Say I have some ordinary portable powerbanks I use for my smartphone, I wonder if all of them are able to jump start a car with flat battery as long as I have a correct connector (e.g. clamp).
Or else, whats the particular requirements for a powerbank to have the ability to jump start a car? Capacity? Must be specially designed for that purpose?
No, you can't. A car battery needs a lot of amps at ~12v, and most USB outlets are rated between .5A and 2A at 5v. Car batteries run in the hundreds of amps at 12v.
Water hoses often make a good analogy for electricity. Think of amps as the width of the hose, and volts as the speed that the water is flowing at. Your smartphone powerbank is a hose the size of a straw, with a slight trickle of water. Your car battery needs a sewer main with a heavy flow of water.
And it gets worse: depending on exactly how the charging circuit is designed, you could end up having reverse electrical flow (your car battery will be sending voltage and amps to the powerbank), resulting in a damaged charger or worse! Going back to the sewer analogy... what would happen if you attached the straw to a sewer main, and turned on the water?
EDIT
Ok, so @JPhi1618 pointed out this thing. It's interesting that the reviews are generally positive, but there a few that say either "this didn't work at all" (in cases of a completely dead battery), or "this only worked once or twice". So, I guess I'm going to slightly change my answer to:
"Your powerbank almost certainly won't work, unless it was specially designed with jumping a car battery in mind, like the above unit was."
You need one of 2 things - volts or amps.
To jump start the car, you need something with 12-14v and able to crank out 100+ amps.
You can charge your battery with anything with a higher voltage. There is a video of someone charging his battery with a bunch of AA batteries in series. You have to let it sit for a bit. This is only going to work if you battery isn't completely dead.
Maybe.
The various answer so far touch on the reasons that it is uncertain but do not 'spell them out'.
It is possible for a vehicle JSU (jump-start unit) to provide all the starting energy directly from the JSU essentially without involving the dead battery directly. In this case current flow from the JSU internal battery to the vehicle's starter motor - so the JSU battery must be able to supply the required cranking amps and volts directly and the cables and connectors must be rated to both carry the current and to not cause excessive voltage drop. While some JSU's can do this they are generally some mix of larger heavier and more expensive than most. These units will generally work with a "completely dead" (= deeply discharged) vehicle battery because they have enough current capacity to boith charge the dead battery and to start the vehicle directly.
More often, a JSU does not have enough current supply capacity to provide the needed cranking current directly. Attempts to start the vehicle from a JSU that can supply say 10A when cranking currents are 100s of Amps will be completely unsuccessful.
What allows relatively low JSUs to work in many cases is that the JSU stores current at a low rate into the "dead" battery over a period of seconds to minutes, and the battery then transfers this energy to the starter motor at a much higher rate over a period of a few seconds. eg
10A x 2 minutes = 10A x 120 seconds = 1200 A.s
Starting 400A x 2S = 800 A.s
So -
IF a JSU can transfer energy at 10A for 3 minutes into a vehicle battery and
IF the vehicle cranks at 400 A and
IF the vehicle starts in 2 seconds and
IF the vehicle battery can return at least 2/3 of the energy input at an adequate voltage.
THEN the vehicle will start.
Which is why my original answer was "maybe".
In many cases the above condition set is well enough met that vehicles can be Jump Started using batteries that are far far far smaller than are required for direct starting. This is much more likely if the vehicle battery is "not completely dead. eg solenoid clicks and engine won't crank but headlights still turn on (maybe not very brightly) and horn may work.
If eg the headlights were left on all night with no low voltage cutout then the chances of success are much lower.
Real world results:
(1) I had a battery drop below starting voltage in a hired camper van. We may have left the lights on or the battery may have just been as dead as the rest of the van was. I had two small 6V lead acid batteries (probably about 1.2 Ah each) joined with (thin) bell wire that I was using to power LED lighting. "At a venture" I connected this very low capability to the campervan's battery and, after a few minutes charging time the van was able to be started.
(2) Two charged but oldish and deadish 12V car batteries were unable to supply enough charge to a 12V battery in a car to allow it to start. Connecting both in series using 3 jumper leads and instant attempt to start once connected gave an immediate start with no charging time to speak of.
It's dependent on a considerable number of variables but easy Ines out of the way first:
If someone stole your car battery, there is no way that a few mobile phone power banks connected in series for a total voltage or about 12v would be able to give up enough amps to start the car
If someone stole your car battery you might be able to connect together many (tens or hundreds) of these power banks in a mixture of series and parallel to get approximately 12v and sufficient current delivery capacity, to start a car. You'd have horrendous problems with this many cells in parallel if they're mismatched voltages
If your car battery was completely dead because you left the lights on for a month, it's likely as good as stolen due to sulphation of the plates, it will unlikely be capable of storing enough energy to start the car and hence you'd have to proceed as above
If your battery was slightly too flat to start the car- you left the lights on for an hour while you were in the gym, it's a bit old, and now your engine is going "rur, rur, rur" but isn't starting then maybe a few power banks could see it running in the end; there would have to be sufficient energy stored in the banks to charge the battery of the car to a point where the car battery was capable of starting the car. How much energy would be required (how many power banks you'd need) depends on how flat the battery is i.e. how much more energy it needs to be topped up with for the car to start (big engines- more, cold climates- more, diesels-more, carburettor cars- probably more because they typically don't start as soon as ecu controlled cars, cold engine/not run recently- more and son on). It would be a slow process; the thin wiring inside the power banks would never flow enough current to turn the engine (the cables from car battery to starter are as thick as your finger) but it would slowly charge the main battery.
How much energy are we talking? Well, the battery on my car is 80 amp hour. The battery in my phone is 1 amp hour. Phone batteries have a higher energy density than a car battery but they have a much lower capacity. In theory an 80amp hour battery would take 80hours to charge on a 1 amp charger, from totally dead. In reality the usable capacity of a car battery is about 20%, as if they get discharged more than about 80% of total capacity it starts to irreversibly reduce the battery capacity.
Now, perhaps your battery that is just a bit too flat to start the car is 50% charged, but maybe only needs a 10% top up on its charge level and it'd be enough to start the car. For my car that would mean getting my battery that currently has 40ah stored, to 44ah. If your power bank was 4v and 1000milliamp hour (1ah) then somewhere more than 4 of them would maybe get my car battery up to the charge needed- they'd need to be connected in series to get the voltage above 12 in order to push charge into the car battery. Also the voltage of the banks would drop as they discharged so it's unlikely to be a straight maths "4 banks, 1ah each, gets the 4ah needed" because you reach equilibrium where the car battery is pushing charge back as hard as the power banks can push it out; charging stops
Hopefully this gives you an idea of what could be possible. Don't try it in practice; you're likely to start a fire, or damage an unrelated system in the car. You can buy dedicated power banks, like a big laptop battery, that are specifically designed to revive a low lead acid battery to a charge level that will start a car. They're about the size of a paperback book and are mostly lithium ion battery
