Converting devices to run on USB-C PD or voltage cheap converters

This is really my page for how you might consider doing things in a slightly more modern way to streamline your low voltage electrical system, and make best use of your camping battery.

Introduction

A lot of people still run inverters to power 240v power supplies for devices. Many years ago this was often the only semi-feasible way to do power conversions for many portable devices, as making up your own voltage converter was relatively inefficient to power devices directly off 12V.

This has changed with the ubiquity of low voltage buck converters, and has been accelerated by the adoption of the USB-C power delivery (PD) standard. Modernising how you power devices or recharge portable devices can make a huge difference to the efficiency of your auxiliary battery setup, saving you a heap of power.

The efficiency gain comes through avoiding the step through an inverter. Inverters are never fully efficient, somewhere between 85 and 95% depending on load (more efficient at higher loads), and often people size the inverter for their highest current draw but this means being relatively inefficient at the lower loads of smaller devices. A classic one might be having an inverter to run an induction cooktop: it’ll be 95% efficient for your use with a cooktop at up to 2000W, but when you’re using the same inverter to run a 65W laptop charger you’re really pulling 75W, or nearly an extra A.

In addition to the inverter inefficiencies, you also have charger inefficiencies. Older power supplies can be also 80-85% efficient; more modern semiconductor technology like gallium nitride (GaN) also bumps this up to 95%. Even so, you can be throwing away a reasonable amount of your auxiliary battery connection

These inefficiencies add up once you do a few devices over a day. Maybe a couple of hours charging a laptop, couple of hours with camp lights that charge at 5v or similar. Even more so would be something like Starlink; some of the early units were supplied with quite inefficient power supplies & converting them to use power converters and power over ethernet standards gave you simpler routing options & more efficient power usage, too.

Voltage converter basics

Let’s start with some basics of voltage conversion (and regulation). I’m using the two terms interchangably here, mostly to simplify the language. Often times devices need both a converted voltage from the nominal 12v provided by your auxiliary power setup, and they also need a better regulated voltage.

Regulation first: An auxiliary battery setup can vary from ~11V to 14.5V depending on it the battery is almost flat all the way through to the typical charging voltages used; some devices absolutely need better power regulation than that.

Voltage conversion is also needed as many small devices run on 3.3V or 5V; think of it this way: 3.3V is effectively a couple of AA batteries, 5V is 3-4 AA batteries. Things like your mobile phone also often charge at 5V, so there is some inherent voltage conversion required.

The key here is making a choice about where you do the voltage conversion. If you’re doing it via a 240V wall plug plugged into an inverter, you are:

  • Pulling power from your battery setup
  • Converting 11-14.5V semi-regulated DC into 240V AC
  • Converting 240V AC to ~5V DC regulated
  • Putting that power back into a device

For something like 12V to 5V, you can do this with a DC-DC converter. Many of these are highly flexible on input voltages, and some can support changing voltages for their output. Often times the current capacity is limited by the ability to dissipate heat, so you may need to do some designing around this or buy something premade with a heat sink attached.

Effectively at this point you’ve made yourself a power supply and all is good. It’s then just a question of connecting it up to the car in some sensible way and off you. However, there’s a more modern way, where you take advantage of more standarised power converters that are a bit more flexible. This way is to take advantage of the USB-C PD standard

USB-C PD basics

USB-C PD is a method by which a USB device negotiates power out of a USB device. You’re potentially already using it and not aware of it: many modern laptops charge via USB-C PD. USB-C PD allows for up to 240W of power (through the PD 3.1 standard), although 65-100W is more typical.

Originally, USB standards allowed for 2.5W of power through the power pins: this was at a fixed 5V and a maximum of 0.5A. This was increased slightly up to 4.5W (0.9A @ 5V), and then the battery charging standard was pushed to 7.5-12W.

Some mobile phone manufacturers also made their own high power standards that were carefully negotiated with specific chargers, e.g. Qualcomm developed the Qualcomm Quick Charge (QC) protocol which allowed for

  • QC1.0: up to 6.3V @ 2A = 10W
  • QC2.0: 18W maximum (9V @ 2A) but higher voltages supported
  • QC3.0: 36W (12V @ 3A) but higher voltages inconsistently supported

There are further refinements, but almost all ‘quick charging’ traditional USB A ports will be QC3.0 these days