Unfortunately, the offset voltage of the opamp for charge current measurement breaks measurement at low currents with a small shunt, so at dusk, when currents become low, it may happen that 0 current is measured while the battery is still being charged. This would falsely trigger the transition to sleep state. To improve this situation, we simply keep the charger active for a longer time.
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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.
- 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)
- Solar panel reverse current
- Battery overvoltage
- Battery undervoltage
- Load current limit
The sensing hardware should be quite accurate, but some minor error always
remains. They can be compensated via the parameters in
To adjust those parameters, first set all
CAL_FACTORs 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
All customizable parameters can be adjusted in
The code in this repository (except submodules) is licensed under the MIT license.