How do I charge a cell phone from a 12V 70Ah solar battery?

Thread Starter

ElectEng

Joined Sep 3, 2019
35
Hello everyone,

How do I charge a cell phone safely using a 12V DC 70 Ah solar battery? I have looked at several cell phone outlets and see different current ratings. Most seem to be around 5V and 1.5A.

Should I just use a LM7805 voltage regulator that outputs 1 or 1.5A?
 

Alec_t

Joined Sep 17, 2013
10,378
A LM7805 is a linear regulator and would have to dissipate (12V-5V) x 1.5A = 10.5W if fed with 12V and supplying 5V at 1.5A. That would require a large heatsink for cooling it. A SMPS might be a better bet and would run cool.
The phone will decide its own charging current, but to do so may require coding of the data lines in the USB connector.
 
Last edited:

Thread Starter

ElectEng

Joined Sep 3, 2019
35
Why not use an automotive lighter-socket 12V to 5V USB adapter?
They apparently are switch-mode devices.
I would like to but this is a school design project. I am trying to go above and beyond to add this accessory to it. In other words I need to be able to recreate the circuit using software like eagle and solder it myself.
 

BobaMosfet

Joined Jul 1, 2009
806
@ElectEng - Some things you need to be aware of- the max ratings for a part aren't absolute. An LM7805 won't support 1-1.5A, unless you're careful with voltage level in, and thermal junction temperatures (keeping the 7805 cool).

If you put 12V in, wanting 5V out, using a heatsink on the LM7805, and you don't want the 7805 to get hotter than about 38.8C (somethng reasonable that won't burn someone if they touch it), you're only going to get about 418mA out of it. Otherwise it'll thermally shut down.

I recommend instead, using the ubiquitous MC34063A . Impress your teachers, and your friends, and learn how to use one of the most widely used, most reliable buck/boost converters around. People will tell you that you can't make boost/buck converters on breadboards. Baloney. I do it all the time and they work with zero problems (year after year after year).

In truth 100mA to 500mA is fine for charging a cell phone.
 

MrAl

Joined Jun 17, 2014
6,507
Why not use an automotive lighter-socket 12V to 5V USB adapter?
They apparently are switch-mode devices.
Hi,

That's an excellent idea and i have to second that.

I just got one myself about a week or so ago and it works great. I got it for the car, plug it in and it has two USB ports that can charge phones up to 2.4 amps each port.

I do have to recommend getting a QUALITY unit though. Dont get a really really cheap one. I got a really really cheap one some years ago and i took it apart to examine the circuit and was very dissatisfied with it because it looked like it was soldered together by someone that just learned to solder yesterday ... ok maybe someone that learned to solder the DAY AFTER they soldered that one together :)
 

MrAl

Joined Jun 17, 2014
6,507
Hello everyone,

How do I charge a cell phone safely using a 12V DC 70 Ah solar battery? I have looked at several cell phone outlets and see different current ratings. Most seem to be around 5V and 1.5A.

Should I just use a LM7805 voltage regulator that outputs 1 or 1.5A?

Hi,

See post #3 in this thread. That's the best idea because it's already made. All you need then is a 12v cigar lighter socket with two leads with alligator clips that are made for 12v automotive batteries. Sold on Amazon and other places. With this setup you just use the same cord that came with your phone to charge with because the ready made device has one or two USB ports you just plug the wire into that and you're charging just like with a USB port or wall wart. These devices support 2.4 amps charge so i think that means they support Turbo Charge and other super fast charge technologies too.
I just got one a week ago and tested it at 5.05 volts and ran a USB light and a USB tester.

Check the output of your solar arrangement though and see that it is not that much above about 14 volts DC.

This is not the one i got but very similar...
Each USB port does 2.4 amps max for a total output of 4.8 amps.

RandomCarCharger_20191011.jpg
 
Last edited:
Top