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All powerbanks that have car-jump function have some thingy between the clamps and powerbank socket: enter image description here

What's inside that thingy?

The manufacturers typically claim 300A of starting current for powerbanks as small as 8Ah, which cannot be drawn directly from Lithium batteries they have inside.

There's a related question I found here Jump start car from lithium ion laptop battery, but I don't believe it's just the protection circuitry, it has to have some kind of "boost storage".

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    \$\begingroup\$ Open and have a look. \$\endgroup\$ – PlasmaHH Jan 25 '18 at 15:46
  • \$\begingroup\$ Depends - there isn't much room for capacitors in there, and Li batteries can put out a huge current for a short period of time. \$\endgroup\$ – pjc50 Jan 25 '18 at 15:48
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    \$\begingroup\$ BigClive on Youtube took such a product apart, see: youtube.com/watch?v=0tGK1nqXr28 \$\endgroup\$ – Bimpelrekkie Jan 25 '18 at 15:52
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Not sure which "thingy" you refer to. The smaller lump has protection circuits. But the secret sauce to these emergency car starter power packs is in the big lump. The reason they can put out so much current (hundreds of Amps) despite being such small capacity (e.g. 8 Ah) is because they use lithium iron phosphate cells.

Lithium iron phosphate batteries have incredibly low internal resistance. This means they can dump their small capacity as huge amount of current (hundreds of Amps) in a short time (seconds instead of hours).

Let's compare a typical lithium-ion cell with a typical lithium iron phosphate cell:

Tenergy 18650

  • 2.6 Ah
  • Impedance <= 80 milliOhms
  • Max discharge current = 2C (which is 5.2 Amps)
  • Nominal voltage = 3.7 Volts

A123 26650 (A bit fatter than an 18650, but similar capacity)

  • 2.5 Ah
  • Impedance = 6 milliOhms typical
  • Max discharge current = 120 Amps for 10 seconds
  • Nominal voltage = 3.3V

Looking at the max discharge current, there's a huge difference, and that is because of the very low internal resistance (or impedance).

Doing the math: if we draw 100 Amps for 10 seconds, how much capacity have we used? Assuming conversion is ideal, that is 100 Amp * 10 sec * (1hour/3600 seconds) = 0.28 Ah.

What? So that much current is only taking roughly 1/10 of the cell capacity because the time is so short! So we don't really need so much capacity to start a car: we need super low resistance for very high discharge current.

Another convenient thing with lithium iron phosphate is the nominal 3.3V and max charging voltage of 3.6V per cell. You put 4 in series and you get a charge voltage of 14.4 Volts. Great! That's right around where lead acid charging voltage is.

So put say 2 cells in parallel x 4 pairs in series and with 8 cells you can handle 200+ Amp discharges, 16 cells for 400+ Amps, at a voltage that is compatible with lead acid systems.

There's also electronics in there that protects from reverse polarity, etc, but what makes truly work are the very high discharge capability of lithium iron phosphate cells.

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The doodad contains protection circuitry, for example against reverse polarity connection. The device can break the positive line with an electro-mechanical relay. See the below screenshot from this video.

enter image description here

There may be short circuit protection as well, the circuit includes a couple of ICs (perhaps comparators) and a transistor to control the relay.

Many lead-acid battery based boost devices have had this kind of protection built in to their (relatively very bulky) enclosure for years. It's far too easy for the user to reverse the connections and you probably don't want it exploding or bursting into flames unnecessarily.

The chap in the video mentions bypassing it.. I suggest that is a really Bad Idea.

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PowerBanks like money banks are just a place to store a charge, but you can't use it all at once to start a car.

It must be transferred at a rate the bank can support for a duration that is sufficient to charge the car battery and start the car.

e.g. This may be 10A for 100 seconds so you can get 100A for 5 seconds or better

They typically use LiPo cells but instead of 18650's they are flat packs.

When a Car starter motor starts at full RPM, I would expect the battery to drop below 12V. When it drops to half normal RPM sound I would expect Vbat drops to 8V under load. Thus I might expect from the photo, and the size, this Powerbank has some electronics but mainly 3 series and 6 parallel or 3S6P with a No load voltage of 3.7x3=11.1V and a loaded voltage of 3.1Vx3=9.3V which is enough to start the motor using 0.6V drop from 24mOhm/6cell or 0.6V/0.008R=75 Amp on a warm day to a 4cyl eng.

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    \$\begingroup\$ This doesn't answer the question. The OP already knows that these 'powerbanks' contain Lithium cells. \$\endgroup\$ – brhans Jan 25 '18 at 16:03
  • \$\begingroup\$ The 300A might be the locked rotor current from best case LiPo batteries but there is no other answer other than LiPo inside that generates the charge. Probably 3S6P 18650's \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 25 '18 at 16:16
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    \$\begingroup\$ The question asks what's inside the thingy between the clamps and powerbank socket... \$\endgroup\$ – brhans Jan 25 '18 at 16:23
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    \$\begingroup\$ What does it matter? The Question does not ask "what's in the powerbank"! \$\endgroup\$ – brhans Jan 25 '18 at 16:33
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    \$\begingroup\$ @markb Capacitor is my guess as well, which makes sense for the need to hold the button for 3 seconds. Hold the button which charges the capacitor at a rate the lipo can handle, then crank the engine and pull the turn over current from the capacitor. \$\endgroup\$ – Ticktok Jan 25 '18 at 16:44

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