My goal is to get as much as I can from my solar photovoltaic system, and at the same time protect my batteries from over-discharging. I want to be able to use battery power as much as possible, and only switch to grid-supplied AC when battery state of charge (SOC) has declined to a preset value. The first step, the installation of a “Transfer Switch”, has already been done. Next, I’ll need a way to switch my inverter on and off, depending upon the battery state of charge (SOC) at any given time. When the inverter is switched on, the load is powered by the batteries. When the inverter is switched off, the transfer switch automatically connects the load to grid-supplied AC power instead.
I’ve considered three ways to accomplish the task, and thought about the pro’s and con’s of each:
1. A simple circuit that allows precise control of low and high voltage threshold settings to open and close a relay.
A considerable amount of time is needed for development and testing. Cost is also an issue. While the finished product may do the job, it may not be as efficient as a commercially available product that can perform the same function. This device will not be easily expandable.
2. A microprocessor-based controller that can easily be reprogrammed to open and close a relay based on battery voltage.
Hardware and software development time will be tremendous, unless I go with a commercially available product, but that will be expensive. However, the result will be a product that performs a simple task at first, but can be easily expanded to include many more functions. A real-time-clock can be included, greatly enhancing control, monitoring, and logging functionality.
3. A commercially available product that can be programmed or configured to connect and disconnect the load based on battery voltage.
The one-time cost will be significant, but the system will be up and running in a short time. Expansion capabilities are considerable, but do not include a real-time-clock (RTC). An RTC would be useful because I will want to take advantage of low night time electric rates to run a battery charger.
I’ve already built a simple device, but it’s going to take more time to get it working as well as I would like. Meanwhile, I’m not making the most of the available energy from the sun right now. Building a microprocessor-based device would take even longer. I want to get things going fairly quickly, and the time I have available for this project is limited. With these things in mind, I’ve decided to go with option number 3. Specifically, I’m considering the Morningstar Relay Driver and MSView software.
Calling the Morningstar product a “Relay Driver” is, in my opinion, a big mistake, and perhaps the reason I overlooked this option earlier. The name implies that its function is simply to turn on and off one or more relays, based on external signals. In reality, it does much more than that. Using the MSView software, the device can be programmed to perform a variety of functions. Most importantly to me, it can control a relay based on high and low battery voltage setpoints. I’ll use that relay to turn my inverter on when batteries are at a high SOC, and switch it off when battery voltage is low, with the transfer switch selecting the appropriate source of AC for the loads. I’ll use just one relay at first, but will add functionality later. The Morningstar Relay Driver can control up to four relays.
Once I get load control set up and tested, I probably want to control a battery charger. I’ll use the Morningstar Relay Driver to close a relay based on battery voltage, and I’ll use a timer to only allow the charger to be powered-up when electric rates are low.
Shown below is my simple circuit in the testing phase. I’ll soon be abandoning this project in favor of the Morningstar Relay Driver.