Solar Safety: High Current Fault video

A common mistake I see some people make when designing a solar energy system is that they will parallel the outputs of the solar panels without using a combiner box that has fuses or breakers.

This works fine in the “yeah, the lights come on” sense, but if you should ever have a fault in one of the modules, you may very well experience a fire at the module that will spread to any other flammable materials nearby….. yes, that means your ROOF.

Note that while the solar panel’s encapsulant and backsheet self-extinguish and only exhibit a couple millimeters of flame spread, the sheet of paper I taped to it to simulate the flammable debris that *will* gather around your panels does not! 🙂

Flammable crap you will typically find around and on your panels includes oily soot from smoke/automobile exhaust, dried leaves, paper, bird nests… anything the wind or animals can bring in!

Under the hood: Morningstar SunSaver MPPT.

Buckin' Bronco

This is the Morningstar Sunsaver MPPT charge controller, capable of pumping 15 amps into a 12 or 24 volt battery system from an up to 75V input. It’s fairly simple, though the 6P6C jack can be used for Morningstar’s Modbus system or Remote Meter to add more control, programming, and monitoring capabilities. The unit is driven by a Microchip PIC18???* microcontroller.

A typical MPPT controller consists of a switching buck or buck-boost converter with the input connected to the solar panel array, and the output connected to the battery system. A microcontroller monitors the solar array voltage and current (and multiplies them to calculate the power) periodically, and adjusts the switching of the converter appropriately to keep the input side voltage at the solar array’s maximum power point, Vmp.

Inside the Morningstar Sunsaver MPPT, there is… a switching buck converter with a micro… etc. Here you go:

* The conformal coating stuck to the top of the chip made it difficult to read. Like the flavor of PIC matters? XD

Control/Logic Board

No fans or other active cooling are needed. The inductor is thermally coupled to the back of the housing, which is a tall metal fin attached to the heatsink/base. The switching transistors are, undoubtedly, potted somewhere in there. The potted construction is also used on the SunSaver PWM controllers.

Simple, elegant, but here’s the big question: WHY does it cost $250?! Rest assured, I’m scouring the market for some *good* low cost MPPT controllers. This is just a very good and not quite as low cost controller!

Under the hood: The Outback FM80

Warning: Engineering porn ahead. All images are clickable to view in full resolution.

The Outback Power FM80 solar charge controller is a high performance MPPT controller which converts a solar array’s output (up to 150VDC, 64 amps) down to charge a 12, 24, 36, 48, or 60 volt DC battery string using a high efficiency switching buck converter and an extremely flexible microprocessor control system. It is field programmable from the front panel and can be linked to other system components using Outback’s communication buss and the MATE controllers for system logging and remote control.

It is extremely well built, and solid as a rock.

More photos below…

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The IndoorGenerator…

The designer of the IndoorGenerator power units just dropped by here at Sun a few minutes ago. Their products are really interesting, especially for those of us down here in the Hurricane Belt.

Their product is, in a sense, like a giant UPS. Sealed maintenance-free lead-acid batteries are paired to a power inverter/charger unit in a nice cabinet. It’s also possible to get a unit from them which features a solar charge controller, and an outdoor rack to pair a couple of nice big solar panels to the system for off-grid operation!

Now, you see where I said “nice cabinet”? I mean it. Go look at their page for the PowerCubes – they look like a piece of fine furniture. You could use one as a nice end table or coffee table.

They also make some less aesthetically fancy but very functional UPS systems for commercial and office use. He was talking about using Outback Power inverter/charger units inside some of his systems, and those would be awesome for such use – they produce very clean power and transfer back and forth perfectly.

For those of you urban cliff dwellers, they also have a very nice looking balcony unit that unintrusively sits out there and can be wired to solar panels.

He’s going to bring over one of their standard series units in a couple of weeks for us to play with. I look forward to putting it through its paces and brewing a pot of coffee or two…

Elkhart, we have a problem

Modified Sine Wave HORROR! This is what I got off an inverter with a bad output filter. I believe this is proof of why you should invest in a true sine wave inverter if you’re running anything other than lighting and a refrigerator (note: a refrigerator with electromechanical controls — not an electronic control system.)

