For the traditional off-gird solar/battery system, there seems to be an inescapable design challenge: the solar PV and battery size must be significantly over-sized in order to ensure reliable power during the winter or multiple cloudy days. Meanwhile, typical AC coupled rooftop solar installations convert DC to AC, just to be converted back to DC for many of our typical households loads (LED lighting, computers, TVs, etc). All at the same time, we buy expensive battery UPS units to backup computers, which add another AC-> DC -> AC -> DC process to provide backup to computers, modems, and routers. This all adds up to increased costs and losses in efficiency.
This instructable tries to provide one solution to these 3 challenges, by connecting a standard 12V off-grid solar/battery with grid-connected power supply to create a hybrid 12VDC power system.
I've written this assuming the reader has a general background in how to wire a 12V solar battery system, and has a basic understanding of fuse protection and diodes.
As will all electrical projects - PLEASE USE CAUTION, and only attempt this if you have the understanding of how to work safely with 120VAC and 12V batteries. Never work with 120VAC while energized, and always unplug from the wall before making any 120V wiring adjustments!!
Step 1: Theory and Circuit Design
To create this hybrid 12V system, I connected a standard 12V solar/battery system in parallel with a 12V grid power supply, using diodes to block either system from back-feeding the other.
Loads are then connected to this dual-supply system, and will automatically draw power from whichever source is at a higher voltage. This means that there is automatic load sharing between the solar/battery and the power supply. When the sun is shinning, the solar charge controller will regulate up to 13+ volts, and will supply power to the loads, sinking any additional power into the battery. When the sun sets, the battery will initially discharge to supply loads, until its voltage reaches about 12.0V, at which point the grid power supply will step in, continuing to supply power until more solar power is available. This "handover" point is controllable by adjusting the power supply voltage (assuming you can), but 12.0V is approximately the 25% state of charge point for a lead acid battery, right about where you typically want to discharge to.
When solar power is available, it is used efficiently, and will supply DC loads (or charge the battery), without the need for a conversion step to AC and back. This also fosters "self-consumption", as solar power is never sent back to the utility, as it is either used by local loads or saved in the battery for later use.
All of this "load-sharing" is done without the use any fancy microcontroller, and doesn't involve any relay or transistor based switching. All that is needed is a couple simple diodes, and we can take advantage of the voltage range of the lead acid battery to determine where power should come from, providing a seamless & continuous power supply for whatever you want to plug in.
Step 2: System Enclosure
As with any 12V solar/battery system, you can come up with a range of different enclosures to house the charge controller, battery, and load distribution (lots of good examples on Instructables!). I've built a few of these systems, and was looking for a low cost, flexible enclosure, which was big enough to house all of the electronics, as well as have space for some moderately sized batteries (30 Ah or more). My chosen material was an $18 bookshelf made of composite wood (bought at the red bulls-eye), which I modified to give me 2 compartments - the top for the various electronics and wiring, and the bottom for battery storage. This is meant for a fixed installation where I don't need to move this thing around, but if you need something mobile, you would definitely want to take a different route.
As seen in the photos, I used a circular saw to cut the top 15" off of the side panels, and then mounted two of the fixed shelves to serve as the bottom and middle shelves. Instead of a fixed shelf on the top, I put hinges on the shelf to allow for quick and easy access to the wiring section.
I then used the spare "wood" to create front and back panels, completely enclosing all sides. This makes it easy to mount various plugs for a solar panel, loads, and a 120VAC input. You can see I've already started to cut some of these holes, but more on that later.
I plan to add hinges and a latch to the panel in front of the battery compartment, so that these can also be easily accessed.
Step 3: Wiring
Once the enclosure was all good to go, I mounted the solar charge controller, power supply, and a fuse block.
I used a pretty standard (marine?) fuse block to wire everything together, as you need to connect a lot of different circuits in parallel, each with a fuse for protection. The 2 sources (solar & grid), as well as the various loads are all wired into the fuse block. Each should be fused with the appropriate current rating, based on the devices capabilities (10A fuse for 10A power supply, etc).
The solar charge-controller that I chose was a basic unit, which included Low-Voltage-Disconnect (LVD) to protect the battery. This is wired in the standard way, with the battery connected to the to the battery input, a Power-Pole socket on the left of the cabinet for connection of a solar panel, and the "load" connected to the fuse block through an inline DIODE. The diode is very important, as it will prevent current from the grid power supply from back-feeding into the charge controller, which it is not designed for.
I tested the system with a 10Ah, 12V SLA battery, but plan on installing one or two 30Ah batteries for more capacity. These are installed in the lower section, and wired with their own inline fuse for proper protection.
For the power supply, I mounted a 120V socket on the right side of the cabinet, thru an external switch, and to the 120V input of the power supply. The 12.0V output of the power supply goes through an inline DIODE, and then feeds into the fuse block.
The diodes that I used are 10 A Schottky diodes, with about a 0.15V drop across the terminals. I soldered these in series with the positive connection from the solar charge-controller and the power supply (2 diodes total).
Step 4: Conclusion
So there you have it - a small design change to a traditional solar/battery system, to provide continued power even when the sun has set and you've discharged your battery. This allows you to plug in loads with confidence, and avoid needing to switch between outlets depending on if your solar system is charged or not.
As built here, it's great for charging phones, tablets, and laptops, as well as powering 12V lighting or powering a radio. I plan on also connecting my router and modem to this system, using a 12V buck-boost converter to regulate the 12V input to these devices. For these types of always-on devices, you instantly shift them to a solar-first, grid-second supply, with a built-in battery backup to keep them going during grid-outages.
This could also be extended to a small computer or server, using a "pico-psu" to replace the PSU and accommodate a 12V DC input.
While you certainly could, I've avoided installing a 120V AC inverter, as I am trying to take full advantage of the DC loads, and avoid unnecessary conversion steps.
Hope this instructable was helpful - happy making!