New Firmware 2.0 supports 12V LiFePO4 batteries

Description

The MPPT Solar Charger is an intelligent solar charging system and power supply designed to provide power to IOT-class devices (Arduino type through Raspberry Pi 3 type) in remote, solar-powered environments. It manages charging a 12V AGM lead acid or LiFePO4 battery from common 36-cell 12V solar panels. It provides 5V power output at up to 2A for systems that include sensors or communication radios. Optimal charging is provided through a dynamic perturb-and-observe maximum power-point transfer converter (MPPT) and a 3-stage (BULK, ABSORPTION, FLOAT) charging algorithm. A removable temperature sensor provides temperature compensation. Operation is plug&play although additional information and configuration may be obtained through a digital interface making it an ideal companion to intelligent or internet-connected devices.

The use of commonly available 12V solar panels and AGM (UPS) lead-acid or LiFePO4 batteries allows for an inexpensive system capable of operating systems 24/7 even in inclement weather and over a wider temperature range than systems using other battery technologies. Lead Acid batteries can be charged down to -20°C and LiFePO4 batteries can be charged down to 0°C.

I developed the MPPT Solar Charger as a result of feedback from my Solar Pi Platter from customers that wanted 24/7 operation of their devices from solar power. The MPPT Solar Charger has been in development and tested in several locations and all seasons for years. If you're interested, you can read about its development on hackday.io.

Comes with a spare fuse and 6-pin header. An optional wiring kit includes battery and dupont cables for connecting to your device as well as remotely mounting the temperature sensor.

Features

• Reverse Polarity protected solar panel input with press-to-open terminal block
• Fused battery input with press-to-open terminal block
• Maximum 2A at 5V output on USB Type A power output jack and solder header
• Automatic low-battery disconnect and auto-restart on recharged battery
• Temperature compensation sensor with internal sensor fallback
• Status LED indicating charge and power conditions, fault information
• I2C interface for detailed operation condition readout and configuration parameter access
• Configurable battery charge parameters
• Status signals for night detection and pre-power-down alert
• Night-only operating mode (switch 5V output on only at night)
• Watchdog functionality to power-cycle connected device if it crashes

Applications

• Remote control and sense applications
• Solar powered web or time-lapse camera
• Night-time “critter cam"
• Solar powered LED night lighting controller
• Power supply with battery backup for Raspberry Pi Server or Router

Supporting Software

The MPPT Solar Charger comes with full software support.

• Arduino library for easy access via the I2C interface. The library will also run on the Raspberry Pi using the WiringPi library.
• Linux daemon pre-compiled for ARM-architecture single-board computers like the Raspberry Pi and Beaglebone Black. The daemon allows logging of data from the charger, configuration, and controlled shut-down on low battery detection. It also allows other programs to access the charger via a simple character-based pseudo-tty device file or over the network.
• Desktop application for Windows, Mac and Linux computers allowing real-time display of data (through the daemon) or analysis of logged data.

Compatible Panels and Batteries

The MPPT Solar Charger is designed to use standard 25- or 35-Watt 12V solar panels with 6-cell AGM type lead acid or 4-cell LiFePO4 7-Ah to 18-Ah 12V batteries. It has a maximum charge capacity of about 35-38 watts. A detailed sizing method is described in the user manual but it is possible to use smaller or larger panels and batteries depending on the application.

Typically a 25-Watt panel is paired with a 7-Ah battery for small systems (Arduino-type up to Raspberry Pi Zero type). A 35-Watt panel is paired with 9-Ah to 18-Ah batteries for larger systems. Larger batteries provide longer run-time during poor (lower light) charging conditions. A larger panel can provide more charge current during poor charging conditions.

Solar panels should be the 36-cell types with a typical maximum power-point of around 18V and maximum open-circuit voltage of 23V (typical 21-22V). Available panels and batteries I have tested with are shown below.

• 35 Watt Panel
• 25 Watt Panel
• 9 Ah Battery
• 10 Ah LiFePO4 Battery
• 18 Ah Battery