There are two kinds of charge controller for solar power systems: Pulse Width Modulation (PWM) and Maximum Power Point Tracking (MPPT). The first kind does exactly what its name implies: It modulates the pulse width of the square wave current pumped into the batteries, discriminating between bulk charge, maintenance charge and float depending on the batteries’ state of charge. It works fine under ideal conditions, not so much when the sky is cloudy because it’s not the most efficient way of doing things but at the time I set up my system 13 years ago it was by far the least expensive so it’s what I got.
MPPT controllers – let me just look at the literature here – “feature a smart tracking algorithm that maximizes the effectiveness of photovoltaic cells.” Don’t ask me to explain that, because back when I was gathering materials MPPT controllers were WAY out of my league and I never really paid them much attention. I can’t describe the difference between a Bugatti’s engine and a GM 350 but I assume there is in fact a qualitative difference.
Anyway, unnoticed by me the price of MPPT charge controllers came out of the stratosphere to visit Earth, and my brother made me a gift of one.
Since I just recently replaced the solar panels on my main rack, this upgrade from my still-working-fine PWM controller made sense. Unfortunately it caused me to have to do something I really don’t like to do: Mess with live electrical circuits that can arc and spark and – if I screw up – wreck expensive electronics and make me look like an ass. Despite my having lived with it for going on 20 years now it’s really not my field.
Still, here we were. So I carefully read the instructions, and it really only came down to four wires anyway so why are you procrastinating, Joel?
Here’s the old controller, with the cover mostly off…
Yeah, just four wires, and I can isolate it from short circuits by disconnecting it from the panels and the batteries before I mess with those four wires, so what could possibly go wrong? Right?
Right. Stop being a pussy, Joel.
So I opened the circuit breaker on the panel rack, turned off the inverter, disconnected the negative lead on my battery bank, and then pulled the wires off the old controller – carefully noting which was which – mounted the new controller on the wall and gingerly wired everything back up. And when I turned everything on again…
…I was rewarded with a little animated show of power going in one end and coming out the other. The new controller seemed to assume I’m the idiot that I, in fact, am, and wished to reassure me that it had things under control.
So great! I wanted to see how this would affect the way my system charges on that panel array with the new controller, and maybe fortunately it looks as though it’s going to stay cloudy all day so I can see if it really makes a difference under not-ideal charging conditions. Which is after all what it’s for. At this moment it’s been running for about two hours with no direct sun and the voltmeter on my wall is showing 14.12 volts and rising, which is higher than expected, so I think I’m going to find out. (EDIT: I haven’t even posted this yet and the controller now shows 100% charge and put the system into Float. At 10:30 on a cloudy morning. Which is phenomenal. So yeah.)
And so now I can address the fact that I have two solar panel arrays, and two charge controllers. And the second one…
…is still connected to the panels on the roof of the cabin.
I think this new controller, being rated for 60 amps, can do the work of my two smaller controllers. But I’m not yet certain of that and there’s no reason not to take this one step at a time.
Hey, interesting post on your solar sys and you cracked me up with, “Right. Stop being a pussy, Joel.” I was a little electrocuted when in the Army and I don’t screw with the stuff ever anymore; total pussy here. Stupid question for ya: does cold hinder the panels? Will be retiring in Wyoming and looking to do a small solar set up for some homestead items. Short winter days, really cold, but usually a ton of sun.
Your voltages sound correct, but ensure that you have set it for your type of battery. There is an option somewhere.
Claudia: Cold doesn’t affect solar panels but does affect lead/acid batteries. They’re much less efficient when cold.
Actually, the voltage on a solar panel will go up when the temperature gets cold. There’s an equation for that. I heard about it from Will Prowse (on YouTube) but I couldn’t say which specific video. This is a potential problem, depending on the upper voltage limit for your charge controller. I assume a web search will yield appropriate answers.
And, it’s likely all the answers you need can be found at https://diysolarforum.com/
Jed, you are absolutely correct. Cold panels put out a *lot* more power than hot ones, due to the voltage rise which can be substantial. I had a string that added up, normally, to about 120VDC tied to my Midnite Solar Classic 150 (150V max), thinking that 30V was enough of a buffer. One brilliantly cold -10F January morning it went poof (fortunately Midnite Solar sent me a replacement unit the instant I told them about it…the fix takes them about 10 minutes).
I dropped the strings to 3 instead of 4 and that solved that problem.
The other thing to realize is that a hazy day will sometimes produce more power than a completely clear one. The panels integrate the incoming radiation over a wide angle (over 60 degrees) and the sun coming in sideways from the haze adds up to more than the sun as a point source. Strange, but true. I’ve seen it go up 15% through that mechanism. Cloud edge glints can do the same thing, to a lesser degree.
@Steve Walton – would a charge controller with a substantially higher capaciy have avoided the problem? And, is there a penalty for doing so during “non very cold days”?
@Irving: “capacity” – meaningless word here. The operative limit is voltage. Charge controllers have a voltage range, and it’s published in the specifications. The voltage of the input has to be within the range. That’s it.
@Irving: Jed is right, but if you substitute “capacity” with “voltage” the answer is yes. The Midnight Solar charge controllers include the Classic150 (150V max input voltage) and the Classic250 (250V max input voltage). However, the 250 has a smaller maximum amperage, so to get the same power you have to run a higher string voltage. I chose to design for the smaller voltage due to the type of panels I have.
When one plays with solar power systems, one gets very familiar with all the system trades you have to deal with in voltage, amperage, and power. There are a lot of things going on over and above “just plug in four wires” when you get larger than a simple low-power system like Joel has. Like the cost of wires to the panels — higher voltage series strings means lower amperage and thus smaller/cheaper wiring. For long runs this can be significant. Plus, there is less resistance heating power loss in the wires at higher voltage.
I’m glad Joel got an MPPT controller. Now he can quit throwing away power as conditions change.
Best to be safe, not sorry…
When I was messing with solar I put battery cutoff switches everywhere, so I could disconnect things without having wire floating around loose. They are not cheap, but a few judicially placed is usually enough.
One battery farm cutoff switch on the main positive terminal (can do dual-duty as a circuit breaker), and one on the solar panel input to the charger (also a circuit breaker, DC rated) is enough.
“Right. Stop being a pussy, Joel.”
I can see where the trepidation comes in. It’s not like you can simply pick up a phone and get an electrician out to do the work. I’m glad that this installation went well, and I hope that it serves you well for years to come.