Solar Generator

Solar-charged backup battery system... It's interesting how a simple thing like a power outage can lead to so many projects. If it had lasted only a few hours, our normal laptop and cell-phone batteries would have carried us through with little notice. However, when it lasted several days, I had to get creative, and, being a tinkerer, or maker, I was soon on to some fun projects. You can see my other Instructables regarding power banks and lights using relatively small batteries.

Keeping several workstation-equivalent laptops charged for several days was going to take more than a few uninterruptible power supply (UPS) batteries. I solved this by pulling out the battery from my car and using that with a stock car-battery-to-laptop adapter. That, along with a few lights, lasted a few hours, and then I had to put the battery back into the car and drive around for a bit to recharge it. That got old soon.

As an Eagle Scout I was sorta prepared. I had the car battery, I had the adapter, I had the 12-volt lights. I had all sorts of low power sources such as USB power converters for my yard tool and workshop batteries. I just wasn't prepared for a longer outage, and I wanted to fix that.

I enjoy learning new things technical, and here was a whole new area I had not looked into before: Solar Power.

I will be referring to an earlier Instructable in which I built an 11Ah LiFePO4 battery since some of the lessons learned there will carry over into this project.

The picture shows some of the supplies and tools I used. AliExpress, Amazon, and a US-based LiFePO4 battery cell supplier were my sources. The attached PDF file lists the prices of the major components.

The main ingredient is the batteries. I chose to use 100Ah cells for this experiment. You can find numerous sources on eBay, AliExpress, and even Amazon. Based on references I found in videos I chose to select the supplier at 18650BatteryStore.com which is based in Georgia, USA.

Next, you need a battery management system, or BMS. This ensures that you don't over-charge or over-discharge the battery bank. It can also even out, or balance, the charge across the cells. Some BMS will sense the temperature and will prevent charging or discharging when the temperatures are out of exceptable ranges. For this type of battery bank, the BMS is going to be a bit more complicated than the ones I used for the 11Ah lithium battery project.

Better BMS include a BlueTooth (BT) interface so you can manage battery settings and monitor the status. The JK BMS I selected does this.

When you buy a plain LiFePO4 battery off-the-shelf, that's what you get: a set of cells and a BMS. I wanted a complete power management system and therefore needed to add a few more things.

You need a battery monitor which can give you some idea of the state of charge. For lead-acid batteries, the voltage is a good indicator. Lithium batteries have a very flat discharge curve so that's not a reliable indicator. Instead, the usual way to do this is to measure the current flow across a shunt in and out of the battery. You need to be sure to get one sized for the maximum amperage your battery will deliver. The KG110F (multiple vendors) provides everything you need. There is a display that shows the current voltage, whether the system is charging or discharging, and a fuel gauge. It also has a BT interface so you can monitor it remotely.

You'll need several "glue" items to make this all work. There should be at least one fuse or circuit breaker. Larger systems will have several. I like to use circuit breakers instead of fuses as master on/off switches. On complex systems the circuit breakers provide protection and can be used to isolate parts of the system while you work on them. You'll need thick enough cables to handle the current. You can buy ready-made cables from various sources, however, I chose to buy the crimpers, lugs, and cable stock, and make my own. Bus bars help tie things together. I planned to use this battery for experimenting and camping and therefore used a variety of Anderson Powerpole connectors for providing flexible connection options.

Other parts that are not directly part of the battery itself are the following:

A mains connected battery charger for LiFePO4 batteries. The 20-amp LiTime charger had good reviews and is large enough for this project.

To actually use the battery for real, you need an inverter that converts the battery voltage to 120 volt A/C. I did not select the cheapest. I got what I consider a good, middle-of-road (based on several in-depth reviews) 2000 watt model from BougeRV. It also includes a BT monitoring capability.

For charging with solar panels, you need a solar charge controller. Again, there are a lot of options, but all the serious systems seem to rely on Victron products. In addition to fixed installations, they also show up in a lot of RV and boat power systems.

And, finally, you need some solar panels. These are relatively cheap parts of the system and I got a few to play with. I keep an eye out for more on eBay and Amazon specials.

Sources, Web Sites, and Videos

LiFePO4 Prismatic (rectangular) cells

JK BMS - be sure to select the correct options: number of cells, total current, BlueTooth (JK-B2A8S20P is the model I used)

Battery Monitor - Amazon

Car USB Outlet - Amazon

Anderson Powerpole connectors - beware of mostly fakes on Amazon.

Battery case connector nuts - Amazon

Will Prowse - has a LOT of videos on solar power and batteries, including instruction, reviews and tear-downs. He's frequently referenced in other videos.

Off-grid Power Solutions - good battery building videos.

First, some background.

Over a year ago I rented a camper van, just to try one out, and to explore the Olympic Peninsula. I read a bit about the power systems and solar panels it used, and, of course, looked at a lot of build-your-own camper van videos. This included how to build the power systems, do the carpentry, selecting appliances, and so on. The camper van trial was fun, but expensive, and I moved on.

You can find more information about my camper van trial.

Now, with the challenge of building better power backup systems, I was returning back to slightly familiar ground. My house, during a power outage, is just a huge camper van, off-grid. So, it was back to You Tube looking at videos for solar power systems. There are a lot of resources! These range from vendors showing off how to build systems for RVs to DIYers wanting to power their detached workshops to households living off the beaten path without access to power infrastructure.

