You might thought I have forgotten poor ol’ Will. No, I have been busy and I think Will is now leaning toward grid tie. He has such a big load and the whole purpose of going solar was to save money when he retires. Grid tie can be a whole bunch cheaper than going off grid. In his case, if the grid goes down, he has a whole house generator that kicks in automatically. That gives him a low power bill and a backup supply.
I will continue with the answers, though, because, regardless of the size, many of the steps and advice in this series of posts will be the same for most off-grid situations. It is just a matter of scale.
We previously determined the load, how many panels we need and how many batteries we should have. Today we’ll go over the charge controller(s) that you need to get batteries charged safely.
Except for a super cheap shed system, I almost always recommend a Maximum Power Point Tracking (MPPT) charge controller. Heck, I even put an MPPT charge controller on my new Boxcar Solar system. What’s that all about and what are the advantages?
If you look at the label on the back of your solar module, it’ll have an open circuit voltage that is at one level and a maximum power voltage that is lower. That is really only valid at full sun. If it is overcast, the open circuit voltage may still be pretty high, but the maximum power point will be completely different. The brain in the MPPT charge controller constantly tinkers and adjusts to find the best charge. Not only is that a neat trick, but you will typically end up getting around 30% more power from your panel. That’s something on the order of getting extra free panels, but without having to haul them up onto the roof.
Another neat trick of the MPPT controller is that you can use a higher input voltage. There are all kinds of advantages here. First of all, you could use a cheaper “24v” panel on your 12v RV or boat AND the higher voltage will start the charging sooner in the morning or under cloudy conditions.
Remember, too, that John sometimes has some odd panels from grid tie service that have weird voltages, but they are dirt cheap. I bought a pallet of those 70 watt, thin film panels he sell for a dime a watt. They output 96 volts! You know how a lot of people will test a 9v battery with their tongue? Don’t do that with these panels!
I have 4 of them in parallel on the boxcar. (It is really a 40′ shipping container that looks like a boxcar) The 96 volts goes into the charge controller and comes out as 12 volts to charge the lighting battery. The panels are flat on the roof and in a forest, making for a challenging installation, but it works.
You can also use less wire. Solar wire is often 10 gauge, which in 120v service is good for around 30 amps. If you run 30 amps at a lower voltage you get substantial voltage drop and power loss that can only be cured with more or larger cables. For Stan the Hermit’s system, we put 4 large panels in series-parallel on each downline to his charge controllers. 4 panels do not exceed his power limit and only use half the capacity of the cable, so losses are low.
Now, what about the controllers themselves? They come in all sorts of capacities. I have 4 60 amp controllers on my system. John sells a 250 amp controller that I know of and I think a larger one. In some ways it might be better if I ran the one controller because then it would have all the knowledge of conditions and all the control over the charging. I got in the habit of using multiple controllers for redundancy on my sokar expedition launch and that turned out to be a good thing one time when some rough water nearly sank the boat and killed one of the controllers
In my home system, on a cloudy day or early in the morning, all of the controllers are running flat out to gather power and that is a good thing. Come 10 a.m. on a clear day, it can be possible to have too much of a good thing. You should only hit your batteries just so hard (the C rate) and I have enough solar to hurt the batteries.
I solved that situation by leaving one controller, my nice FlexMax, at factory setting and then step down the ratings of two of the others. That way, when we get into the range of bulk charging the two shut down. I go out at 10 a.m. on a sunny day and the two might be at or near zero output if the load is light. If I turn on the electric heater in the control room, then the dormant controllers spring to life, providing the extra 70 amps DC to the inverter to run the heat, but no more to the batteries.
Some of the newer models have a feature that let you link the controllers together so they can effectively think as one. That’s great, especially if they have a menu item where you can specify just how much battery capacity you have available. It is always best when the system components are working together.
On a system the size Will would need, you’d use the biggest controller you could find and use enough of them to have all that power covered. And, really, the same goes with smaller systems like mine.
I think I mentioned early on in this series that the higher your system voltage is, the more efficient you will be with your charge controllers. Here are some examples. The boxcar system is 12 volts and uses a 40 amp controller. 12 X 40 yields only 480 watts. If used in my 24v boat, then the same controller could handle 24 X 40 or 960 watts. On a 48v system I can run 1920 watts through it. Think how much money you can save buying one controller instead of four!
One last thing I should mention is that MANY, if not MOST, of the inexpensive “MPPT” controllers sold on Ebay and Amazon are not really MPPT controllers. Some are pretty good Pulse Width Modulated (PWM) controllers but not true MPPT. Some are dangerous. There are some bargains, but if you want to make sure of getting good gear, talk with one of Sun Electronics’ sales engineers and they will fix you up with the good stuff.