Thursday, February 11, 2010

The batteries are charged & I didn’t even know it

I’ve been keeping an eye on our solar electric system the last few days in order to watch how it’s performing. It’s been overcast every day it seems, and we even had some snow. However, the panels have been outputting power every day for about 9 hours a day. I was looking forward to seeing what kind of numbers we would get on a sunny day.

Today the sun actually shown through. However, I was disappointed and concerned to see an input value of only about 150 watts. Naturally, I wondered what was going on. I noticed that when I turned the photovoltaic array back on after momentarily disconnecting it via the circuit breaker that the power input would peak around 1,000 watts and then rapidly decrease.

For the previous days, I had noticed that the peak power input seemed to occur quite early and was much higher than the average input during the day. It seemed like something was bleeding off the power. So, I started checking my wiring on the panels. Interestingly, the input power was the same whether I had all panels connected or only some. I should mention that the source of my readings is the display on the Xantrex XW60 charge contollor.

Maybe I’m a bit slow, but I began to wonder if maybe the explanation had something to do with the solar charge controller. I couldn’t find anything in the manual, though. So, I called the technical assistance number and spoke with a representative, explaining what I was experiencing. She told me that the charge controller was adjusting the input because my batteries were fully charged or nearly fully charged.

I liked this answer, but it surprised me. I was sure that the batteries were a bit low based upon my voltage readings on them. I guess I was wrong. We’ve not connected the system to our house yet; I wanted it to fully charge the batteries first. Since it appeared that they were fully charged already and we had good sun today, I told the charge controller to equalize the batteries. This means that it purposefully charged them at a high voltage in order to mix the battery acid inside the cells. The reason for doing this was so that I could take specific gravity readings for each cell to use as a baseline for determining battery health in the future.

So, the batteries have been equalized. During the equalize cycle, the input was around 350 watts. When the cycle ended, I watched the input drop to zero watts. Confirmation that the charge controller was indeed adjusting/limiting the input. Very cool.

Only, I really have no way of knowing what kind of realistic input I can expect on cloudy days, now. What I’ve observed the last few days has been the charge controller adjusting the input to float charge the batteries. I guess it will be at least as much as we’ve seen so far, maybe more. Over 3.5 cloudy days, we registered 1.5 Kilowatt hours. Today only added half a kilowatt even with good sunshine and an equalize cycle. The possibilities are looking good, actually.

I’m going to take my specific gravity readings tonight. Then, tomorrow, I hope to complete the wiring so that we can flip the switch and be off the grid by tomorrow night.



Very cool.

I think what you were interested in is how much power could be delivered on a cloudy day (if the batteries were not fully charged).

I wonder if you could cut away from the batteries, and place the system under a load, and see how much could be delivered directly.

But a safer way may be to do this after you are connected to the house wiring, and then just put on a load, and as long as your load is greater than the solar cells can produce at the time, then you should get a reading of their full output at the time. Right?

Fun stuff!



Yeah, I want to know how much power can realistically be expected on a cloudy day. I thought I was seeing that because I believed the batteries were not fully charged. I put them on a charger for a few hours several weeks ago because they were low when I got them (probably sat on a shelf in a warehouse for a while without a float/trickle charge connected). I'll find out what the system can really do once we have things connected to the house wiring and apply a load, like you say. It shouldn't take long to connect to the inverter. I'll post about it when we throw the switch.

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