Solar Sense: MS 12

Description	Record MPPT data
Target release
Epic
Document owner	Titus Chow
Project leads	Titus Chow, Taiping Li
Team members	Pei Liang Guo, Mena Labib (Unlicensed), Hanna Muratovic (Deactivated), Ayushi Shah

Goals

Isolate ADC
Interface with Nomura MPPT
Record voltage and temperature per-module and current per-string

Background and strategic fit

We're planning on using Nomura MPPTs. Each MPPT is designed to connect to a smaller number of cells and output up to around 30V. A few of these MPPTs are then put in series. Theoretically, this should maximize the performance of our array in non-optimal conditions.

We're like to record data on the array to optimize our strategy and charging.

Assumptions

Our MPPTs will output around 30V at max
Our array should generate around 1kW at max
Our MPPTs will be integrated into the array modules
We will need to deal with communications over 1m+ cables

Requirements

#	Title	User Story	Importance	Notes
1	Work with Nomura MPPTs	We're using 10 Nomura MPPTs, 5 in series for 2 strings total. They don't have any public way of interfacing with them, but we'd like to collect data on our array.	Must Have	Using a master board for current sensing and LIN slaves for each MPPT
2	Record voltage and temperature per-module	We'd like to know the voltage and temperature of each module so we can tell how each module is performing and any potential hotspots.	Must Have
3	Record current per-string	We'd like to know the current provided by each string so we can calculate the overall power of the solar array.	Must Have	We will probably use a dedicated current slave to record the current of the string.
4	Isolated	We want to make sure that our analog measurement is isolated from the rest of the car.	Must Have	Look into analog isolation using a class D amplifier + digital isolator
5	Low-cost/Simple	We'd like to make these boards as simple as possible to reduce development time.	Must Have	Once the LIN stack is written, we can use the built-in ADC to record voltage and temperature.

Questions

Below is a list of questions to be addressed as a result of this requirements document:

Question

Outcome

What kind of architecture are we using?

Master node with controller board living in the roof enclosure

Slave nodes (LIN) attached to each MPPT

How will our boards communicate?

Originally, we were planning on using differential I2C. However, after doing a cost analysis, we have decided to use LIN instead, with low-cost LIN slave controllers on each board. See Solar Slave - System Architectures for a comparison between the two.

What's powering the boards?

The MCUs and LIN transceiver will be powered from as 12V rail provided by the solar master node. To save on costs, we plan on using LDOs since our MCUs are relatively low-power. On the isolated side, we will likely use an LDO to convert the MPPT output to a voltage usable by our isolator.