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An old solar-powered house

Joey roasts venison While my sister is experimenting with Plug and play solar backup, I am in the process of learning to live at a isolated, off the grid, solar powered house.

This house was built in the mid 90's, and designed for both passive solar power (with its southern exposure and earth-sheltered rear), and solar power, with a wiring system that is entirely 12 volt throughout. But 15 years is a long time for a solar system to age, and as it degraded, the occupants adapted to living with less and less power.

That is an adaptation I have not made yet, so will this aging system be able to meet my needs? Like Anna, I am starting off with some experiments. The first was to go live there for a week and use as much power as I wanted.


Today I'm back in town. After approximatly 4 full days of use, the first battery bank dropped to 9 volts, my cutoff point for safe use. Which turned out to be below the safe use point of my laptop power adapter, which burnt out the last evening I was there.

I decided to come back while the other bank is still relatively full, leaving the low one connected to the four 64 watt solar panels to charge.

My modest PV array

Hurrying downtown to grab lunch in between work on Branchable, I noticed it was a beautiful sunny day, and I realized that this makes such days even better, because besides enjoying them, I know I'll be enjoying the yield on chilly nights sometime later.

Well, in theory. Actually, the very antique charge controller in the house was dead and bypassed, so I removed it. I called its manufacturer wondering if it could be refurbished, but they suggested it belonged in a museum. So I've ordered a new controller, a Xantrex C-35. Until that comes, pretty days like today will charge, or possibly over-charge the batteries, which will then drain back out at night.



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Just in case anyone doesn't know, Joey is my brother who also created the software that this blog runs on. Click on the link to Branchable above and you'll be routed to his new business where you can create your own customized blog (or wiki.) Branchable is a great way to go beyond the cookie-cutter blogs you can make on blogger or livejournal (and add your own Google ads to make a bit of cash). It's easy and fun!
Comment by anna Fri Sep 10 16:52:02 2010

I'm jealous. One day...

So do you have Internet at that house? :P $DAY_JOB requires me to be connected at most times.

Comment by Shannon Sat Sep 11 00:36:59 2010
Sounds interesting, how about some photos of the homestead. If the batteries are bad you should be able to find removed UPS batteries at a scrap yard. I needed a battery for my diesel generator and the scrap yard north of me had a whole pallet load of sealed UPS batteries. The batteries are removed from service after a period of time even though they are still good. They also come with a six month guarantee. Just take your voltmeter with you and pick out the batteries with the highest voltage.
Comment by zimmy Sat Sep 11 07:57:01 2010
Joey does indeed have internet access --- I think he's going to make another post soon about how he got his internet access going with this system. I'll pass on Zimmy's wish for more photos too!
Comment by anna Sat Sep 11 16:46:55 2010

The batteries are old (and were recycled into this house from a vehicle), but have been taken good care of and still seem to be doing well for their age. Almost all the ones in use charge up to their original 6+ volts.

Still, I don't know if they're holding a charge as well as some newer batteries might. If the system can't meet my needs, I'll be looking at newer batteries first. And probably last, as more PV panels is outside my budget.

Comment by joey Sat Sep 11 17:16:26 2010

Joey,

Does your charge controller include a maximum power point tracker? It doesn't appear so from the specs I found.

If not, you might want to consider getting another charge controller! Unless the battery voltage is close to the maximum power point of the panel (which varies with irradiation, by the way), efficiency is going to be suboptimal.

I had to make one for the solar-powered battery-less vehicle that was the graduation project for my B.Sc in the early nineties. I've read they're increasingly built into panels nowadays to optimize individual panel efficiency. But your panels are old enough that they probably haven't got them.

Comment by Roland_Smith Sun Sep 12 03:31:52 2010

MPPT is a neat optimisation, especially since it improves efficiency in winter. But controllers with it cost 3 to 4 times the one I bought.

If I had to choose between more panels and MPPT, trading up to a better controller with it would likely be a good choice.

Comment by joey Sun Sep 12 19:36:18 2010

An MPPT isn't that hard to make. You need the following parts;

  • sawtooth waveform generator
  • variable resistor to bring the panel voltage down to something you can compare the output of the waveform generator to
  • an op-amp as a comparator
  • a mosfet to switch the outgoing power from the panel.

I can send you the schematic if you're interested. I cannot recall the precise components I used then, it was almost 18 years ago. :-/

Comment by Roland_Smith Mon Sep 13 13:54:55 2010
You lost me at the op-amd and the mosfet :)
Comment by joey Tue Sep 21 15:16:46 2010

An op-amp is a component that produces an output signal based on two input signals. It is used to produce the pulsed DC signal based on the voltage of the panel and that of the sawtooth voltage generator.

A mosfet is basically a transistor (electronic switch) that is switched by a voltage instead of a current like a normal transistor. In the MPPT it is used to control the total current from the panels which would fry an op-amp. Think of it as a kind of small and super-fast relay.

I'd guess you would be able to find schematics for an MPPT in electronics hobbyist magazines. The mppt article on wikipedia has some pointers as well.

Comment by Roland_Smith Wed Sep 22 14:28:07 2010