DIY Solar Cart: Cheap solar power

The Solar Cart Project started as a test of a solar panel. Simply run a single LED light in my room with an two alarm system 12v batteries in parallel. Giving a total of 12AH of power.

Here is the proof of concept back when I thought it would be a quick project.

Initial Proof of Concept

 

Besides the two 12V batteries the system also contained: A single 5amp inline automotive fuse on the positive end of the battery, a cheap PWM charge controller, and a 150W monocrystalline solar panel. This was all wired using 14AWG landscape wiring.

Upgrading to a central (wooden) chassis and adding fuses

Obviously this system can’t power much for very long, so I continued to expand it. I purchased a sealed 12v AGM 100AH marine battery and found a piece of wood to begin attaching more components on too. With such a large battery there is a greater chance of a short circuit allowing serious current flow, and potentially starting a fire. So I needed to also add more versatile protections such as removable fuses. I also added a single current meter that read the total load on the battery using a single shunt.

 

You may note two oddities. The first being the black wire connecting to the fuse box. This has nothing to do with polarity, it’s just the landscape wiring that connects to the lights which are not dependent on polarity. Secondly, I connected the charge output from the charge controller back into the fusebox to charge the battery instead of just connecting directly to the battery bank. This allows for there to be some level of protection if the PWM charge controller fails to cut power at the correct voltage. I’m honestly not sure how or if that can happen, but you can’t be too careful.

The next logical expansion was to allow for the powering of A/C components using an inverter. There are two ways of going about this – Connecting directly to the panel, or backfeeding into the grid. Since I didn’t have a grid-tie off, I opted to be totally isolated. There are ways of getting around this (see self-islanding microinverters which sync to the sin wave of the grid and then output into your house via a suicide plug) however these are of questionable legality and certainly against SDG&E’s rules.

 

 

Inverter for AC power

As you can see, I added a 1000W inverter (Yes far more powerful than I needed). I fused it at the rating of the wire so it would blow far before reaching enough current to be hazardous. I also added a switch for the lights.

Because of the growth in complexity, I needed some kind of common return for the ground. I had started using the shunt for this, placing things I didn’t want measured on the final bolt of the shunt. However, this became unsustainable and had started to look very cluttered. So I knew I had to once again expand the system to include a common negative bus.

 

Another issue that presented itself is the fact that the amp-meter is one directional. This meant that if the panels were receiving energy while powering something, the meter would get stuck at zero as the net movement of power to the batteries is positive. I had initially solved this by putting the solar power ground after the shunt measurement. This only moved the problem, as now I had no idea how much power the panels were delivering.

 

I explored a few ways to fix this, such as a relay that switches the connections on the shunt when it detects power in the other direction or perhaps a new meter all together. However, I thought for cost, time and diagnostic reasons it would be advantageous to have two meters. One for incoming solar power, and one for all the draw on the entire system.

For some odd reason I was unable to use a common shunt here. I figured if I reversed the sensor connectors on the shunt it would measure the reverse amperage. It just seemed to give me gibberish readouts. Not sure what was going on there…? I ended up giving up and just using two.

 

 

 

 

 

 

 

 

 

 

 

 

 

Leaving the wooden testbed

After this was done I realized that having so much electrical equipment screwed and glued into wood is a pretty large fire hazard. I decided I needed to quickly rectify this. Around this same time, I began toying with the idea of making the entire setup portable. Thus I settled on the only logical conclusion: A

service cart with an electrically isolated conduit box. I also bought two more 100AH batteries. Bringing the total up too 3 12V 100AH batteries. Or 3600W

 ,                            Final Result

 

 

It turned into a project to make a solar ‘crash cart’ able to sustain an area with a loss of power quickly.