Monday, February 25, 2008

I've Automated my Off-Grid PV System

Because over-discharging batteries can ruin them, I only connect loads to my photovoltaic (PV) system on weekends and evenings, and only when I’m home and awake so that I can carefully monitor battery voltage. I’ve been getting 10 to 12kwh of production out of my system each month, but knowing that it’s capable of producing 45kwh or more, I’ve been looking forward to this upgrade.

When the solar panels don’t provide enough power to satisfy the AC load, energy stored in the batteries is used instead. This will occur at night, of course, but also when it’s cloudy during the day. Under these conditions battery voltage will continue to decline, and eventually the inverter will stop working. My inverter stops functioning at about 11.6 volts. Allowing the voltage to dip that low can damage the batteries, and the risk is even greater if the batteries are not recharged quickly.

Shown below is a simplified diagram of my system before the upgrade.

This is my system after the upgrade.

I’ve programmed the voltage-controlled-switch to close a relay when battery voltage is above 13.75 volts. Closing the relay switches the DC to AC inverter on. The AC transfer switch is wired to use AC from the inverter as the default, only switching to grid-supplied AC when the inverter is switched off. The voltage-controlled-switch opens the relay when battery voltage drops to 11.95 volts. When the relay opens, the inverter is switched off and the AC transfer switch connects the load to grid-supplied AC. Once the load is removed, battery voltage will gradually rise. However, the voltage-controlled-switch will not close the relay until battery voltage once again reaches 13.75 volts.

I currently have a data logger connected to measure battery voltage. If I find that the battery does not fully recharge during the day, I’ll reprogram the voltage-controlled-switch. Likewise, I may need to reprogram the low-voltage threshold for better efficiency or battery protection. I’ll determine the appropriate settings after reviewing a few days worth of data logger readings.

Shown below is the complete system, with the new components.

The voltage-controlled-switch is the device at the top-left of the picture. It’s called a “Relay Driver”, and it can be programmed for four independent functions. It gets its information (battery voltage in this case), from the TriStar Charge Controller. Mounted just to the right of the relay driver is the automatic AC transfer switch. AC from the inverter, and AC from the power grid feed in to this device, and the selected AC source is applied to the AC outlet just below the transfer switch.

The off-white aluminum box mounted just below the relay driver contains the relay. For convenience, relay inputs and outputs are wired to the terminal blocks. A relay-override switch is mounted near the bottom of the relay box. To facilitate future expansion, I’ve installed two additional relays, and an LED (barely visible on the top of the box). I’ve wired one of the relays for AC. I have some expansion ideas that I’m kicking around, and I have some ideas to improve efficiency.

I’ll post additional details after I’ve had a chance to see how the system performs. Check back for an update.



d said...

Hi SJ,

I'm betting that you'll need to move that upper threshold down a bit at least!

My grid-tie PV should go live in the next few days, but right now my Internet server is off-grid with my existing PV system anyway...



jelizondo83 said...

Hi, did you have any trouble with the inverter while switching?? I had some problems because when going from grid to the inverter, a current peak was required, specially with inductive loads although also with resistive ones... Did you experience this?

"Guppy" Honaker said...

Oh man. This blog on solar power is awesome. I want to get at least our refrigerator and airconditioner on solar power. Our little in-law house is just over 770 sq.ft. We live on Phoenix where the sun shines the majority of days. Any idea about how much I'd need on terms of pv cells and all? I know nothing about electricity and really don't understand 90% of your blog. Am in poor health and need reliable air conditioning when we have power outages in our 120 F. days of summer heat. I was on the net for a few hours before finding this blog from your link in a comment in another blog. Pls somebody call me toll-free with advice on how to go forward the least expensive way possible. I can be reached at 1-866-750-ALOE (2563) or via e-mail at
Thanks - David