Tuesday, September 15, 2020

Portable Power

Recent events have gotten me thinking again about emergency and portable power solutions. Storms here and elsewhere have left friends without power for extended periods of time; in addition, medical developments in my life in the past few months have made it almost a necessity that I have daily access to 120v power. No, I'm not dying, at least not any faster than the rest of you; I'm on a CPAP machine to help me breathe while I sleep. My wife says it makes me far more pleasant to be around.

The obvious answer to long term power is a generator. We've discussed them in the past, and the topic may be worth revisiting in the future, but they have some weaknesses. Two of the biggest, and definitely most applicable to me, are portability and the inability to run them at night. Running a genset during the day is all well and good, but I need power at night, and that means batteries. If I'm in my camper or have a vehicle handy, I can use the battery power in those so long as I don't overdo it, but I'd rather have an independent option. Today, we'll look at the theory behind constructing a portable power box, and later we'll actually assemble the device.

Generating 120v power is easy, if you have a 12v source. You simply hook up a device called an inverter, which converts the power through the magic of electrical theory. (Seriously, the theory involved takes up an entire year of electrical school, and it probably still best described as wizardry.) Small inverters can be had very inexpensively, and will provide enough wattage to power small medical apparatus, charge cell phones, and keep other small devices running. Larger inverters can power heavier equipment, but that will murder your battery life.

Our 12v power supply will be batteries. A 12v car battery would be simple, and have plenty of energy to run our inverter for ages, but car batteries are large and heavy. By the time our whole box is done, a car battery could push it to 80 or more pounds, and nobody wants to lug that around. Motorcycle or "powersport" batteries are an option, weighing in at 5-7 pounds, but they aren't sealed, meaning they can leak acid if the power box tips over. They're also designed to start an engine, meaning they discharge best in short, heavy bursts, and not in long, slow loads. Instead, we will use a set of 6v lantern batteries. They're light, fairly inexpensive, and are designed for the kind of draw we're going to put on them. By wiring them in series, we can bump the voltage to 12v, getting us the power we need for our inverter.

Rechargeable 6v batteries have about a 4.5 amp-hour rating. What that means is that they'll give you 4.5 amps for one hour, or one amp for 4.5 hours, or anything in between, based upon your current draw. My CPAP, since it is the device currently in question, draws 0.75 amps, meaning that in a perfect world two lantern batteries would run it for 6 hours. The world isn't perfect, though, so for that device, a more reasonable expectation would be 5-ish hours. We can extend that with a second set of batteries, doubling our available time.

To figure how many amp-hours you need, look at the devices you intend to power. Either the power supply for the device, or the device itself, will list the wattage or amperage draw.
  • If it's amps, divide by 4.5 to find out how many hours you have available;
  • if it's watts, divide by 120 (the nominal voltage draw) to get the amperage draw. 
Then figure out your available time. If one set of batteries is enough, wonderful. If not, we'll get around to adding a second. I'll cover the specific wiring method during construction, since those are kind of "show-me" items.

Using four of these batteries and this inverter, plus a few bits and bobs, I'll be into the project for about $100 but it will give me all the power I can use in a day. It will also weigh in at a svelte 10 pounds, making it very convenient to grab and go. I could save some money on both the batteries and inverter, but I like the attachment points on these batteries, and the inverter has some nice protection features built in that make it worth a few extra dollars to me.

Do some figuring on small devices you need to keep running when no utility power exists. Once we come back to this, you can figure for yourself how many batteries to stack in to keep your critical items powered.


1 comment:

  1. Don't forget that if the device has a power adapter brick that bypassing the power adapter makes the process so much more efficient. Turning DC into AC then to DC in order to supply a device that needs DC anyway is very inefficient.


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