Buying the Hardware: Inverters

If you’ve followed this project from the beginning, you know that one of the reasons I wanted to go to a 48V DC system was for generating AC power.  At 48V, generally you get more capacity (in terms of power) for your money; because of it’s popularity in off-grid installations, there’s arguably more choice, particularly as you move to higher capacities.

Why is this?  Well, to start, most of the internal components (everything from wires to traces on circuit boards, transformers, etc), have to be sized based on current (remember that the power, P, dissipated across a wire is equal to the resistance times the square of current through it).  When we quadruple the voltage, we quarter the current.

That means a given component–like a wire–at 4 times the voltage carries 4 times the power with a quarter of the relative losses.  It also means that we can buy inverters with much larger capacities, which just aren’t practical at 12V.

Since we’re wanting to be able to run everything through the inverter, we’re going to need a big one.  Two air conditioners, water heater, microwave, washer and dryer, dishwasher, refrigerator, TVs, ice maker…you get the idea.  If you start looking at pure sine wave inverters, you’ll notice a few things:

  • There isn’t much price difference between an inverter and one that also includes a charger.
  • Broadly speaking, there are 2 classes: Chinese-made, cheap-looking stuff, mostly found on eBay and similar sites, and more expensive stuff from brands most are familiar with (e.g. Magnum, OutBack).
  • Once you start getting to 6kW and larger, split-phase power is pretty common.
  • There’s a big range in surge ratings, and the intervals over which they can be sustained.

I was uncomfortable with most of the cheap stuff.  When you notice errors, even as simple as spelling things right, and you know there’s essentially no warranty or support, it’s a big risk.  Not to mention possible consequences if something goes wrong.

I also wasn’t interested in a lot of the bells and whistles on the higher-end stuff–I’m building my own energy management and automation system, and that kind of stuff would be under-used.  In the process, I ended up stumbling upon AIMS Power, a company based in Reno, Nevada, that’s been around for about 15 years.  They offer a 10kW split-phase model that would do everything I was after, and was pretty reasonably priced at about $2350 delivered.

That was still a lot of money to spend, though, on a project still in its early stages, and with a company where I really didn’t have any independent reviews or first-hand knowledge of their products.  But like with my own business, they make their stuff available on Amazon.  Though I won’t speak to his engineering credentials, Terry Bradshaw has talked about their stuff.

As I poured over spec sheets, there was one thing that gave me some hesitation about the big inverter–it’s standby power consumption.  At 200W, it would put a real dent in how long I could go in boondocking mode.  So I decided to do two things: test their product out with a smaller version, and tweak my system design to include 2 inverters–one for all of the big loads, like air conditioning, and a smaller one that would handle the always-on stuff, like certain outlets, computers, and the refrigerator.

The model that I’d use for that task is the AIMS PICOGLF20W48V120VR.  I could get it delivered in 2 days with Amazon Prime, which also meant I had some assurance it wouldn’t be a total dud and could return it without hassle if there was a problem.  And instead of a $2000 roll of the dice, this was only $628.

AIMS 2000kW 48V Inverter/Charger

Let’s start with the basics on this model.  First of all, it’s outputting 120V AC power at 60Hz.  Be careful as there’s a lot of off-brand stuff that’s set up for “world” markets, at 240V and 50Hz, which won’t do you much good here.  A dead giveaway that you need to pay really close attention is a universal receptacle, that takes a variety of international plug types.

It’s capacity is 2kW, which by itself is enough to run a single large appliance like a microwave, and it’s a 48V nominal inverter.  It’ll limit what we can use in terms of capacity, as it’ll only operate down to 40V (the Volt battery pack’s usable range goes lower), but at this stage it’ll also serve as a secondary battery protection mechanism–it’ll shut down well before we need to worry about the battery.

It also has built-in charging, though it’s not specifically for our battery pack and would over-charge if left on its own.  But it is a 15A max charger, with a variable current limit (remember, 4X the voltage means this is equivalent to an 60A charger at 12V).

Getting a little more into the smaller details, it has a pretty reasonable (for our use anyways) power consumption at 60W max. Under load, it boasts a 6kW 20-second surge rating–which means there’s plenty of time to shut something off should we get the microwave and two hair dryers going at the same time.  It also allows selection of “grid” or “battery” priority–this allows some configuration of how the transfer switching works.  All of the circuit boards are conformal-coated, making it resistant to dust and moisture that can cause damage.   I don’t particularly care for the style of terminal strip they use–it’s not easy to reliably torque the flat head screws as the whole strip buckles and twists. But several other big-name manufacturers use the same ones. Perhaps most importantly, I can download a user manual on their website, and it’s well written in English.

While I’ve only been using it for testing so far, I’m pleased with the purchase.  It’s doing what I need it to do, seems well made, and price and delivery were just right.

Buying the Hardware: Battery Monitoring/Charging

I mentioned in the previous post that a new battery  monitoring system was going to be built to allow use of run-of-the-mill charging equipment, and that it’s cost would be less than $100.   It’s also going to be internet connected–we’ll be able to log state of charge, voltage, and a number of other performance metrics to a Google spreadsheet if we want (and that’s easier than it might sound).  How do we do this?

Continue reading Buying the Hardware: Battery Monitoring/Charging

Buying the Hardware

Here’s where things start to get pretty interesting–and where money really starts leaving never to be seen again.  I’ve talked briefly about the battery pack itself, and still plan to write more about that decision process, but it’s in-hand and looks well suited to do the job.

That’s the easy part.  Now we’re into figuring out how to charge the battery, how to protect the battery, and how to use energy from it.  Before we go too far, let’s look at a couple of things about the battery pack.

Continue reading Buying the Hardware