In order to see how well this works you also really need a good measure of the voltage and current at both the battery and the charging input. Which means adding the INA219 modules back again after I took them out.
Having considered reworking the previous board it's just too untidy and with the SD card reader attached the ESP8285 doesn't have enough pins to go around. Using socketed modules allowed me to just make a new carrier board and swap them over. I also made a big effort at tidier wiring, the last one was offensive.
The supply load tracking feature on the MCP73871 is called Voltage Proportional Charge Control (VPCC) and reduces load to the supply if the voltage drops below a threshold. VPCC needs a simple voltage divider that provides 1.23v (or more) at a pin while the supply is OK. If this voltage drops, it reduces current draw to bring it back up. On the new board I've fitted a small socket header so I can fit different values to the voltage divider.We're also getting to the time of year where it might go below freezing and the thermal protection circuit needs testing. So I've added another header for the thermistor network, temporarily bypassed with a resistor while I fiddle with the VPCC in isolation. I'm going to move my test location to near an indoor window where there's more sun for more of the day and hang the solar cell out of the window. Once I've investigated the practical worth of VPCC I'll add the thermistor network and do some tests with heating and cooling it.