Pros: Small size, lower cost than other MPPT controllers, flexible charging options
Cons: Limited to 15A charge current
A Maximum Power Point Tracking (MPPT) controller is very useful for when you have PV panels that work at much higher voltages than the batteries you wish to charge. The power delivered is the product of the voltage and the current. Solar panels that are used on a battery with a regular controller have their working voltage reduced from 15, 16, or commonly 17.5 Volts down to the battery voltage. This could be as low as 10.5V for a flat battery. The panel puts out the same current regardless of what voltage it works at so a panel rated at 15W at 17.5 Volts will deliver 0.86A at 17.5V and 0.86A at 10.5V, which is only 9W.
An MPPT controller uses inverter technology to convert the voltage that the panel wants to work at for maximum power and turns that into more current at the voltage level the battery is working at. So 15W at 17.5V is delivered to the controller but about 14.5W is delivered by the controller to the battery, charging at 1.38A into 10.5V. About 3-5% is lost in the conversion.
That's what it says in the sales blurb. Does it work? Yes. I have measured the input and output currents on the thing and it does exactly what it says it can. The bigger the difference in input panel voltage over the actual battery voltage the more the current is boosted.
This also presents some interesting possibilities. I had bought some 17.5V panels and used them with a normal controller but after collecting a few of these panels I got this controller as it allows you to use much higher voltages than the battery wants. As the panels are some distance from the controller, a lot of power can be lost in the wires to them. Either that or you have to spend more money on thicker wires. By running pairs of the 17.5V panels in series to give you twice the voltage but at half the current, you deliver the same power (17.5V x 1A = 17.5W and 35V x 0.5A = 17.5W) but with lower wire losses. The power lost over the wire is proportional to the square of the current flowing through it so by halving the current, you reduce the power lost on the line by 4 times!
The Morningstar MPPT-15L can accept solar panels or series strings of up to an absolute maximum of 75V input. Now it's not actually possible to run it this high as solar panels put out a lot more voltage in frosty weather so you have to use a lower rated panel. I've got two Sharp ND-170E1F panels and have run them in series to give me a maximum voltage of about 59 Volts.
I actually have two of these controllers charging one battery bank in parallel. This is because some of my panels are 35V series strings and this conflicts with the much higher voltage Sharps. You can use different voltage panels at the same time on one controller but the controller picks the lower voltage to work at and so quite a lot of power from the higher voltage strings would be lost and you also need blocking diodes for the low voltage strings to prevent them being back-fed.
One thing to watch is that the controller can only output 15A of battery charge. You can use the controller with either 12V or 24V batteries but the current limit is the same. This means that for a 12V battery, you can only have up to 200W of solar panels. You can install more but the output will be limited to 15A. With a 24V battery, you still get 15A of charge current but that means you can have up to 400W of solar panels.
Incidentally, I run the controllers with 264W and 340W of panels and in winter you never see more than 12A of current on the small array as the light in the UK is so dim. I did however overload the new controller with the 340W string. That was on a very clear February day and did reach and exceed the 15A limit. At first nothing happened other than the controller limiting the current to just over 15A. As the sun got to it's peak, the controller started to cut the output current on and off in a cycle but later resumed just limiting the current to 15A. I wouldn't recommend running the controller this hot for long, even with it's protection.
The MPPT-15L, in common with most other Morningstar controllers uses an advanced 4 stage Pulse Width Modulated (PWM) charge scheme. This also includes very short discharge pulses that are supposed to aid charging by dislodging bubbles that form on the plates in charging and to help break down sulphate deposits that reduce the battery capacity.
The battery will charge up to a preset voltage (bulk charge) and then be maintained there for a variable number of hours, depending on how discharged the battery was (absorption charge). Finally, it will then drop down to a lower float charge level to stop self-discharge of the battery. The fourth mode is a variable timed equalise charge at a high voltage that can help keep all the cells in a battery topped up to the same degree. This happens after deep discharges or every 28 days. It can be disabled.
Battery types supported are: Flooded, AGM and gel. These have appropriate charge voltages set in memory but they can be changed by an optional PC interface (not included). You choose between the preset battery types by a screw terminal jumper and some little switches.
I've not used it but the controller has a data port for talking to either a dedicated meter or to Morningstar provided PC software for reading and setting parameters. The controller also retains 30 days of performance data in memory that can be downloaded by this means.
There are load terminals on the controller and it can supply 15A of battery power or solar power (although the battery must be connected for the controller to work). The load controller also has programmable shut-down and restart voltages so that batteries do not get run flat.
The controller doesn't have much of a display, just 4 LEDs to show whether there is a fault or whether the unit is sleeping or charging and the approximate battery level (low - med - full-ish). It's not reliable as a state of charge gauge and I bought a dedicated gauge for that purpose. The good thing is that the controller doesn't draw much current when sleeping at night (measured about 35mA).
Installation was pretty easy. The thing comes with screws to mount it (but not wall plugs). The base is actually the finned heat sink and so the fins are not visible from the front. Make sure to leave air space above and below the unit on a wall so that air can circulate. Having said that, I haven't noticed it get more than just warm to the touch even when maxed out at 15A output.
All in all it's a great controller, well built, good at charging 12 and 24 Volt batteries and pretty much bullet-proof in terms of its self-protection against wiring faults and general abuse (although they do say that they won't cover replacements due to abuse of the unit). Otherwise, they'll guarantee that the thing works for 5 years.
I did look at the Outback MX-60 but it's a lot more expensive and bigger and if you run a 24V battery, this little Morningstar will handle up to 400W of solar panels. If had bought the MX-60 I could have put all the panels on one controller and had up to 60A charge current possible but then I would have had the problem of wasted power on the different voltage panel strings.
I just wish that Morningstar would come out with a more powerfull version that could do say 25-30A at 12V.