Without going into particulars, I've listed some of my favorites in the Supplies section above.

Side note: As with all things Internet, you do have to keep the BS detector finely tuned. Some information is flat-out wrong, and sometimes I wonder if it's intentional, because anyone with any amount of science background will know it. So watch out! If in doubt about something try to find videos on the same subject from other authors. And check the reviews.

Based on the information I eventually trusted, I built the parts list shown above. Everything came from Amazon, AliExpress, a US battery supplier, and my own warehouse 13 parts bins.

Goals: My goals for this project were:

1. See if I could actually duplicate what I had seen in the videos.
2. Build a battery system for experimenting with solar power generation.
3. Make the system portable for easy experimentation and practical use.

I ordered the cells from 18650 Battery Store. They came well-packed in foam and look great. I checked the voltage of each cell and they were all within 1/100 of a volt. My cat gave its sniff of approval.

According to my research building the 11Ah battery, I need a case that will keep the cells compressed. I pretty much followed the construction directions in this video when I designed and built mine. I used some scrap 1/2 inch MDF, 1/4"-20 threaded rods, polyethylene tubing and furniture connector nuts. The trickiest part is measuring the length of the threaded rods because there's not much thread in the connector nuts.

The threaded rods are covered with plastic tubing so the threads don't rub against the cells. I placed thin cardboard sheets between the cells so they don't rub directly against each other, and by inserting more or less I could adjust the spacing so the nuts fit onto the rods correctly. The cardboard is the backing from used notepads.

Be sure to line up the cells in alternating fashion so the bus bars can connect the cells in series.

Once the sides have been firmly pressed against the batteries, I attach the bottom with wood screws, being VERY careful not to miss the wood and hit the batteries. The thin strips at the top provide a finger grip.

It's not shown in the picture, but I keep strips of tape over the battery terminals while I'm assembling them.

See the pictures above for the wiring details listed below:

1. Connect bus bars and BMS sensing wires to the battery terminals.
2. A red cable goes from the positive terminal of the battery bank to the circuit breaker.
3. Another red cable goes from the other side of the circuit breaker to the red (positive) bus bar.
4. From the negative terminal of the batter bank a cable goes to the B- terminal of the BMS.
5. The P- terminal of the BMS is connected to the shuntfor the power monitor.
6. The other side of the shunt is connected to the black (negative) bus bar.

At this point the battery is complete. Additional connections are made to the bus bars. These include:

1. Heavy-duty connector that goes to the power inverter.
2. Connection for an automotive USB charging port and voltmeter.
3. Charging connector for connecting a mains-powered charger or charger(s) from solar panels.
4. 15/30/45A Powerpole connector for 12V devices.

The BMS is attached to a shelf just above the battery cells. A 3D-printed cover hods the batter monitor display and USB charging connections.

The charging connector can connect to the 20-amp charger, or the Victron solar charge controller, both shown in the picture. I use Anderson Powerpole connectors for both flexibility and safety. I follow the Anderson recommended colors and use the yellow connectors for the 12-volt system. I will be building a 24-volt system and will be using the recommended red connectors for that. That way there is no risk of, for example, plugging a 12-volt inverter into the 24-volt system.

Woohoo, everything works. I plugged lights and appliances into the inverter and everything worked as expected.

I did notice some of the wires getting a little warm. I have a dedicated dual-input electronic temperature guage, and my voltmeter also has one and I connected several of thermocouples to the cables. Most of them were getting into the 30's Centigrade so I wasn't too worried about those. However, the cable going to the inverter was over 50C. That cable was one that came with the inverter. As I was doing additional research on inverters I did notice some comments about manufacturers providing thinner cables than would be required when the inverter was used at maximum. Some reviewers recommended just throwing out any cables that came with inverters since they were usually too thin, and most likely made of aluminum.

Anyway, I upgraded all the cables to at least one gauge larger, and then they got only slightly warm.

The second picture shows the complete setup next to the window where the cable from the solar panels comes in. In the summer, the two panels fully recharged the battery in a few hours. The solar tracker helps to maximize the radiation received. Unfortunately, I live on a small property surrounded by high trees so I only have a few hours of usuable sunlight. Good enough for testing, but not for real use. In an emergency situation I would set up the panels on the sidewalk in front of the house.

I look forward to my next camping trip where I will have plenty of light, an electric blanket and coffee-maker.

Some analysis and comments...

So how does this compare to some alternatives?

During the multi-day power outage that inspired all this I use a 50Ah AGM car battery to power our laptops and some lights. This battery weighs 39 pounds and costs $250. In actual use, it is not supposed to be discharged below 50% capacity, yielding only 25Ah at 12 volts, or 300Wh.

My new lithium 105Ah battery weighs 27 pounds and is roughly the same size as the AGM battery. Parts cost is about $450. It can be safely discharged to 20%, providing over three times the capacity at 1000Wh.

Off-the-shelf alternatives are available. I don't know much about them but the prices vary greatly and specs can be misleading. Just consider my 50Ah AGM car battery.... I looked at some in-depth reviews for systems in this capacity range and I don't regret making my own. I know the quality of the parts and I have control over the features and interconnections.

Next up... an Arduino-based monitoring and logging system for my solar panels and battery. Also, a 300Ah 24-volt system. Both have been running for six months - but the documentation is behind :)

Happy building.