I am converting a wheelchair access vehicle into a campervan. Having removed the old wheelchair ramp lift mechanism, I'm left with a couple of 12V cables in the back.

I'd like to turn one of these cables into a phone charger, i.e. a simple micro-USB cable (see below).

enter image description here

The question is, knowing that I have 12V coming through the cable, could I simply attach a DC to DC converter to reduce the voltage to 5V (which is needed by the phone), followed by the USB cable? Are there any other specifications I should check?

Here's the DC-DC converter I'm thinking of using:

enter image description here

I have only a (very) basic knowledge of electronics, so wanted to check this project seems feasible.

  • \$\begingroup\$ Honestly the only concern you would have to take in account of is the current of the 12V supply. The current matters when using a step down converter, as too much current can blow out the step down. You may also want to consider a buck-converter type 12V to 5V voltage converter as they are more energy efficient. \$\endgroup\$ – Bradman175 May 31 '16 at 9:39
  • \$\begingroup\$ It would not surprise me if that CPT module is a buck converter. So yes, this will work fine. Do pay attention to how you connect the USB cable to the 5 V and ground. Also before connecting an expensive phone, test it with something cheap, a simple rechargable powerbank for example. \$\endgroup\$ – Bimpelrekkie May 31 '16 at 9:51
  • 2
    \$\begingroup\$ Why don't you simply buy a car USB charger and a micro-USB cable? \$\endgroup\$ – Dmitry Grigoryev May 31 '16 at 9:56
  • \$\begingroup\$ @Dmitry mostly because I'd like to see if it works... \$\endgroup\$ – CaptainProg May 31 '16 at 10:27
  • \$\begingroup\$ @CaptainProg I don't get it. You could buy a car charger and see if it works just as well. \$\endgroup\$ – Dmitry Grigoryev May 31 '16 at 11:11

I've built a car charger using one of those CPT modules, out of curiosity as much as anything. They are pretty good, cheap buck converters. The noise on the output of mine is within the spec for USB charging. It is not isolating, so 12V ground is connected to 5V ground - which can cause noise if it's plugged into the stereo with an aux jack.

The tricky bit is "fast-charging" your phone. If you just provide 5V to a phone, using the power and ground wires in a USB cable, it will usually charge at 500mA. That's the same as when plugged into a PC, and it'll take a while. Most phones these days can charge at least four times that fast, if the charger supports it. They check whether the charger supports it by looking at the connection of the data wires in the USB cable, and sometimes the connection of the metal housing of the connector. There is a standard, but frustratingly many phones don't use that standard.

If you have one type of phone/tablet you want to charge you can usually look up how to wire the data leads to get fast charging to work for that type. Most brands use the same pattern across all their devices, but Apple use different patterns for different devices. You can also get ICs which detect which type of device is connected and pretend to be the right type of charger, but that's quite complicated for a beginner.

Or, if you want to do it the simple way, buy a charger with the fast-charging features, connect it to your cables, and hot-glue it out of sight behind some trim.

Oh, and whichever way you do this, work out which fuse in the fusebox feeds those cables and swap it for something appropriate - a wheelchair lift would probably take a lot of current, where one of these modules only needs about 1.5A max.


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