"MPPT" Solar charge controllers
Since my alternative power system at home is growing, I wanted to try out MPPT charge controllers since I'm having more and more loads added to it to maximize the available power and speed up the return for our investment on the system.
I looked around online for some MPPT charge controllers to try out. I stumbled upon these on a well known online auction site and for about $14 each (the 10A and 20A versions) and about $50 for the digital one with LCD screen, I said, why not? Let's give it a try.
I went ahead and ordered them and studied about how different MPPT techniques are applied, Looking at Google images, I couldn't find the models I bought. I wonder why? Then they arrived. When I held the small ones, You feel what $14 buys you. It's very light and feels hollow.
Once I got home, powered up the one with LCD screen and it seems to work. So, on with the screwdriver!
Surprise, surprise. Look at the pictures I took to have an idea of what you get for that price.
The three charge controllers that I ordered. The top one is 30A rated, the two below are 20A and 10A. All are supposedly MPPT type:
The manuals clearly state MPPT in the model numbers. The online ad clearly says
"MPPT solar charge controller MPPT10-60: Compared with normal solar charge controller, this MPPT controller could increase efficiency by 10%-30%. " - yeah right:
Let's start with the 30A with LCD:
Back side is nice extruded aluminum with fins as heatsink. This one weighs a bit and doesn't feel hollow:
Top cover removed:
The LCD with the LCD driver is on a small board that can easily be unplugged.
Note the lack of a huge inductor. MPPT charge controllers are SMPS based and are usually buck or buck-boost topology so they WILL have a huge inductor somewhere.
There is one on the upper right corner but that's for the 5V USB port power supply so these are PWM controllers.
It does use beefy high current shunts (copper bars on the PCB), nice buss bars to handle the current and no diodes carrying the full charge and discharge currents. All are switched by MOSFETs only:
One small MOV to absorb lightning surges on the input line.
The two multiturn trimmers with OP07 op amps for calibrating the current readings.
There is no means as far as I can tell to calibrate the voltage readings as mine was off as much as -0.4V:
The temperature probe is a barrel connector with a thermistor inside:
On with the smaller ones.
Both are the same only difference I think would be the MOSFETs used
- I can't confirm as the numbers aren't readable, and the fuse. 25A on the 20A version and 15A on the 10A version:
Back cover removed. The back cover is a punched aluminum sheet about 1mm thick. The MOSFETs are simply held on with light pressure.
There are no screws clamping them tightly to the "heatsink":
Front of the PCB. Note the same lack of large inductors present in a true MPPT controller. The PCB is neatly laid out though. I do see three SMD trimmers. They don't appear to be factory set as they are still in the middle position as it came from the reel:
A puny transorb to absorb input surge?
Self resetting fuse? Polyswitch?
I don't think so!
One thing I do like though is that they all use decent quality rising clamp terminal blocks:
To end this writeup, The electronics do look decently assembled but only time will tell if they will last or even meet the current capacity as advertised. The manuals mention of automatic selection of 12/24V systems but I don't think I'd want to believe that. Another thing I don't like is that both input and output are switched on the negative side so consider that when using a common negative ground system. Also, in this case, the saying "You get what you pay for" still holds true. Cheap and true MPPT? Not a chance.
14 July 2013:
I tested the two smaller ones which have trimmers inside but doesn't seem to do anything when I tried turning all of them. I measured that the controllers cut off at 14.5V battery voltage, the low voltage cut off is at 10.80V and resets at 12.15V on both the 10A and 20A version.
I could not stand the voltmeter display being off on the 30A unit so I modified it. Turned out I only needed to add a 100ohm trimmer to be able to calibrate it.
This is the part we are interested in. Main microcontroller is the 32pin QFP IC marked U1:
Here are the modifications I made to be able to calibrate the voltage display. The trimmer I added is a 100ohm Bourns 3296Y:
Trimmer in place. I also added metal and fiber washers on the three screws holding the MOSFETs onto the heatsink to distribute the clamping force and not deform the PCB:
Once I had calibrated the voltage display, it is fairly accurate. Interestingly, When charging at high currents, the display is accurate even with some voltage drop across the line between the controller and battery. This was possible because the controller charges in pulses by turning off the panel for about 6ms at 10 times per second and measuring during the off period. As for the amp displays, the output current display is fairly accurate against my external display. But for the input current display, I'm having a hard time calibrating it as during input, current is pulsed so it is difficult to get an accurate DC reading without an oscilloscope + shunt or a good DMM but a few days of observation shows it follows closely the reading of my external large display meters.
But the output control is quite versatile. If set at 24H, the output is always on as long as battery voltage is above the LVC (low voltage cutoff). When set at <24H, it will turn on when there less than 4Volts input voltage from the PV panels. Once it is on, it will turn off in any of the three conditions: there is >4Volts PV voltage, LVC is triggered or the time set is reached. In all these modes, the output can be manually turned on or off with the 'lightbulb' button.
Page updated and copyright R.Quan © 09 July 2013.