There are several mobile solar power stations available today. Their emphasis is on portability and emergency back up power. However, Their long term or higher power use is limited. Many are rated at 1500 to 2000 watts of output. But when you read closely, the rating is the capacity of the built in inverter, not the capabilities of the solar panels or the batteries.
We were most impressed with the ECO1800 by Ecotricity because it is a complete portable power package. This includes 90 watts of solar panels on wheels, an 1800 watt inverter, and four 115 VAC outlets, a sealed lead acid battery, and charge controller, making literally plug and play. However, it has a couple of drawbacks. Being that it is a complete package, it does not take advantage of today's lower solar panel prices. The system can be found online for around $1600. Solar panels today are selling at between $1 and $2 per watt. So a 200 watt panel can be had for $200 - $400. You can get two panels and still have plenty left over for a good charger, battery, and inverter. Secondly, with only 90 watts of solar panels in a 12 volt system, it takes 12 hours of full sun to charge the battery inside. Lastly, the batterycan supply a hair dryer or small electric heater for only five minutes from a full charge.
We ended up buying a Xantrex 1800 Emergency back up unit because it was only $500. It was designed to be used as back up power in the home. So you would keep it plugged and charged up until the main power went out and you could use it to power lights and small appliances for a short while. If you wanted it to be charged from solar power, you have to add a solar charge controller and panels. The 12 volt to 115 VAC inverter is built in. So with this power plant you could build a simple portable system using higher power solar panels for around $1000. However, the SLA battery has very limited capacity. The Xantrex included battery jacks similar to those on a car battery so that you could charge additional batteries. There are a few examples of this posted on youtube.com.
However, with such limited battery power, we decided to build our own portable power center that would take advantage of today's lower solar panel prices, have a much higher output capacity, and offer some additional shelving space for our cordless power tools. So we spent lots of time shopping for components and parts, so here is our story of how it was built.
We wanted the portable solar power station to include the ability to keep our power tools charged while working off-grid on our remote property. The power station should also be able to provide enough juice to supply one or two lights at night and maybe keep a lap-top computer going for a couple of hours before bedtime. The unit should also provide enough shelving space for several power tools and charging modules.
Throughout our search for components we found that you can do most of it from one chair using the Internet. No need to drive around from shop to shop. You can even use the Internet to search for hardware such as bolts, nuts, and screws. We found that Lowes has their entire inventory listed one their web site. You can search for items, build a list using their shopping cart, then place the order and they will have the items waiting for you to pick-up in 20 minutes at the nearest Lowes store. I'm sure Home Depot offers the same service. For hardware that you can not find at the big box retail stores, there are many other online retailers that can provide unusual parts.
Since lead acid batteries are very heavy, we found that its best to buy local. However, you can search the net first to see what brands and capacities are suited for your application. Then compare prices with local battery stores. We got lucky and found that one of the largest battery sellers on ebay was physically located only 2 miles from us. So we were able to shop their ebay store, print the list then go and pick them up. The CEO was very accommodating in helping us find the right battery at the right price.
The real jewel, though, was google shopping. Using it for items offered by multiple shops, you can reveal the lowest ultimate cost. Just search for an item, such as "battery charger" on google.com, then select "Shopping results for battery charger". Google shopping provides a list of items available from many retailers. Then sort by "Price: Low to High". We found many great deals this way and were assured that we were getting the best price. The google shopping feature pulls in items from both Amazon.com and ebay.com, as well as many other big online retailers. If the search tool brings back too many unrelated items, refine the search by adding more keywords.
We chose Trojan T-105RE batteries because they were designed and are the most popular battery for use in solar off-grid applications. (The "RE" in the nomenclature stands for "Renewable Energy". Its a little more durable than their normal T-105 marine version.) Their cost is reasonable compared to other batteries available. They are heavy, but not so heavy that one person could lug them around for short distances, until they could be anchored into a home position. They weigh 67 pounds (30.39 kilograms) and can be found for around $160USD each.
They are rated for 6 volts output, so we had to buy two to use in a 12 volt system. Each battery has 225 Amp Hours of capacity, which means they can supply 225 amps for one hour or one amp for 225 hours or anything in between. If you place two in parallel you can increase this to 450AH. However, the voltage output is still 6VDC. If you place them in series, you increase the voltage potential to 12VDC, but the current capacity remains at 255AH. We ended up getting four to be arranged in a serial and parallel configuration for more capacity and higher voltage.
Lead Acid deep cycle batteries should be kept fully charged at all times. The water levels should be checked frequently. Distilled water should only be added to a full charged battery. Be careful to prepare all your cables before making electrical connections with live batteries. You never want to short out the positive and negative terminals of any of the batteries. If they are powerful enough to start a car engine, they can really lay you out flat on the ground like Joe Frazier.
Since the batteries are very heavy (67lbs x 4 = 268lbs) we thought it would be best suited to use a heavy duty cart. We found Advanced Display Systems' custom welded nursery carts to be the most stable for carrying heavy loads. The uses very few bolts for construction. It is mostly welded together. The tires are tubeless and can never go flat. They are expensive, tuning in at a mere $400, but they are built to last forever.
