The other week, my friend was trying to jump start a friend's very dead car battery with one of the portable power bank car jump starter kits. It looked something like the below:

portable car jump starter kit

Image source

If I were to have been jumping the dead car with another car, I would have given the dead car's battery a good long charge from the donor car before trying to start it.

I am wondering, can one "charge" a dead battery by leaving one of these portable jump kits connected for like 10 minutes?

Or are these jump kits meant solely to provide a enough juice just for a jump.

  • 3
    As a bit of contemporary commentary: I think this question will have a lot of views as folks in quarantine go to start their car.
    – mario87
    Commented Jul 17, 2020 at 22:14

8 Answers 8


As Miracle Max would say, there's two kinds of dead batteries, and there's a big difference between them.

Mostly Dead

This is the most common kind of dead battery. Let's say your battery is rated for 300 cold cranking amps. After several years of use, it degrades until it reaches, say, 250 amps. Your battery isn't out of power, it just can't get your engine over the hump of cranking amps. Incidentally, in this scenario, "letting the battery charge" won't help because the battery is no longer capable of holding sufficient charge.

Jumping in this scenario works by borrowing the remaining amps from an external source. This is why even a small car can often jump a large pickup. You can even buy battery chargers that can plug into a wall and will give you that boost as well (both batteries and wall chargers do this via a capacitor). These smaller jumping batteries are designed to give you surge in cranking amps for a single moment to give you that extra push. Depending on the device, that could be as little as 100 amps or as many as 2000.

All Dead

After you've gone through the battery's pockets for loose change... wait, we're talking batteries. Sorry, Max.

An "all dead" battery is really a good battery that is mostly or fully depleted (i.e. you ran your car radio with the engine off for a long time or your alternator died). This is a less common problem, but it still happens. A pure surge battery isn't designed help you directly with this problem because it's generally designed to offer a one-time boost. Your lead-acid car battery really wouldn't like getting 500 amps all at once.

Lead acid batteries like a slow, steady charge. Most wall-powered chargers run 15-30A in charging mode and you'll need to charge it for some time (meaning several hours) to get it back up to full.

If I were to have been jumping the dead car with another car, I would have given the dead car's battery a good long charge from the donor car before trying to start it.

The reason people are taught to "let the car charge a bit" first in jumping is that you need the dead battery to provide some power in the attempt to crank. Since you're probably not carrying a battery tester with you, there's no way to know why the battery is bad, and there's no harm in letting a "mostly dead" battery charge for a minute.

If you're using a battery pack, just hook and crank. If the battery is all dead, the alternator will recharge it naturally. If the battery is mostly dead, it will provide the missing amps. Go take it to a car parts place and they can test it for free and tell you if you need a new battery.

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    I was tempted to upvote this just for the Princess Bride reference. Instead, I upvoted it because it also has relevant info. Commented Jul 16, 2020 at 18:50
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    "revving the engine on the donor vehicle doesn't make it charge faster" True, but it can make it charge at all. My '97 Jeep does not charge at idle, but it does while driving.
    – AaronD
    Commented Jul 17, 2020 at 6:28
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    This is, overall, a good answer. However, there are parts of it which look to be technically inaccurate. A bit reductive.
    – Mast
    Commented Jul 17, 2020 at 7:57
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    I am doubting the "revving the engine on the donor vehicle doesn't make it charge faster". An empty battery can take lots of current, in the order of magnitude of what the donor battery can deliver. The current may be high enough that the "inner resistance" of the donor battery comes into play, leading to a voltage drop. Trying to start the receiving engine on top of that will drop the voltage even further (you see your lights dim when you start, or your car radio rests). Modern alternators can deliver hundreds of amperes, which will keep the voltage up. So by all means, run the donor engine. Commented Jul 17, 2020 at 10:54
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    @Peter-ReinstateMonica The charging system will never produce more than about 15 volts, unless it's faulty. (14 and change, with some tolerance) Usually, that's regulated by adjusting the field coil as it's far more efficient that way, but I've also seen a separate linear regulator, which is basically an automatic series resistance with its own heatsink. If you're already at the regulated voltage, then running faster will not increase it. But if the donor is loaded down to where the regulator can't adjust any more and the voltage sags at the regulator, THEN you can give it some more juice.
    – AaronD
    Commented Jul 17, 2020 at 14:35

Those types of chargers are designed to provide short bursts of power to start a car, most do not have a battery charging mode. Once the car is started the alternator will complete charging the battery. Some of these chargers have a USB output for charging phones, but they don't have the capacity to charge a car battery back to full.

  • 3
    Agreed. Most of the time the reason a car is left to "charge" when jumping off of another car is because the jumper cables themselves are not large enough to carry the amperage needed to get a car started. If you have larger cables which can provide the juice, there's no need to wait. Commented Jul 16, 2020 at 10:39
  • In fact, the alligator clamp fob on these guys generally has a "boost" button, which gives you about 30 seconds of extra power - specifically for the case where the device's normal output isn't enough to get the car started.
    – Tacroy
    Commented Jul 16, 2020 at 19:09

Miracle Max notwithstanding.

