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Thursday, October 5, 2017

Emergency Cell Phone Charging

Today's topic is whether or not you can use a USB car charger with anything other than a car's electrical system to charge a cell phone.

One of my friends posted a video of a simple hack that shows how to connect a regular 9V battery to a car charger, and the video was automatically denounced by an electrical engineer as impossible or even dangerous. The ensuing pissing match led me to doing my own research and trying to find the truth.

Just to make it perfectly clear, nobody is telling you to plug your phone into a 9V battery. The video showed them connecting a standard car charger to a 9V battery and then plugging a phone into the charger.

Some Basic Facts
  • Watts=Volts*Amps.
    • Amperage is the amount of electrical flow.
    • Voltage is a measure of the force behind the electrical flow.
    • Wattage is the amount of work done by the electrical flow.
  • Battery storage is measured by the amount of power they can produce over time, milliamp-hours (mAhr).
    • Milli- is a prefix that means “one-one thousandth” or 0.001.
    • One milliamp is 0.001 Amp. 
    • 2000mA=2A.
  • Standards have changed, but USB chargers are rated at 500mA (slow charge) to 2.0A (fast charge).
    • Cell phones and other devices that charge through their USB port require about 5VDC (4.4to 5.25 VDC).
  • A common smart phone battery is going to be rated for 2000-3000 milli-Amp-hours (mAh) of power. This is why even a fast charger will take a few hours to fully charge your phone.
  • Normal car batteries produce about 12VDC (it varies from 11.5 to 13.0 VDC depending on the state of the battery) and amperages that are measured in the hundreds of Amps.
  • A common 9V battery, like you'll find in a smoke detector, is rated at 400-500mAh (lithium versions are around 1000-1200mAh).

Now For Some Math
(I heard that groan, Erin)
  • A dead cell phone (smart phone) is going to need around 2000mAh at 5VDC. 
    • 2000*5=10,000 mWh.
  • A common 9V battery can provide around 500mAh at 9VDC. 
    • 500*9= 4500 mWh, or about half of a full charge on your phone. That's enough to make a few calls for help. 
  • A lithium 9V battery can provide at least 1000mAh at 9VDC.
    • 1000*9=9000MWh, or almost a full charge.
There is some inefficiency in the charger and wiring, so you're not going to get absolutely everything out of a battery; as a battery discharges, the voltage drops off (less of a problem than you'd expect) and it will reach a point where there isn't enough voltage (pressure or force) to push the electricity into the phone battery.

That Video
This is the video that got and ripped up by a sanctimonious Electrical Engineer (EE) who didn't think it would work.



A lot of people assumed he knew what he was talking about, because, “Hey, he's an engineer!”. Let me tell you a secret: engineers are like doctors; there are so many specialized fields that no one person is going to know everything. Just in the field of electrical engineering alone there are 299 different journals published by the IEEE. An engineer who designs power plants isn't likely to know any more about semiconductors than I am, and the engineer in question is an “adjunct professor” from Boston who specializes in radar and remote sensing technology (I looked him up). How much he knows about batteries and charging is up in the air.

(For what it's worth, Nye is a Mechanical Engineer and his opinions on anything outside that field should be taken with a grain of salt.)

Being an obstinate old man, I dislike people “in authority” stepping out of their narrow fields of research and handing down pronouncements like some petty gods. I do my research before I stick my foot in my mouth. Here's what I've found.

For something that won't work or can damage your phone, there are dozens of hackers posting videos on YouTube showing how to do it. Here's an example, with almost 3.5 million views. This is not a new hack; people have been building battery-powered phone chargers in Altoids cans for years. Links to similar videos are usually on the right-hand side of the page when you view it.

For those who don't use YouTube, I'll describe the basic wiring.
  1. You'll need a car charger (the kind that plugs into a cigarette lighter), a battery, and some way to connect the two electrically. You'll need a USB cable, too, if your charger doesn't have one.
  2. Connect the “+” post of the battery to the center post of the car charger
  3. Connect the “-” post of the battery to the spring clips on the side of the car charger.
  4. Plug your USB cable into the charger and your phone.
  5. You're done.

How It Works
Inside the car charger is a circuit that converts one voltage to another. Here's a diagram for those who like to see how things go together.

http://www.electroniccircuitsdesign.com/battery-charger-circuits/usb-car-charger-adapter-circuit-design.html

Most simple chargers like this use a chip to do the work, with a few other pieces to control the chip output and protect it. This diagram uses an LM317L fixed current voltage regulator that puts out 100mA. The input voltage can be anywhere from 3VDC to 40VDC, and the output voltage is set at 5VDC by the resistors marked R1 and R2. Here's the data sheet for any engineer that doubts me. This is a slow charger, it only puts out 100mA and would take a long time to charge a smart phone.

Other common chips are:
  • 7805 (78xx series voltage regulator, the xx designates the output voltage), which doesn't require external resistors and has an input range of 5VDC to 18VDC.
  • 34063A, which is capable of putting out 5VDC at 1500mA from a supply ranging from 3VDC to 40VDC.
  • LTC1174, another 5V, 100mA converter with an input range from 4VDC to 18VDC.


Here's my experiment. I prefer real-world results over some educated fool's pronouncement, so I grabbed a spare car charger, a 9V battery that's been on a shelf for a few years, and about three inches of scrap 12ga solid copper wire from when I moved an outlet.
  1. I stuck the wire under the spring clip on the side of the charger and the other end on the "-" post of the battery, with the center post of the charger resting on the "+" post. 
  2. I have an old cell phone that isn't worth using any more, so it is the guinea pig. I plugged the phone into a regular charger to make sure it would still take a charge, brought it up to 5%, and then switched it over to the battery/car charger. 
  3. In five minutes got it up to 10%; 10 minutes got it to 15%. I let it run for a while, as I wished for a real electronics bench with the proper tools to measure the voltage and amperage while it's running. 
  4. At 25 minutes and 15%, the phone booted up. The charger LED started to flicker a bit, but it isn't a very good charger (there's a few reasons it's a back-up). 
  5. 40 minutes in, it shows 20%. 
  6. I let it run for an hour and it stopped at 25%. Not bad for an old phone with a weak battery and a 9V that had dust on the package! 25% is more than enough to make a few calls to let people know where you are and that you're still alive.
So, no, setting something like this up is not going to make your phone blow up. Yes, you can use a DC-DC converter with a variety of input voltages and still get the same output voltage; that's what they're designed to do in the first place. Four AAA, AA, C, or D cells would give you 6VDC if wired in series (+ to -), which would be enough to power any of the converter chips I was able to track down. Any battery that produces more than 5V but less than 18V would be safe to use. Look for them in emergency lights, toys, etc.

Personally, I'd use one of the low amperage chargers to slow down the draw from the donor battery. Pulling too much out of a small battery too fast can make them heat up, and that reduces their efficiency.

The Fine Print


This work is licensed under a Creative Commons Attribution- Noncommercial- No Derivative Works 3.0 License.

Creative Commons License


Erin Palette is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to amazon.com.