today my car couldn't start beacuse of dead battery. The battery is not old, I simply left my dash cam attached for a few days without using the car... I'm alone at home and my car is in a difficult position to place another car beside and charge the battery through the wires... I have not external battery charger or anything to charge the battery with. This is an annoying sitiuation. So I got an idea (for future similar events): basically, all you need to charge the battery is something that can provide a tension greater than 12v, with a good amperage, for a while and attach it to the battery, wait and start the car. I saw people using laptop charger, AA batteries, etc. I was wondering if I could do this using a power bank, or even a mobile charger. My phone can handle 12v and the output of the charger is 12v 1,5A, quite high. My power bank is 5v 2.1A instead. By having a USB transformer I could attach it into the power bank and have something like 15V 0,7A... Is this possibile? Could this give the car battery enough juice to start? How long would it take to recharge the battery enogh to start the car? Which is the best voltage to charge the battery? Where can I find a USB 5V to xV transformer? Thank you and sorry for my bad English

  • 15V at 0.7A would work, but would take several days to fully charge a car battery. What is a "USB transformer" and how would you get 15V out of it?
    – Bruce Abbott
    Commented Jul 8, 2016 at 20:30
  • USB supplies put out 5vdc, so a transformer will not work. You need to re-think this from a different angle, and that is buying a off-the-shelf battery charger. Your thinking in terms of USB, which has maybe 1% of the power you need.
    – user16540
    Commented Jul 8, 2016 at 21:25
  • Take a look at your insurance policy. More than likely it has a "road assistance" provision which will provide someone to jump-start your car if the battery has gone dead.
    – EM Fields
    Commented Jul 8, 2016 at 21:36
  • The guy I saw was using 12 AA batteries (about 18V) with an amperage going from 2.2A to about 1.4. So it was about 40w and the battery charged in 10 minutes enoght to set the engine on (I don't qant to charge the battery at 100% of course). If my power bank can provide 10w, then, if I can manager to get a voltage higher than 12v, I should ne abile to charge the var enough to start in about 40 minutes, am I wrong?
    – Franz Tesca
    Commented Jul 9, 2016 at 3:48

8 Answers 8


Any car jump starter device will do, such as http://www.walmart.com/ip/Schumacher-Instant-Power-500-Peak-Amp-Jump-Starter/13005748

enter image description here

Cheaper, simpler, and more reliable than playing with stuff you find around the house.

  • 1
    Ask a simple question, get a simple answer. :) my thoughts exactly. Commented Jul 21, 2016 at 18:01
  • 1
    Lousy answer. Why lug around this 10 lb device when you can get one that's 1/4 the size and 1/10th the weight and even costs less and works as quickly? See Jason's or my answer... Commented Feb 7, 2017 at 6:40
  • 1
    Yep. A "lousy answer", but timely. I'm pretty sure the OP has started his car by now.
    – PeteCon
    Commented Feb 7, 2017 at 15:52
  • 1
    Yep, but the main goal of SE is to have the best answers most accessible to lots of people. Not to answer one-offs. Commented Mar 31, 2017 at 3:27
  • This doesn't answer the question.
    – SusanW
    Commented May 14, 2020 at 14:14

First of all, I realize your goal is to charge the car battery, but note that the typical approach is to just start the car (you need to output a few hundred amps for a short amount of time to start a car). Secondly, assuming your alternator functions properly, you don't need to charge the car battery, you just need to get the engine started. The alternator will take over from there and charge the car battery back up.

Anyways, I know this doesn't directly answer your question of how to build your own, but there are dozens of products like this that already exist, are small enough to fit in your pocket, have a USB port for charging devices as well as cables for jump starting a car, and charge in just a couple of hours, e.g.:

enter image description here

The one pictured above cost $40 USD, is a 8000mAh power pack with a USB output port for charging devices, a USB and barrel input port for charging itself, a jump starter output with 300A crank current (you can do about 10-ish starts with it when it's at full charge), and it fits in my pocket. I own this one it's great (I don't want to turn this into an ad though so I'm not going to mention the product name, but these are easy to find).

So you might want to consider an existing product.

Also, depending on the type of vehicle you have and the terrain around you, you might just be able to push start it, too.

  • 1
    I'm a newbie here so my votes don't count yet, but this is by far the best answer. I have a similar device (different brand and it works great.) Is it not OK to post an amazon link? You mention not wanting to turn this into an ad, so I won't either. Commented Feb 7, 2017 at 6:28
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    Well, the current top-voted answer has a link to an utterly obsolete product at walmart, so it seems links are OK. I have one of these: amzn.to/2le9uec works great but there are competitive products for half the price now - $39 ! Commented Feb 7, 2017 at 6:30
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    @MatthewElvey It's totally OK to post an Amazon link. Me not posting them is just personal preference, and isn't really related to this site. My picture already has some free advertising in it but I prefer not to further that by making it easier to buy. Also there may be better products out there, I don't want it to look like I'm implying that only one product is suited (esp. if it's actually a crappy product and I just didn't realize it). Plus, like you noticed, the products can become obsolete. In this answer I only wanted to say that products like this exist.
    – Jason C
    Commented Feb 10, 2017 at 2:35
  • @MatthewElvey Btw, since we've broken the seal on Amazon links (lol), I also have one of amazon.com/gp/product/B01D42TYFC, which is a bit pricier, but is 18000mAh with a 600A crank output, and also has a variable voltage generic DC output, and charges the hell out of my cell phone, the USB ports have an ungodly max output current. The one in my picture, though, is cool because it fits in my pocket.
    – Jason C
    Commented Feb 10, 2017 at 2:40

