Lesson 2: Buy your Solar Battery Charger Equipment
At this point, you know what your project array power will be or you have already acquired your panels (if not, read Lesson 1: How to Size Your Array first). In this lesson, we will help you with the following:
Before we start spending money, it is important to make a general list of items you need for your array. Make a schematic drawing of your system, showing voltages and maximum currents you’re expecting on your wires and electrical components. Each component has its own range of voltages and currents in which it can operate, and you should therefore first check if all components are compatible with all the others. So read this whole page first, before you order your PV array parts!
Keeping the ‘System Configuration’ theory in mind, make a nice drawing on a piece of paper where you outline your system. See below the drawings Garrett and I made for the example array we built. Each PV module has a voltage Vmp = 32.9 V. Since that is relatively high compared to our 12 V battery bank, we decided to parallel-couple the panels. Both the battery bank as well as the load operate at that 12 V level. With some cheap cigarette lighter converters you can get for a car, we built a very universal load bank, where people can plug-in their iPhones and Blackberries for a free green charge!
The charge controller is the brain of the system that regulates voltage levels for your array, battery and load. It is handy to give it a central place in your drawing as well as in your actual array.
Watch out! Do not buy the very first ‘BIG DISCOUNT’ offer you see when you browse the web! Many solar panel retailers sell their products with huge profit-margins, and it is beneficial to spend a fair amount of time browsing the web for good-quality panels available at wholesale prices.
$/Watt, that's what you think…
The best thing you can do is browse the web and try to find an offer for below $2.5/Watt delivered. Ecobusinesslinks.com provides a good starting point: it compares offers from multiple vendors. Unfortunately, the list does not include shipping costs in the $/Watt indicator.
One thing is important to note when buying panels for a small (< 0.5 kW) battery-connected system like ours: You should buy panels with a fairly low maximum power point voltage (Vmp), preferably under 35 V. This is because many charge controllers built for smaller arrays can’t handle PV modules generating power at higher voltages. Read more about this in the MPPT section.
In the first lesson, you learned how to size your solar array by using PVWatts to find the average daily yield in each month. Of course, this electricity has to be stored somewhere unless your load is directly connected to the solar panels. So lets take a look at what size and type of batteries you should get. Again, you can save some money here on making a good decision.
Size (a.k.a. capacity)
It is obvious that you should at least get a capacity that exceeds the battery capacity of the electrical device you plan to charge with your solar charger. It makes sense to oversize it with a factor 2 or 3, so that you are prepared for possible battery performance loss and you can store more juice in those sunny summer days. The size very much depends on the application of your system. If you need to run some 5 Watt LEDs for 10 hours a day, you need (!at least!) 50 Wh of capacity in your battery. Garrett and I chose for a 40 Ah @ 12 V lead-acid battery that will allow us to run some lights and charge our laptops and phones during our Solar Journey from NY to CA. With our 135 Watt array, we can charge this battery in a day with approximately 5 full sun-hours, taken into account losses from the wiring, panels and the battery.
As per the type of battery to get, we would recommend buying either sealed lead-acid batteries or li-ion. Other battery types like NiMH and NiCd require special charging algorithms that are not supported in most charge controllers. These batteries are also more sensitive for high/low temperatures and NiMH has a fairly high self-discharge rate. Read more about comparison of batteries here. So the choice is between lead-acid and li-ion. Lead-acid is the cheap but heavy option. This is a great pick for stationary applications, where you don’t care about the battery’s weight. If you want to get all fancy, buy the more expensive li-ion batteries. They are very light and easy to handle, but keep in mind that not all charge controllers support this technology (like our Morningstar Sunsaver MPPT).
A Maximum Power Point Tracker (MPPT) is a special type of charge controller that optimizes the energy yield of your array by controlling the panel’s operating voltage.
The MPPT arbitrarily sets the voltage when the system is connected and then it tries out different operating points until it finds the MPP. The device simply multiplies the current and voltage for a point and then compares it to the last operating point. Depending on your configuration and component specs, you can increase your yield up to 30% compared to a conventional charge controller. All these extra kWh come with a cost though: the MPPT version of a $50 charge controller can cost you $200.
For small systems (<100 W), it doesn’t make sense to buy an MPPT. It would be better to spend your money on a 30% bigger array. For large arrays (>250 W) it is worth the money. In our case (135 W), we decided to buy one so that we can get experience working with the device. After all, we are building a 7 kW system later…
The market for small MPPTs can be divided into two markets: Good quality and bad quality. There are a couple of good manufacturers of MPPT charge controllers out there although only three (Morningstar, Blue Sky Energy and BZ Products) build MPPTs for smaller systems:
Cheaper MPPTs are typically from Chinese manufacturers like:
Here’s a specs list with typical MPPT prices for <500 W systems:
This table displays small (<500 W) Maximum Power Point Trackers (MPPT) that are available on the market now.
You can see that there is a big price range between same sized MPPTs. We have bought two of these so far, the Morningstar SunSaver and the Wellsee MPPT-15. If you hold one in each hand, you can tell that the difference in quality is big. The Morningstar feels tough, it is a little heavier and not assembled in a simple plastic casing, held together by four screws. Lets not judge MPPTs by their looks and feels though, we should test and compare their performance!
We will post a comparative review of the Morningstar compared to the Wellsee and maybe later the Juta Solar MPPT as well. Stay tuned!
Besides the main electrical components for your solar battery charger, you need to have cables, connectors, fuse boxes etc. Lets go over the parts we bought for our array and explain what type we got and why.
This table shows the complete shopping list of components (with prices) that were used in the example array. Found cheaper items? Let us know through our Facebook comments plug-in. *The Wellsee charge controllers was bought for the comparative test, the Grand Total does not include this item.
For sizing of battery and panels I refer to 'Lesson 1: Sizing your Array'. We found a great online store that ships the battery for free (at least our shipment). For your panels, begin your search with the provided link in the table. Below, we give some smart tips on some important components.
The junction strips with jumpers from Radioshack were a little hard to find online. We didn't really know what word to search for. The jumpers are little metal plates that connect multiple sockets together and allow for parallel-coupling. We decided to get some cigarette lighter sockets as loads, since these are universal connectors for electric components used in cars. We ended up buying 3 items 3-in-1 sockets from FocalPrice.com, which turned out to be a bad idea. When opening the box, I found that all package material of the product was opened already. Two out of three were not working at all, and the other had a malfunctioning USB charging port. Next time, we'll get better quality.
Under 'Miscellaneous' you find items that you might already have: some screws, cable holders and double sided tape, all used for mounting the components to the plywood board.
Comments, Questions? Let us know and we will help.