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

• A charge controller which regulates the incoming raw electrical power from the generator as it charges up the battery, and smooths out the battery energy output

• A power distribution interface which allows for connecting appliances to the batteries in a safe standardized way.

My particular electric system has a 200w 28v solar panel for power generation, two 20ah lifepo4 batteries connected to double capacitance, and the charge controller doubles as a very basic interface with two usba slots and a car cigarette port.

AC vs DC

Now let's talk about AC and DC. Theres essentially two kinds of electrical power people deal with.

The difference between Alternating Current and Direct Current is in the way the power flows. Direct current moves in a straight path. Alternating current moves power back and forth in three perfectly spaced cycles.

AC The one most people are more familiar with is AC power. it comes to your home from power plants through power lines and transformer boxes. You move around extension cords and plug the three prong outlets into a wall.

Alternating Current (three phase) power is very easy to transmit long distance however its very high voltage. So only certain power hungry devices like kitchen appliances, washing machines, dryers and AC compressors use it directly. Most of your consumer home devices need to convert AV power down into more manageable DC power.

DC Offgrid electrical systems with batteries are Direct Current by nature. All your power comes from the battery banks. The power moves straight from battery terminal negative to positive. It flows right through your appliances in one way out the other.

The battery banks tend to be arranged into 12v, 24v, or 48v depending on the systems power draw and transmission needs.

The popular standards for delivering direct current are:

• 5v 2.4a usb (15 watts)

• 12v 10a car cigarette plugs (120 watts, can be rated to supply 24v fused 15a I believe though not common at all)

• circular dc barrel plug connectors, the most common size is 5.5mmx2.5mm but there are dozens if not hundreds of slightly different barrel plugs. Part of what makes usb so great is reducing arbitrary manufacturing complexity like this.

• usbc-pd various voltages depending on charger, cable, and device at up to 100w for current protocol.

• solar quick connects tend to be for connecting and transmitting high voltage DC power to charge controllers and power banks. Its worth mentioning but not that relevant to what were talking about.

Most consumer devices in your home dont actually use wall outlet AC power directly, it uses wall power thats been converted and stepped down to 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% total 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 and every watt hour counts.

Let's say I want to power a laptop on my offgrid DC system and my only means of powering it is the AC power brick 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 slightly different 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 away by this needless double converting. 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 can convert one DC voltage to another. They still introduce efficency loss but way way less only 10% total.

Traditionally you would hope your device had a commercially available 3rd party travel adapter for 12v car batteries. The dc to dc converter is built in and uses car plug.

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. Oh wait, you dont have to. Here's what that looks like.

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$ usbc-pd to barrel plug cable that manually selects the voltage needed and some barrel plug adapter bits to fit into the appliance. Energy efficent and simple wiring. All the dynamic controller stuff is abstracted away in a safe way. Powerful enough to deliver 100 watts of power, and its going to be more powerful over time.