My rustic writer's retreat: portable power (part IV)
Great, we've got our battery. Can't do much with that, though. We need to be able to plug in appliances like computers and margarita machines. We'll need an inverter. A power inverter changes direct current to alternating current, which is what you get from those plugs in your wall. I obviously know exactly how inverters work (electricity gnomes, right?), but if you're curious, here's a more detailed breakdown from everyone's favorite good-enough source.
Power inverters come in lots of shapes and sizes to serve lots of applications. Maybe you've hooked one up to your car's cigarette lighter outlet to give you power at your tailgates (seriously, has anyone ever used a margarita machine?), or maybe you use a small inverter to charge your phone in the car.
The trick is to get an inverter with a high enough power rating to accommodate your maximum power needs. I'm primarily using this to charge my laptop, which, according to my power brick, has a maximum draw of 3.5 amps at 18.5 volts. I'll need an amp that can deliver that peak. To Home Depot!
I actually had a Home Depot gift card burning a hole in my pocket (thanks mom!), or I would have done some smarter shopping. When I got to my local store they only had a couple inverters that were suitable for my needs. I ended up choosing a 500 watt inverter from Black & Decker. According to the package, 500 watts will put out 4.17 amps at 120 volts (note, the inverter changes 12 volt DC to 120 volt AC).
You may have just noticed something interesting (though probably you just glossed right over those numbers, didn't you? Didn't you?!). The inverter changes direct current to alternating current. But my laptop power brick, which draws alternating current, is sending direct current to the laptop. So when you plug your laptop in, the power brick changes your wall outlet's AC to DC. What I'm doing with this big ass battery and inverter setup is changing DC to AC, which is then turned back into DC by the power brick for my laptop. Okay ...
If that sounds massively inefficient, it is. And if the only thing I planned to plug into this charging station were the laptop, I'd skip the inverter and buy a special laptop cable designed to draw DC directly. But, like I said, I want to be able to plug any home device into this thing, so I'm going going to have to live with inefficiency.
Putting it all together is pretty simple. You attach the supplied jumper cables to the battery terminals and voila, you've got a couple plugs to plug things in. The Black & Decker inverter also has USB outlets, which is great for charging a phone or an iPod.
I wanted something a little more stable than the cheap clamps on the jumper cables, so I went to a boating store nearby and bought 12 gauge electrical wire. I crimped some connectors on and screwed them to the battery terminals for a more permanent fix. I also bought a battery box from the boating store to hold the whole thing together. Then I drilled some holes and screwed the inverter to the outside of the plastic box. Pretty snazzy!
The last element is the battery charger. This system won't be much good unless I can replace the energy I've taken out. I returned to Amazon, our retail overlord, and asked for an offering. After reading lots of reviews, this bad boy [LINK] seemed like my best buy. To charge the battery I just hook the supplied charging cables up to the terminals, plug the charger in and select the appropriate voltage and battery type (12V / Absorbent Glass Mat), and let 'er rip.
This charger tells me how full the battery is with a percentage readout, which is great (although probably not spot on).
Initial tests with this thing are promising. It'll take a few cycles for the battery to settle to its true capacity, but so far so good. I've noticed that my laptop's charging rate with the battery is a little slower than from the wall, probably because the inverter isn't putting out as much current as a typical 120 V AC outlet. But the battery is supporting multiple charges and I'm eager to do some real tests in the field.
I deem this project a success ... and now I'm one step closer to my rustic writer's retreat.