Power architecture
Solar power is measured by an INA228 before it enters the BQ25176. The BQ25176 is a linear battery charger with input voltage dynamic power management (VINDPM) which prevents solar panel collapse, but does not perform true MPPT; there is still room for more effective power extraction, especially if a switching charger is chosen.
The BQ25176 then charges the LiFePO4 18650 battery at up to 480mA, with a termination voltage of 3.5V.
The battery is also protected by a HY2112 with overvoltage, undervoltage and overcurrent protection.
The MAX17260 monitors the battery, which communicates an accurate state-of-charge % and other metrics to the MCU. An 0805 sized NTC is placed directly underneath the battery holder part, and makes direct contact with the cell.
After passing through the second INA228, the battery is connected to a pair of TPS63802 buck-boost converters that provide a 3.8 V rail for the Particle module, and a 3.3 V rail for the sensors and other peripherals.