No, the scope isn’t lying here… see the last pic…

I almost missed this because I had a resistive load on the inverter. Apply a resistive load and it looks like this. Apply an inductive load and it still has those ugly spikes.

They really are that bad.

Set to trigger on the very start of one of those negative pulses. What you’re looking at is essentially a DC offset of -170vdc… with THAT on top of it. YES, THAT IS 200 VOLTS/DIV. HOLY ASS, BATMAN.

Solar Panel Specifications Explained

On a solar panel, module, cell, or laminate, there are a number of different specifications given. Here’s what you will find, and how to interpret it.

Here are some sample specs I, uh… stole for purposes of illustration. These are for a Canadian Solar CS6P-190-PE.

Power (W): 190 Watts
Open Circuit Voltage (V): 36.00 Voc
Short Circuit Current (A): 7.42 Isc
Maximum Power Voltage (V): 28.60 Vmp
Maximum Power Current (A): 6.64 Imp

Now, here’s what these mean to you!

First off, the Open Circuit Voltage (Voc). This is the voltage you will see present at the solar panel’s output when it is exposed to full sun and is not loaded. While this is not really relevant to the panel’s power output, it should be taken into consideration for two reasons: First, you should ensure that any equipment connected to the panel (meters, charge controllers, etc) is capable of handling the full Voc of the solar panel or the string of solar panels connected. Otherwise, equipment damage may occur when the sun hits the panels and they’re not loaded down. Consult the documentation on your charge controller if in doubt. Also, for your safety, be sure that any overcurrent protection devices or disconnect switches are rated for Voc or higher! Upon unpacking and installing your panels, if you’re lucky enough to get full sunlight hitting them, check the open circuit voltage – it should be close to Voc. If it’s too low, the panel may have a problem or be miswired (check the junction box).


Short Circuit Current (Isc). This measurement is useful for testing the panels and determining the sizing of your wiring and controller.  Set your multimeter to amps, and connect the leads across the solar panel’s output terminals. In full sun, you should get Isc. If you do, the panel is operating correctly. Any wiring to the solar panels, and the charge controller itself, must be capable of handling the Isc of the array. Do not expect to load the panel down to Isc in normal operation, as you will be getting almost no voltage and extremely reduced power. (See I-V curve below!)

I-V Curve. Source: pveducation.org
Maximum power voltage (Vmp) and amperage (Imp). These levels are very important to consider in selecting panels and components for your solar energy system! In short, please keep the voltage as close to Vmp as possible. The reason for this is that the solar panel has a certain internal impedance, and you will only receive maximum power when the panel output voltage *under load* is allowed to remain near Vmp. If you load the panel down to a lower voltage, it will become severely inefficient.
If you are using the solar panel with a conventional charge controller to charge lead-acid batteries, the ideal Vmp will be near the absorption charge voltage for your batteries. If you are using an MPPT charge controller, Vmp should be anywhere within the controller’s MPPT tracking range. This may be an extremely wide range of voltage, allowing you a lot of flexibility in choosing panels!
To illustrate the importance of the Vmp point, see the above I-V curve and power curve for a solar panel. Note that the power curve tapers down towards zero as the voltage falls below Vmp, and abruptly falls off as the voltage approaches Voc. If you buy solar panels and operate them too far from Vmp, you might as well be throwing money down a hole.

If you have an MPPT charge controller, it will periodically sweep the array voltage to find Vmp, which actually varies a little with different sunlight levels. For the best possible power output under all conditions, use an MPPT controller. The only exception I should point out: if you are using HF radio equipment, the switchmode boost/buck converter inside an MPPT controller may cause excessive noise on the receiver. Consider using a simpler charge controller in this case. The Morningstar ProStar series charge controllers have an internal jumper (really, a 0 ohm resistor that you can cut) to disable their PWM charge control to reduce RF noise to minimum.
Don’t toss your system efficiency and money down the toilet – choose your solar panels wisely!