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
Any car jump starter device will do, such as http://www.walmart.com/ip/Schumacher-Instant-Power-500-Peak-Amp-Jump-Starter/13005748
Cheaper, simpler, and more reliable than playing with stuff you find around the house.
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.:
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.
- The currents involved will need thicker gauge cables or they will melt.
- 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.
- 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.
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!
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.