Ask Lemmy

Im happy to explain pastermil. So first off let's talk power.

Electrical Power Systems

Most off-grid electrical systems have a few major components. A device that generates electrical energy, a battery that stores excess electrical energy for later, and a power distribution interface which allows for connecting appliances to the batteries in a safe standardized way.

My particular electric system has a 200w solar panel for power generation, two 20ah lifepo4 batteries for capacitance, and the charge controller acts as a very basic interface with two usba slots and a car cigarette port.

AC vs DC Appliances

Now let's talk about AC and DC appliances. Theres essentially two kinds of electrical power people deal with. The one most people are familiar with is AC power it comes to your home from power plants through power lines and transformer boxes. Its very easy to transmit long distance however its very high voltage so only very power hungry devices like kitchen appliances and washing machines and AC compressors use it directly. It's why american homes have a seperate 240v circuit for kitchen and basement.

Offgrid electrical systems with batteries tend are DC powered by nature. The difference is technical but the way the power flows through the system is different. Direct current moves in a straight path while alternating current moves back and forth.

Most consumer devices in your home dont actually use wall outlet AC power directly, it uses converted stepped down DC power. Desktop computer power supplies, Laptops, monitors, vaporizers, led lights, DVD players, audio speakers, your phone. everything that can powered by usb and batteries. Everything that has barrel plug inputs and power bricks plugging into it.

If you look closely on the power bricks plugged into the appliance you'll see that it has an input and output voltage rating. The input tends to be 120vac here in america 240v over the pond, and the output tends to be either 5v, 9v, 12v, 15v or 20v DC usually up to 5 amps.

Device vs Voltage Examples

Laptops and computer monitors tend to be 20v, fast charging smart phones and the Nintendo switch docked are 15v, very bright home LED lights can be bought that are powered at 12v directly, the ps2 could be powered with 9v, and most usb devices charge at standard 5v. Would you like to guess which voltage profiles the USBC-PD protocol is capable of? Its all of them.

Energy Conversion Efficency Losses

Now let's discuss energy efficiency. Converting from AC to DC eats up some of your power. So does converting from DC to AC. And its not small losses either, each time you convert its about a 15% loss in efficency.

This loss through conversion doesn't matter when you pay cents on a kilowatt and have unlimited power at the tap. It adds up very quickly when you have a limited power supply.

Let's say I want to power a laptop on my offgrid DC system, and I only know how to power it with the AC cable that it came with. I would need to

1. Convert the DC power of the batteries to AC through an inverter. 15% efficency loss.
2. Then convert that power right back down into DC with the power brick plugged in. 15% efficency loss.
3. The inverter and power brick are both parasitic draws. They eat a bit of power just sitting there even if nothing is being powered. Lets add 5% total system efficency loss each.

Add these up and you get 30-40% of your power eaten up needlessly double converting the power. Wouldnt it be really nice if we could convert the battery DC voltage directly to the appliance DC voltage without those power hungry inverters and transformers?

What DC-to-DC Converters Are

Thats where dc to dc converters come in. They still introduce efficency loss but way way less only 10% total.

Traditionally you would hope your device had a 3rd party travel adapter for car batteries and use car plugs. If you were SOL you has to wire up boost converters to raise up voltage and add resistors in series to lower it. You ever try to wire and solder your own circuts before? Its a tedious experience. Imagine doing that for each device voltage.

A USBC-pd 100w charger that plugs into a cigarette port or is built into a power bank can convert a batteries 12vDC into 5v, 9v, 12v 15v, and 20v dynamically depending on the device.

Do you know how magical that is? How much trouble that saves when it comes to mcguyvering a DC appliance that only came with AC cable to supply proper power directly? All I need is a 10$ cable to manually select the voltage needed and some barrel plug adapter bits to fit into the appliance.