Three things:

  1. The currents involved will need thicker gauge cables or they will melt.
  2. For about the same cost of tinkering with all this stuff, buy yourself a battery charger. If you don't have an electrical socket near where you usually park, either get a proper extension cord or learn how to disconnect and remove a car battery, charge it in your home.
  3. Since you say your dashcam is what killed your battery, you may want to change the power wiring of your dashcam. Right now, it sounds like it is getting power directly from the battery, instead use power wires that are only on when the ignition key is at the "accessories" or the "on" position. The radio, cigarette lighter and heater fan use such connections.
  • 2
    Not all lighter sockets do. For example in my ford transit the lighter socket is always on (so I've added a double socket+USB switched by a relay run off the radio power line).
    – Chris H
    Commented Jul 21, 2016 at 15:11

In theory something like this might work but we are talking charging times in days not hours. Go buy yourself a cheap battery charger. If you really insist on trying it you are looking for a 5-14V dc-dc converter.


Basically, a power bank is designed for low power applications and a car battery would fall into high power category. So you would be using the power bank for something that it is not intended for. I would definitely not recommend it.

Getting into the technical aspects of it:

First thing you would want to consider is the Ah ratings of your power bank and car battery. Typically, a mobile phone power bank(assuming a very powerful one) would have charge rating of approximately 10,000 mAh(that is 10Ah). A car battery generally has a charge rating of approx. 70 Ah. Even under ideal conditions and loss free energy transfer, you would need 7 such power banks to charge the car battery. You need not completely charge the car battery to start the car. So, let's say you try to charge the battery to 50% of it's capacity. This would still need 3.5 times the charge content of your power bank.

Let's assume you have 4 such power banks. Now the charge problem is solved(ideally!!). But, the critical factor here is whether your power bank can supply power at the required power rating. You have stated that your mobile power bank has 5v 2.1 A rating.

So it's maximum output power rating is 5 X 2.1 = 10.5 W.

Generally a car battery would require a charging current of atleast 1A at a volatge higher than it's terminal voltage. Car batteries are generally charged at a voltage of approx. 13.5v. So, the power required to charge the battery is 13.5 x 1 = 13.5 W (ideally).

Therefore, your power bank will not be able to charge the battery even under ideal conditions since power ratings do not match. In real time, charging circuitry would involve lots of power dissipation which makes things even worse. I would not recommend this!

  • Thank you for tour answer. I don't want to charge the battery at 100%, not even 50%. Just the minimum to get the engine start (which I suppose it's not that high). I saw a guy which with 12 AA batteries gets the engine on after 10 minutes of charging. That's about 18v at 2,2 to 1,4A for 10 minutes (che showed alla the infos about the charging)... Why would you say that the battery requires 1A to charge? With 0,7A wouldn't it simply charge slower? What about the wall USB charger which outputs 12v 1,5A (18w). Would it be able to charge the battery? Thank you
    – Franz Tesca
    Commented Jul 9, 2016 at 3:55

Such a product already exists and is available, at least in the UK.



No, your normal cell phone charger (wall wart) won't put out nearly enough current to jump start a car, even if it puts out 2.1A at 5v or 1.5A at 12v (that would be one weird cell phone!!! I bet you're misreading the specs.) But one of jumbo-sized cell phone power banks can put out enough current to start a car- which is why some come with the adapter to make this possible. If you tried to run enough current to start the car through a cigarette lighter, you'd likely blow a fuse - (or worse melt a wire and possibly start a fire) so the devices that work via the lighter port instead need to slowly feed enough charge to the battery 'till it has enough to start the car.

What you want is what Jason C provided in his existing answer (we could combine answers...)

I have one of these: http://amzn.to/2le9uec works great but there are competitive products for half the price now - $39 !

This one is $19.99 on Amazon. Prices keep dropping! (but currently it's only 4, stars and the one Jason links to is 4.5 stars...)


This is going to go against a bunch of other things on here.

Batteries do regenerate some current by chemical reaction if you have an engine that has a low compression start cycle AKA not high performance you may be able to let the battery sit disconnected for about five or six hours and try to start the vehicle with your headlights off. You disconnect the battery so that parasitic drains don't hurt your chances this will help if your battery is weak and not completely dead. If your battery is old you may not be able to do this. I have successfully done this on the road. Also if you have a manual transmission you can pop start the car scents that will let you cycle the starter and get the engine turning but that also will not work if your car battery is completely dead.

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