Sunday, July 10, 2011

Solar electrical notes, lots of them

Long post, originally intended as an answer to a vandweller mailing list post. It was too long, so I decided to put it here instead.

I started vandwelling with just a vague notion that I rely on electricity and internet a lot for my work, so I knew I needed "lots" of electricity. I overdesigned and underdesigned as a result.

After some years of thrashing around, I found out that the solution was to actually MEASURE stuff. Like, with an ammeter. And then do some math. Then I can make useful decisions to make everything work right.

For example, laptops suck for vandwelling, IMHO, especially if you rely on them a lot. Mine uses 2-5amps at 12v! When I started out in my van, I was also stupid enough to hook mine up to a large-screen monitor which required an inverter, for a year. The inverter was inefficient and used up current itself, as well as having somewhat low efficiency in converting 110v to 12v and then the monitor's own power supply converting it back to whatever the monitor used inside (probably 12v and 5v and 24v). Not smart.

What I learned is that netbooks and smartphones are nearly ideal for vandwelling. They hardly use any electricity and can be easily charged up off of your alternator (if you drive a lot) or a modest-size, relatively-inexpensive solar panel or genset and house batteries (if you stay local a lot).

If you use a 12v netbook, DEFINITELY get a car-adapter for it. Your batteries do not put out 12v-- they put out whatever they have left in them plus whatever is being used to charge them. Your alternator or solar charger or genset or whatever will shove 14V or more at your netbook. Mine took the abuse valiantly (probably has its own internal 12v regulator) but started making buzzing sounds when running in high-voltage situations. I got a DC-DC converter and all is well. When your batteries are low, they'll put out less than 12v. Get a DC-DC car charger/converter/adapter for the netbook that'll give it a nice regulated 12v no matter what.

Likewise, electric refrigerators are GREAT for a solar-based van setup, if you buy the super-efficient Danfoss (TruckFridge) models that only need 2A to run and only run a few hours a day, and keep your van and refrigerator insulated well enough that they hardly need to run at all. If you don't have enough charging or battery capacity, get a propane or 12v/propane/110v hybrid kind. My TruckFridge uses up 5-10Ah a day, and most of those during the day when the sun is shining bright, so they're hardly any drain on the batteries.

I had made the mistake of buying a cheap thermo-electric cooler and ran it for a few months-- it used 4A, ran half the day, and didn't even cool the food properly (only 20 degrees F below ambient). Dumb dumb dumb-- I trashed my very expensive battery array that way. This was before I'd learned to actually measure anything before deciding whether (or how) to use it. The TruckFridge works a lot better and uses less amps.

Having a separate "house" battery from your starter battery is a good idea in all cases, and essential if you are charging anything more than a smartphone. The House battery (or batteries) should be an AGM or other deep-cycle-- designed to be run down deeply over a long period of time. Starter batteries are designed to provide cold-cranking amps (to start your engine) quickly and then expect to immediately start being charged back up by the alternator. They do NOT like being deep-cycled or run down slowly over time, the plates get sulfated (ruined).

Also, make sure you match the battery size to the amount of charging power you have. Solar panels only trickle-charge a large battery array-- it depends on how many amps it generates-- though a large solar array could bulk-charge a small battery no problem. I was dumb enough to buy 400Ah of battery capacity for a solar panel array that could only generate 15A when at full sun at noon in summertime. Some simple math would show why that was moronic: how long would it take for a 15 amps to charge up a battery array cycled down only part way, say to 300Ah, which would need 100Ah to fully recharge? It can't do it in one day for sure; there aren't enough hours of full sunlight, even in summer. Also, batteries need a few hours of continuous charging EVEN AFTER THEY ARE FULLY CHARGED to stay in good condition. You don't want to let deeply-discharged batteries-- even AGM's-- sit around for days or weeks (or months or years, as I did), before hooking them up to something studly enough to fully charge them within a few hours.

It's OK to have a larger number of amp-hour capacity in your batteries than you can charge up in just a few hours of sunlight at whatever amps your panels put out, but you'd also need an AC converter or genset (or both) that'll put out enough amps to really bulk-charge your batteries should you run them down, and be able to do that a day or two after running them down. (I bought a 75A 110V converter a few years ago and finally hooked it up at the beginning this year. Dumb of me to wait so long.).

My old G1 Android phone has 1.4Ah batteries, so charging it up from completely discharged state uses only a couple amps. I don't worry about it either. My panels generate 15A in sun in summer, and at least 5A even in fog and winter. The phone is a non-issue.

Back to the original question (from the vandweller's list) about leaving the charger plugged in: best to measure it and find out. I put an ammeter on my smartphone charger and found out that it uses 0.01A (10mA) when idling; so it's not crucial that I turn it off or unplug it all the time. Likewise the charger for my netbook uses 0.01A when the netbook isn't plugged in, and the netbook itself uses the same amount, so to have my netbook on and sleeping only uses 0.02A (20mA)! So that's something I don't worry about most of the time.

However, my dual-core 64-bit laptop and the charger for it uses almost 0.5A (500mA) when the computer is TOTALLY OFF (weird, or design flaw), and can draw up to 5A when running all-out! So I rarely if ever use it anymore, and only when in full sun and my house batteries are all charged up. I don't even have the big-screen monitor hooked up anymore.

Very long answer, but figured I'd share some of this at some point, and this seemed like a good enough place to do it.

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