There are a relatively few (more like very few) jump starters which directly start the vehicle with little or no involvement from the vehicle battery. Most jump chargers which advertise large starting current capabilities are marketing hype with about no relationship to reality.

The majority of jump chargers transfer charge at a relatively low rate into the low impedance but "empty" main battery which then transfers the charge out again in a starting surge at much higher current in a short period of time. This is similar to Machavity's "All Dead" scenario, but applies to a variable extent to any battery which you can jump start within minutes with a jump-start unit.

I have jump started an "all dead" battery using 2 x 6V, 3Ah (= wimpy & small) lead acid batteries in series plus very thing "bell-wire". The vehicle battery was in fact in a fairly solid state but was still able to accept and then pass on the charge. It took several minutes to trickle enough charge into the battery and then a very short time to start the vehicle (fortunately).

What would I know?
It varies :-) - but here are some other of my other stack exchange answers re lead acid charging, some of which will have relevance.


I am going to be in the minority here, but I have had success jump-starting my SUV with a larger version of what you are showing only after letting it sit connected to the battery for 10+ minutes

My car has a power drain issue that no one has been able to figure out in 7+ years. So I've needed to jump start my car frequently, so i bought a larger version of what you are showing. And when I connect it to my car and try to start it, it almost inevitably fails to start my car. But if I connect it and walk away for 10 to 15 minutes, when I come back, there is enough juice to start my car.

So in my experience, these types of devices will charge your battery a little.

  • Some of these devices can even work through the ciggy lighter socket in the dashboard. For your needs, consider installing a big master switch between the battery's negative terminal and the chassis - simply pop the hood/bonnet and flip the switch, or spin out a bolt, and you're completely unplugged meaning no-drain. Might mess up your radio, clock, etc.
    – Criggie
    Commented Jul 19, 2020 at 3:25

There is a reason why a car battery is big and heavy, and the portable kits are not - they have not nearly enough power to charge a battery.

But there is no need or use to charge a dead battery before starting the car - just jump-start the car, and once it is running, its alternater will load the battery just fine.

It is a good idea to run the car for 15-30 minutes at least, depending on how dead the battery was, or you might need another jump start (so don't just drive to the mailbox and shut the engine off...)

  • Not always - some cars have intelligent charging which will not respond well to a dead battery and systems that are very sensitive to voltage.
    – Solar Mike
    Commented Jul 16, 2020 at 17:05
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    The reason why a car battery is big and heavy, and a portable kit is not, is that a lithium battery has five to ten times the energy density of a lead-acid one.
    – Mark
    Commented Jul 16, 2020 at 22:21
  • @Mark why are care batteries not lithium ion? Cost?
    – Tim
    Commented Jul 16, 2020 at 22:37
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    @Tim, many reasons, including cost, durability, performance, and history. Weight isn't really a consideration in a car; putting out sufficient power in the winter while not catching fire in the summer is.
    – Mark
    Commented Jul 16, 2020 at 22:57
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    @Tim: Lead acid batteries can be damaged by overloading, but the consequences of brief overloading for a battery will generally be limited to premature aging (e.g. a few seconds of overload may take months or years off a battery's useful life). Consequently, vehicles can use lead acid batteries without any protection circuitry. By contrast, abusing a lithium battery can cause it to turn all of its stored energy into heat within a matter of seconds. For large batteries, that condition is sometimes called "vent with flame". For something like a cell phone or computer...
    – supercat
    Commented Jul 17, 2020 at 15:09

If the voltage on the battery pack is higher than the voltage on the car's battery, it will charge the battery to some extent. How much depends on the voltage, capacity, and charge level of the 2 batteries. It is unlikely that the jump pack will be able to charge the battery enough to start the car without the jump pack. As others have said, the intended usage is to have the jump pack start the car, then allow the alternator to bring the battery up to full charge.


I learned long ago to not charge the battery but go directly for the engine (one contact to the postive pole and the negative to the enginge block to avoid damage from small flashes. not sure this is at all a good idea or even possible today, maybe someone with knowledge can evaluate?

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    That has nothing to do with charging the battery, as the power goes into the exact same circuit. The reason for doing this is to avoid causing sparks near the battery, as they can ignite hydrogen (resulting from electrolysis of water in the battery) and cause explosions. To avoid that, connect the negative pole last, and to the engine block rather then the battery. There will still be a spark, but further away from the battery.
    – user149408
    Commented Jul 18, 2020 at 22:23

It's a bad idea to let those kits charge batteries in advance. Either the problem is that the battery does no longer hold enough charge to jump start the car in which case the kit will not help. Or the battery is good but in a state of significant discharge. The jump start kit will not be able to make much of a dent in the state of discharge and it will detract from its own ability to jump start the car. But you will likely make the situation worse since a fully discharged lead battery tends to have a comparatively high inner resistance and it takes some initial charging to break that down and get it to charge well. If you get the battery to that stage before starting the car, it will suck up most current that the jump start kit can deliver when you try starting the car.

So there is some point to getting the car started before the lead battery wakes up and clamors for all the current it can get. If the motor is already running, the alternator will provide that current. Otherwise the starting kit has to do that.

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