Monday, June 09, 2008

Adjusting to New Batteries and Higher Electric Rates

Every PV system upgrade has had a noticeable effect on performance, and my recent battery upgrade was no exception. The larger battery bank takes longer to charge on a sunny day, but it also powers loads for longer periods at night. Because of the battery bank upgrade, I decided that it would be wise to review system settings. My goals are:

• Prevent battery overcharging.
• Prevent battery under-charging.
• Prevent batteries from discharging too deeply.
• Get as much energy from the system as possible, reducing my electric bill.
• Maintain safety.

A charge controller prevents batteries from overcharging, and mine is adjustable via internal switches. Since my old batteries were sealed, and my new batteries have removable watering caps, I’ve readjusted charging parameters to accommodate the new batteries. I’ve adjusted the battery voltage upward a little. Some “gassing” shouldn’t be a problem, since I’ll be checking battery fluid level on a regular basis.

My batteries are in a windowed sunroom, and therefore I’ve set “Equalization” to manual instead of using the automatic equalization setting. I’ll open the windows when I equalize the batteries, allowing hydrogen gas to escape. My charge controller keeps track of the days between equalization, so I’ll know when it’s time to do it.

My automation settings help me get the most from my system, while protecting the batteries, by performing two basic functions:

• Wait until batteries are nearly fully charged before switching on loads.
• When battery voltage declines to a preset value, remove the loads.

The automation settings turn the inverter on and off, and therefore control the loads. I’ve adjusted the settings as posted below:

• Battery High Voltage Threshold = 13.85 volts (Turn inverter on)
• Battery Low Voltage Threshold = 12.25 volts (Turn inverter off)

I have a Transfer Switch, which is nothing more than a relay, wired to use AC from the inverter as the default. It switches to utility-supplied AC when the inverter is off. With this setup I can be pretty sure that my refrigerator and freezer will always be powered up.

Since I’ve just spent a great deal of money on the new batteries I’ve decided to be more conservative with the settings in order to prolong their life. I’ve adjusted the Battery Low Voltage Threshold upward, minimizing the depth of discharge. This adjustment not only prolongs the life of the battery, it means that I’ll have more reserve energy available in the event of a grid power failure. I’ve adjusted the Battery High Voltage Threshold upward, providing a little more assurance that the batteries will be fully recharged each day.

Even with these conservative settings I’ve observed a significant performance increase with the new batteries. I’ve also learned that it is beneficial to keep the load relatively high. A heavy load reduces the amount of time that the charge controller spends in the “absorption” and “float” states. In other words; more of the available sun’s energy is used to power the loads and therefore less of the sun’s energy is unused.

Adjusting to “PowerSmart Pricing”

My electricity rates vary on an hourly basis, depending on the demand at the time. I might pay as little as 1.5 cents per kwh when demand is low, and more than 17 cents per kwh when demand is high. Now that summer is here, and demand is greater, electric rates tend to be highest from about 10:00am until about 8:00pm. This has caused me to adjust my strategy somewhat. I want to make sure that I’m using electricity from the PV system as much as possible during times when rates are highest. In addition to using as much energy from the sun as possible I can take advantage of this large price differential by charging my batteries at night when rates are low, and use the stored energy during the day when rates are high.

In other words:

I sometimes buy electricity for less than 2 cents per kwh (to charge batteries), and use that stored energy when utility rates exceed 17 cents per kwh. Even with system losses considered, I suspect that I’ll come out ahead. I shift loads to the PV system when electric rates are highest, hoping to have enough energy from the sun and stored energy to last until rates drop.

To avoid manually switching things on and off in the early morning hours I use a timer. Here's a picture of my timer, charger, and battery bank.

And here's a simplified diagram of my system with the charger and timer:

I’ll have to compare this summer’s electric bill to that of last summer to see if my efforts have paid off. If so, I don’t think I’ll have any problem convincing the wife that we can benefit from a couple more panels by the end of the year.


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