# Firmware for the LNSC-2420 Solar Charger

The LNSC-2420 is a PWM-based solar charge regulator for 24 V systems supporting
up to 20 A charging current.

Noteworthy features:

- High-side switching with N-Channel MOSFETs:
  - Gate voltage is provided by a charge pump which doubles the battery
    voltage.
  - The MOSFETs are controlled using GPIOs.
- Current and voltage measurement:
  - The solar current, load current, solar voltage, battery voltage and charge
    pump output voltage are measured using the internal ADC of the STM32F030.
- Low power usage:
  - The charge pump is disabled if both power FETs are off.
  - Additionally, the microcontroller is put into deep sleep (Stop Mode) if no
    solar power is available and the battery voltage is in undervoltage
    protection.
- LED status display:
  - 6 LEDs show the internal charging and discharging state.
  - The LEDs are controlled using Charlieplexing (i.e. only 3 GPIOs are needed)
- Protections:
  - Solar panel reverse current
  - Battery overvoltage
  - Battery undervoltage
  - Load current limit

## Calibration

The sensing hardware should be quite accurate, but some minor error always
remains. They can be compensated via the parameters in `src/calibration.h`.

To adjust those parameters, first set all `CAL_FACTOR`s to 1000, recompile and
flash the firmware.  Then watch the RS485 output, which contains all
measurements in a line starting with `MEAS`. Apply a known current/voltage for
each measurement and calculate:

                              Actual value
    CAL_FACTOR = 1000 * -----------------------
                        Value shown by firmware

Round to the nearest integer and replace the 1000 in `src/calibration.h` by the
calculated value.

## Customization

All customizable parameters can be adjusted in `src/config.h`.

## License

The code in this repository (except submodules) is licensed under the [MIT license](LICENSE).