My rustic writer's retreat: portable power (part III)
We won't get anywhere dicking around with tiny lithium ion batteries. My portable power solution has to be cheap, powerful, and capable of charging my laptop six to nine times. So where does that leave us? I briefly toyed with the idea of a solar solution. Cool as these things might be, they're not practical for my purposes. By way of example, here's a great solar kit called the Sherpa 50.
I love the look of this thing, especially that shot of the power unit sitting on the grass next to a bunch of gear (I'm a softy for carefully staged photos of gear), but for $350 (sale price) this only extends the life of my laptop battery by two hours. It would then take 8-16 hours for the solar panels to recharge the device's internal battery. There are plenty of applications for something like this, but a power-hungry writer's retreat ain't one of them.
I decided to look at a big honking 12V battery, similar to the kind that powers the starter in your car. Typical automotive batteries, while easy to find and relatively cheap, are designed to deliver a lot of current very quickly. But I'm looking for something that'll give me a steady draw of 3.5 amps at 18.5 volts over a lengthy period of time.
Deep cycle batteries do just that. Wholesale Solar explains the difference between deep cycle and shallow cycle batteries here. Suffice to say that I'm going to want a deep cycle battery if I want my portable power system to last. So where do you find deep cycle batteries?
I found this bad boy, a 50 Ah deep cycle battery designed to power electric scooters. It's $100, so not cheap, but it should do the trick. My laptop draws 3.5 amps at 18.5 volts. This is a 12 V battery, so we'll have to recalculate to accommodate the higher 18.5 volts (3.5 amps x 18.5/12 = 5.4 amps). This is a 50 Ah battery, so if I'm drawing 5.4 amps the battery should last 50 / 5.4 = 9.3 hours. Since my laptop takes an hour to charge, this system would theoretically give me 9.3 charges. Excelsior!
Of course this battery won't actually charge my laptop for 9.3 hours. Batteries aren't 100% efficient, for one thing. And there's another issue: AC/DC isn't just some old guys in goofy face paint. Your home appliances are designed to use alternating current, but batteries deliver direct current. If I want my portable power system to run any home appliance without a special adapter (read: margarita machine), I'm going to need to change DC to AC.
Up next: a power inverter, a battery charger, and voila!