Testing the Nomura MPPTs
In MSXII, the MPPTs were never fully validated to source the error for inconsistent charging of the battery. If the below block diagram is to be implemented, extensive testing must be conducted to verify the MPPTs are not the source of the issues. The following will be the testing procedure:
Unit testing of MPPTs
Each MPPT will be tested individually. They will be isolated, then attached to an E-load to draw constant current . A 240 Watt LED bulb will be shone to imitate sunlight on a cloudy day or in the nighttime testing.
The following table should outline results from varying the E-load and intensity of the sunlight. The first cell in series was isolated, and attached to an E-load.
| Light Source | V(in)oc | E-Load Setting | Measured Voltage Output | Measured Current | extra notes |
|---|---|---|---|---|---|
| Sunlight | 25.131 | ||||
| Clouds | 16.2 | 7.0kohm | 10.03 | 0.0014 | |
| Clouds | 16.2 | 10ohm | 0.155 | 0.0152 | |
| Clouds | 16.2 | 70W | basically 0 | 0.015 | |
| 240W light | 9V | 5ohm | 0.227 | 0.0422 | reading the vout from mppt was 0.56V while output from solarsenseslave was 0.056V |
| 240W light | .866 (dmm) 25V(eload off) | 5ohm | .285 | 0.0566 | |
Note: when the Mppts are lit by the light outside on a fully cloudy day, the mppt goes into short circuit mode where it provides basically no output voltage increase for a normal resistance on the line. However, using the 240W flashlights provided by Micah, we are able to see a huge increase in the voltage input and the voltage out on the line (from 15V(out)oc when cloudy to 25V(out)oc). On a sunny day outside the voltage output from the mppt is still 25V which is pretty good.
Another note: the voltage on the input of the mppt changes drastically when the outside environment does not even change. For example, the voltage output(oc) was 15V, which changed to 25V after several minutes and changed to 8.8V after several more minutes of doing nothing (same light source and intensity).
Due to the variance in the measuremntes, each mppt was taken out of the holders and tested with the bench supply and DC E-Load. The following table summarizes the results.
| MPPT# | DC E-Load Settings | V(in)oc | V(out)oc | Vin | Vout | Iin | Iout | max temp (deg C) | Notes |
|---|---|---|---|---|---|---|---|---|---|
| 1 | 6ohm | 15.98 | 25.245 | 10.01 | 18.516 | 6.264 | 3.087 | 44 | 24-25V has audible frequency oscillations (can hear) |
| 1 | 7ohm | 15.98 | 25.245 | 33.01 | 32.339 | 4.608 | 4.62 | 31.3 | When in audible range, heat increases really quickly. With an increase in resistance on the E-load is a increase in the voltage output with a significant decrease in tehb output current for a total less amount of output power. Maybe this has to be configured so the output voltage we want will get drawn, and the |
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Note: I started testing in constant current mode to match the following diagram in terms of power out and in.
I found that the output voltage started fluctuating and making the input also fluctuate to maintain the current. Not sure if it is supposed to operate like this, but it may be an issue if other mppts in the string force one mppt to be at a constant current. I will continue testing this issue by looking into the suggestions on the user manual of the mppt:
V(in)oc & V(out)oc were measured without an electronic load connected.
Note, the maximum output power for any mppt is determined by decreasing the resistance on the DC load until the power supply goes into Constant current mode. This limits the maximum input power knowing the mppts are limited to 7A on input
what is the minimum power out that we expect from each mppt?
what is the maximum power out that we expect form each mppt?
were each of them meeting this requirement?
String Testing of MPPTs
Each MPPT once unit tested, will be tested in series for the total array output power, and continuity. Like unit testing, this will be run for at least half an hour for full validation.
SPI Communication Testing of MPPTs
After obtaining a SPI logic analyzer, we will record the SPI messages given by the SPV1020 MPPT IC.
The team is using 11 Nomura mppts (same model as used on MS12). Unlike in MS12 in which solar sense was broken up into two boards (a solar sense slave that was placed beneath each mppt and a solar slave master that brought the power from each solar sense slave in series for each array → two masters for each array of solar cells in series were paralleled to provide a power path to the batteries), for MS14 it will be combined into one board for the following advantages:
- Easier to assemble and debug two boards as opposed to 11 different solar slaves
- Easier wiring as we will need to only harness the thermistors going to each solar panel section, isolated spi communication, and each power line going to each mppt from solar sense board
Other features of solar sense:
- current, voltage and temperature sense of the outputs of each mppt
- isolated spi communication between the board and the mppts
Block Diagram:
