And if not, what is fundamentally different about the type of battery they use in the linked product that causes it to withstand the startup current?

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  • \$\begingroup\$ I would not do that if you were intending to use the laptop battery afterwards, the extremely huge startup current of the start-motors would destroy the battery in my opinion. As in, hundreds of amps. \$\endgroup\$
    – KyranF
    Commented Oct 16, 2014 at 13:39
  • \$\begingroup\$ @KyranF edited to include related question about the "why not" \$\endgroup\$
    – Lessac
    Commented Oct 16, 2014 at 13:47
  • \$\begingroup\$ numerous factors I can see: The contacts (physical connections) of the laptop battery, possible built in current limiting protection/cut-off, the fact that Lithium Ion batteries often have rated maximum discharge values (like, 5C or whatever, which is only 40 Amps for your 8Ah battery), they might instantaneously heat up and explode too. The batteries in the product you showed MAY be lithium ion based, or they have have Ni-MH or Sealed lead acid, with some ultracapacitors which are charged up ready to dump on the output contact/push button triggered. Their wires and some ultracaps are better! \$\endgroup\$
    – KyranF
    Commented Oct 16, 2014 at 13:53
  • \$\begingroup\$ What @KyranF said, But if you had a big bank of capacitors (and maybe a way to limit current draw from the battery) it might work. \$\endgroup\$
    – George Herold
    Commented Oct 16, 2014 at 13:56

1 Answer 1

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Yes, it can be done, with due care.
The laptop battery described could typically provide quite a few jump starts. Getting it to do so safely and effectively is "the trick". Read on ...

What is often not generally appreciated is that energy to turn the starter motor during a jump start does NOT come directly from jump-battery to starter but is first transferred from jump-battery to car-battery and then from car-battery to starter motor. (See note at end)

The starter motor draws hundreds of amps - probably 100-200 Amps in a very good situation and 400-600-800 Amps in various other cases. Even when the car-battery has been run flat by lights left on or whatever, it has the ability to accept energy from the external source and then to return it to the starter motor.

A 11.1 V LiIon battery probably has 3 cells in series.
11.1/3 = 3.7V/cell. Actual cell voltages will be about 4.2V fully charged and 3V dead dead, so the battery = 9V to 12.6V. It is OK to use down to about 9.3V. At 9V if it does actually charge you are in the area where it is doing damage to the battery to use it.

Issues are

  • If car battery is very flat its terminal voltage may be low enough that the laptop-battery provides more current than is wise. The laptop-battery can be damaged and lose cycle capacity and worst case can "vent with flame" - spectacular high energy melt down.

  • If the car battery is very high impedance due to eg major internal sulphation the voltage may be around 12V when open circuit. The laptop battery open circuit voltage may be lower than the car-battery voltage and no energy transfer may occur. This will probably not usually be what happens.

  • If the laptop battery was rather flat (around 9V) and the car battery had modest life in it (eg starter relay goes click-click-click but starter will not turn, headlights light dimly) then the car-battery voltage may be usefully greater than the laptop-battery voltage and the charging may occur in the opposite direction. Max safe charging voltage for your laptop battery is probably about 8.4A (A = Ah rate). A battery that may not start a car may happily deliver 10 x as much current as the laptop battery can safely handle.
    "Vent with flames" / fireball / pyrotechnics may well eventuate. you'd be needing a new laptop battery. The car battery would not have noticed.

In many cases you may be able to 'just connect the batteries " +ve to +ve, -ve to -ve and get an acceptable result. But, as above, maybe not.

Adding a series diode of suitable current rating will protect against charging the laptop battery. You can still get excessive current the other way. And voltage available is reduced. The laptop battery would be good for this purpose if properly used.
What is needed is some limit on safe maximum current, and a voltage for charging that is suited to the car battery in any state. You could make such but you can but them on ebay for less than you can make them for - or from China for perhaps slightly less again.

What you need is a power supply with Vin is at least 9V - 13V, Vout is say 13V-14V and current out is limited to say 5A. If the car battery loads down the supply then the current limit should work. such supplies probably cost in the $US5-10 range on ebay. You can pay more.

Note: You will find much discussion on whether jumpstart energy is first saved in the car battery. I'm wholly convinced that it is. I have started cars from battery packs joined with twisted connections and thin wire and connected to the battery by 3 feet + of "lamp cord.
Eureka! Very satisfying when it works. A starter motor would not even notice that such a battery was there.

If you have REALLY heavy jump start leads, big solid connecting clips and some luck then you may be able to jump start directly from the remote battery. I've seen it done once when all normal attempts failed. The AA was called and he had a monster battery and lead set. It only just worked and his leads nearly melted. It charged and ran OK after that and I have no idea why it behaved that way. ALL the jumps start packs sold have no chance of starting a car directly.

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  • \$\begingroup\$ added link in the question to the actual battery model, which says 9 cells. So yes probably 3 cells each in series of 3. \$\endgroup\$
    – Lessac
    Commented Oct 16, 2014 at 14:28
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    \$\begingroup\$ so are you suggesting that these devices are connected, allowed to trickle/fast charge the car battery enough to then attempt to start the car? Rather than a 10 second "attach and push button"? \$\endgroup\$
    – KyranF
    Commented Oct 16, 2014 at 15:07
  • \$\begingroup\$ @KyranF Essentially, yes. They may dump a supercap to get more available energy quickly but the leads, cllips, plugs and internal battery cannot handle typical CCA levels at voltage drops that are useful. They say "TIPS for using the PowerAll: 1. When Jump Starting a car with a dead battery, you often need to leave the PowerAll connected for 10 seconds (or longer) to build up enough of a charge in your car's battery. Wait 10 seconds between start attempts as well." \ One user comment said "20 times without a battery". \$\endgroup\$
    – Russell McMahon ♦
    Commented Oct 16, 2014 at 15:18
  • \$\begingroup\$ Note that quite some products like APA 16547 Power Pack provide 600A direct starter power, but those are mostly low Ah starter batteries with a convenient package. The same way you can just get a (charged) spare car battery from somewhere else ( I have been doing this quite a lot of times). \$\endgroup\$
    – PlasmaHH
    Commented Jan 29, 2015 at 15:32
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    \$\begingroup\$ @RussellMcMahon does it mean that 10 seconds of charging from a lithium power bank can bring car battery back to life? What would be the current during those 10 seconds of pre-charging? even at 10A which is already quite high for Lithium battery at 12V -- it would pump just 0.02 Amper*hours into the car battery. That's 0.05% of capacity of a small car battery - how can it make any difference? \$\endgroup\$
    – Gleb
    Commented Jan 25, 2018 at 15:28

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