Charging a car battery with a phone charger

scubascuba3

Standard Member
My battery is dead and my charger isn't doing anything. I've read that if the battery is so dead that the charger sometimes doesn't recognise that it needs charging.

Now, I've read another forum where a guy connected a mobile phone charger to the battery to give it a small charge, then the proper charger kicked in and away you go.

My question is, I might try this,there are 4 wires in the USB cable. Black, white,green,red. Which should I connect to the positive and negative terminals of the battery? Anyone have a clue? I will quadruple check before I try.
 

Wardy257

Well-known Member
Never heard that one. Is your charger working? Is your charger suitable for your battery as some cars use weird voltages? Some battery charge better if you light up a bulb between the terminals.
 

scubascuba3

Standard Member
The charger seems to be working if I put my ear to it I can hear a low hum. Yes its a 12v battery and charger is set to that.
How do you light up a bulb? Will that still work if the battery is flat?
 

twoeyedbob

Active Member
You'd effectively be trickle charging the battery... which would take about a week.... at least..
Although phone chargers are usually 5v's(usb standard) afaik... cant see it doing any harm to the car battery, the charger however may not be so lucky ;-)
 

scubascuba3

Standard Member
Its not to do the whole charge its to give it a slight charge to kick in the proper car battery charger, the key though is the wires. I don't really want to kill myself, I'm too young to die :)
 

scubascuba3

Standard Member
Interestingly wiki says the following:

The standard USB A plug (left) and B plug (right)
Pin 1 VCC (+5 V, red wire)
Pin 2 Data− (white wire)
Pin 3 Data+ (green wire)
Pin 4 Ground (black wire)
USB - Wikipedia, the free encyclopedia

So does this mean I can just connect black to black and red to red and ignore the other two wires?
 

Mr_Ceee

Active Member
Have you checked the fuse in the charger? Not the mains one, there is usually another fuse to protect the 12V side. Don't connect your phone charger unless you want to have an ex-phone charger.
 

car-man

Well-known Member
If your car battery is completely dead it isn't unusual for the charger not to register anything until the charger has been charging for a while.
 

John7

Well-known Member
A phone charger is designed to charge a Lithium Ion battery, a completely different chemistry to a lead-acid battery in a car, which requires a different kind of charger. So, it won't work.

First thing to check with a car battery is that the electrolyte (sulphuric acid) is covering each of the cells. If not, top it up. If a cell is bone dry, the battery is usually kaput.
 

noiseboy72

Distinguished Member
If you leave a lead acid battery totally discharged for any length of time, it is unlikely to recover. The chemical changes within the battery make charging it or getting it to retain charge impossible. It certainly won't have enough power to start the engine.

You won't do any harm to the battery connecting a usb cable to the battery, but if there is any charge in the battery, you could be sending 12V BACK down the USB cable into the charger! This could well be a bad thing if it is not well protected from such abuse.

Bottom line? If your knowledge of battery charging, voltage, current and basic electrical theory is so limited, I really would not try it. You could cause a fire in the charger or burn yourself with electrolyte.
 

scubascuba3

Standard Member
If you leave a lead acid battery totally discharged for any length of time, it is unlikely to recover. The chemical changes within the battery make charging it or getting it to retain charge impossible. It certainly won't have enough power to start the engine.

You won't do any harm to the battery connecting a usb cable to the battery, but if there is any charge in the battery, you could be sending 12V BACK down the USB cable into the charger! This could well be a bad thing if it is not well protected from such abuse.

Bottom line? If your knowledge of battery charging, voltage, current and basic electrical theory is so limited, I really would not try it. You could cause a fire in the charger or burn yourself with electrolyte.
I would only attempt to charge for less than a minute perhaps just enough for the main charger to kick in? I might not do it, just buy a new one.
 

John7

Well-known Member
Safest to take the battery to a motor factor - they will usually test the battery for you for free.
 

probedb

Banned
Your phone/device/whatever controls how to charge it's battery, you're not just connecting the USB power to a device's battery. In this situation you're attempting to connect two power sources together!
 

nvingo

Well-known Member
...In this situation you're attempting to connect two power sources together!
This is probably the theory behind the suggestion.
Battery output is below the threshold for the car charger to accept it is connected to a car battery.
Alternate DC source (eg. 9v PP3 battery) connected in parallel tricks car charger to start.
Once battery is charged to above threshold it can take over and alternate source can be removed.
 

scubascuba3

Standard Member
This is probably the theory behind the suggestion.
Battery output is below the threshold for the car charger to accept it is connected to a car battery.
Alternate DC source (eg. 9v PP3 battery) connected in parallel tricks car charger to start.
Once battery is charged to above threshold it can take over and alternate source can be removed.
I'd say that's a flaw with modern chargers. You'd think they would have an extra switch for that.

I decided to buy a new charger, battery appears to be charging. I also bought a new battery which is on its way.
 

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