2014-10-07 Fast Charging From Car’s 12V Socket

Earlier this year I bought a cheap cell phone charger for plugging into the car’s cigarette lighter socket.  It works, but it’s s-l-o-w.  I can drive for a half hour with the cell phone plugged in and it says it’s charging but it only charges a few percent.  As a result, I don’t use the charger that much; I charge it in the house. The Samsung Galaxy S4 with the Google version of Android tells the percent of charge on the screen when it starts charging.

I decided to try to speed up the charging in the car a lot by maximizing the charging current at the micro USB plug. Two important factors that determine charging rate are the maximum current that the charger will put out, and the DC resistance of the wire between the charger and the micro USB plug. Some chargers say they put out 2 amps, but I could not get them to put out more than 1.2 Amps due to the resistance of the wire.

I decided I would try to minimize the resistance of the wire. I used a very short – only 2 inches – long cord. It sped up the charging to over 1 Amp. This showed me that the cable’s wire resistance is a major factor in how fast the device charges.

Even so, the charger itself is going to limit the maximum current. Chargers may do this more than one way. When the charger reaches maximum current it may reduce the output voltage, or it may shut the output off. The output may go off when the temperature gets excessive. Or the charger may sense the current and turn off. This may happen so rapidly that the output may consist of a series of pulses.

When the charger is running at its maximum, then the cable is doing a good job and is losing very little power. If the charging current is less than maximum, it could be limited by the resistance of the cable’s wires. Or the device itself may reduce the current.

I also wanted to be able to plug in a laptop AC adapter, which means that I need an outlet that puts out 120 VAC at up to 100 watts. I found an inverter that puts out 120 VAC at up to 150 watts and also has a USB port for charging. I put a very short micro USB cable between it and the phone and it charged really fast! Well over 1 amp.

But I really didn’t want to drive around with a box with heat sink fins sitting on the console between the front seats. So I went online to look for a reasonably priced 12 V to 5 V DC to DC converter, and found this small box that puts out 5 V at up to 3 A. It and other DC – DC converters can be found at ProDCtoDC.com. I decided to order two types: one that has red and black leads about a foot long with bare wire ends that go into a small plastic box about 1 by 2 inches. Then a short cable comes out of this box with a USB A socket on the end. Any charging cable can be plugged into this USB socket. The other cable I ordered has a ten foot cable with bare wires to connect to 12 v battery terminals, that go into one end of a small inline rectangular box. Coming out of the other end is a foot long cable with a micro USB connector that plugs directly into the device to be charged. This arrangement allows for charging a long distance (10 feet or 3 meters) from the car’s socket yet has minimal losses in the 5 volt wiring. Any loss in the 12 volt wiring is compensated by the converter. This inline box holding the circuit can be popped open with a thin screwdriver blade or knife.

I went to Orvac Electronics in Fullerton and bought several cigarette lighter plugs ($1.49 US) to put on the ends of the 10 foot cables. I cut off an odd jack and soldered the cable to the wires and covered the wires with heat shrink tubing.

These converters draw about 2/3 to 3/4 amp from a supply set to 13.8 volts. The load I used was a Samsung Galaxy S4 cell phone that was charging at about 50 percent. I could not measure the 5 volt current because the 5 volt output goes directly to the micro USB plug. It wouldn’t do much good if I could measure it because the phone’s load varies a lot as the CPU goes about its business. But I was impressed by how fast the phone charges compared to the cheap cigarette lighter charger plugs that are for sale in stores. The much higher current cuts the charging time to less than half. I was driving for less than an hour, probable about 50 minutes, and the S4 charged from 19 percent to 64 percent, about 45%, or almost 1 percent a minute. Excellent!

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