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Friday, March 12, 2010

12 or 24

Twenty years ago I installed a solar electric system. I had some choices to make back then. One choice I've regretting a bit was going with a 24 volt battery bank. It made sense at the time, but it's proven a bit inconvenient ever since.

My solar array is a fair distance from the house. Low voltage DC lines lose a lot of power over fairly short transmission distances. Bigger wire reduces the loss, but good wire is expensive. Another way to limit power loss is to up the voltage. A 24 volt system has a lot less line loss than a 12 volt system.

Back then, the charge controller had to be 24 volt and inverter was 24 DC to 120 AC. The battery bank consists of 12 heavy duty 6 volt batteries. They are wired in series, 4 in a row, to add up to 24 volts. Picture 3 of those rows (3X4 = 12). The groups of 3 are wired in parallel.

Here's the thing I regret. There are all kinds of nifty things that run on 12 volt DC. Think of all those things that plug into a car cigarette lighter. Add on all the gadgets built for motor homes. Hooking things directly to 12 volt DC would eliminate the efficiency loss caused by the inverter. I wouldn't want to wire everything in 12 volt, as it takes a lot of heavy wire, but some things would be nice.

Almost nothing runs on 24 volt DC. It's possible to get a few things, but not many.

There are a couple ways around the problem. It's possible to get a DC to DC converter that changes 24 volt to 12. Problem is, they cost money, and have inefficiencies of their own.

Some of you are thinking that it wouldn't be too big a deal to just pick two batteries in the battery bank and tap off 12 volts. I actually did that for a while. The problem is that the battery is now discharging unevenly. Those two batteries you've tapped into may never take a proper charge. It could shorten the life of those batteries, thus hurting the performance of the whole battery bank. To spread the damage around, I'd move the 12 volt tap on a rotating cycle. The idea was to spread the load around. Also made sure to put the battery bank on an equalization charge fairly often. It was a pain. Sometimes I'd forget to move the tap. I was never happy with it.

A few years ago I changed the old charge controller to an Outback charge controller. It could be programed to accept one voltage and put out a different voltage. The solar array was rewired to run at 48 volts. That reduced my voltage line loss. The controller could easily take in 48 volts and put out 24.

Now It's just as easy to have it put out 12 volts instead of 24. It wouldn't be that hard to rewire the battery bank to run at 12 volts. The one big remaining problem is the inverter. They aren't cheap. The expected life of a quality inverter is about 20 years. I've had it for 20 years so I should probably think about a replacement. One thing holding me back is that it still works fine. I hate to fix something that's not actually broken.

Money is pretty tight. However, things might loosen up later in the year. Should that happen I'll probably upgrade my solar electric system -especially as I'm probably going off grid completely sometime this summer. I'd like to add a small wind turbine to my system. We get too many cloudy days in the fall and winter. Do I get a 12 volt wind turbine or a 24 volt system? It might be the time to go to 12 volt. That decision can be put off until the fall . . . unless the inverter gives up the ghost.

When you make a substantial part of your own power, you tend to keep thinking of improvements. You think of what can go wrong. You develop backup plans.

For example, should my inverter die and I can't afford a replacement, I've got a plan. I'll rewire the battery bank to 12 volt. I've a 2000 watt inverter in my truck (12 DC to 120 AC). I'll pull it from the truck and install it in the basement.

Now the current house inverter also has a charging function built in. Very useful for topping off the batteries when the sun doesn't shine. The inverter from the truck doesn't have that charging feature. There is a work around. I've a good quality high amperage battery charger. (12 volt) I could charge the battery bank from a portable generator or other power source.

Yes, I've got backup for my backups.

-Sixbears

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