By contrast, the Harbor Freight flat bad carts are build it yourself kits with mostly nuts and bolts. They are very cheap, but only last about a year of good work, then fall apart very quickly. You are constantly fixing flat tires and the steering assembly breaks easily.
The charge controller manages the flow of solar PV generated electricity into the batteries. Since the sun's intensity hitting the panels fluctuates throughout the day, the charge controller smooths the flow and assures the battery pack does not get overcharged. A good charge controller also senses the type of battery being used, has a thermometer, and processes the best charge rate to get the most charge into the battery pack. After doing much research we found Morning Star charge controllers to be the most commonly used in Solar PV battery systems.
The Morning Star Prostar Series of chargers were specifically designed for solar PV off-grid systems. They are very much plug and play, offering connections for solar panels, batteries, and loads. A single lighted button allow you to disconnect the solar panels or the loads from the batteries. The LCD models rotate the display showing battery voltage, solar panel current, and load usage current. Its even got screw holes for easy rack or wall mounting so its easy to see the display.
An inverter takes the DC (direct current) from the batteries and solar panels and converts it into AC (alternating current) to power normal household lamps and appliances. There are several popular brands in the renewable industry available. However, the two most popular seems to be Outback and Xantrex. Xantrex offers enough sizes and models of inverters to be used in any size system.
We chose the Xantrex PROWatt 2000 Inverter, which has a 2000 watt capacity. It has a power button, simple display, and two grounded AC outlets. The rear of the unit has two fans, positive and negative battery connections, and a ground tie. We don;t anticipate using 2000 watts from the mobile power station, but plan to use the extra capacity in a larger system later.
The following photos and captions illustrate the assembly process.
Being that the Morningstar charge controller is the wiring hub of the system it was mounted next. Then the batteries were connected to the plus and minus tie downs at the bottom of the charge controller. It was the first device enabling verification of the battery pack voltage.
Next, the Xantrex inverter were mounted on the opposite side of the plywood vertically. This was done for convenience, space saving, and ability to view the inverter display, as well as have access to the AC plugs.
Also, Xantrex recommends using a high power fuse on the positive line between the batteries and the inverter. This is to protect equipment and load in case of a short circuit event and will prevent the batteries from run away current. Many of the principal used in high power car audio and golf cart systems are directly transferable to off-grid renewable systems.
So we added a 300 amp audio fuse with case capable of harnessing up to 0 gauge wire. We used google shopping to find the best price on ebay.
After wiring everything up except for the solar panels, we were able to power a lamp from the remaining power left in the four Trojan batteries.
The mobile power station should be able to take advantage of falling prices of solar panels. So we built the frame so that one panel could be mounted on top and another could be mounted on one side. The platform should be able to accommodate various sizes of solar panels as they some down in price. The mobile station should also be able to tilt the panels so that they can always face full sun. The structure should be strong but light weight because, after all, it is portable. The solar panel platform should minimize the use of wood because the panels get very hot in the summer.
With those desires in mind we set out to build our first iteration. After building the shelving structure, we used basic door hinges to attach the panel base to the structure. We then employed gas powered trunk lift supports used in hatchback type cars. The intent was to have the lift supports stay in any position set indefinitely holding the solar panel at an optimal position to catch the sun light. However, the lists were too stiff. Once installed, they would not budge. So we reworked them to just be used as tilt supports in one position by drilling holes in the side walls and panel platform and using quarter inch bolts.
We have cabinet supports coming in and will try those next. If they don't work, we'll try screen door closers, which can be locked in any position by a small metal tab.
We selected a 100 watt, 12 volt panel just to get started charging the batteries. We paid about $2 per watt, which was not too bad for a smaller panel. It was certainly better priced than the $1600 plug and play portable systems on the market.
The panel is mono-crystalline, which we have found needs full sun to make power. We used a shelf organizer from Walmart as the panel support until we find something better. The shelf was turned upside down so that the bottom inch and a half cup would keep the solar panel from sliding off the platform.
The first exposure was in mid-December, so the sun light was very weak. In addition, it was a cloudy day, so not much charging was done. However, we did verify that the systems works. Now we just need to beef it up a little bit and add the finishing touches.
At mid day just before winter solstice, we could get the solar panel to produce 1-2 amps, just enough to get the batteries off empty, but not enough to fully charge four high capacity batteries in three days. So, one of our first objectives is to replaced the 100 watt panel with two panels producing a maximum of 300 watts of power. The 100 watt panel will be used to charge the Xantrex emergency back up module. Then it can be used on camping trips or for even more mobility at the work site.
Well, most of the time spent on this project was deciding which parts to use and how it would be constructed. That took a couple of months. Once we actually started building it, it only took three days, and most of that was for the platform construction. The electrical wiring took minimal time.
Several items need to be completed. The top solar platform will probably be rebuilt. All metal pieces in the assembly need to be grounded to the batterynegative terminal as well as the ground terminal on the Xantrex inverter. Trays need to be added to the lower and middle shelves for storage of tools.
However, we've got something that is usable and was completed for about the same cost as the Ecotricity system, but with much higher capacity.
There are very few active components in the system:battery pack, charge controller, inverter and solar panels. So it is very simple to build one your self.
Tags: DIY Solar Power, Mobile Solar Power.