I’ve been working on the plan, or what passes as a plan for the soil sensor. Right now it looks something like this.
I don’t know if it will work yet, I’m not really clever enough to work through the numbers, but I’ve ordered all the components, so they should trickle in over the next few weeks from china. While several people have mentioned that trickle charging lithium cells is a really bad idea, I’m hoping that two layers of protection firstly within the 18650 cell and then on the TP4065 charging board will cut down the risk of explosions and fire to a manageable level. The 5v Solar Panel has already arrived and looks to be suitable for the job, but as I’ve nothing else yet, we’ll have to wait and see.
I may yet drop an Arduino into the picture in place of the ADS1115 i2C if the ESP8266 seems to be a bit too unstable and needs supervision or an external watchdog. It never really sat well with me before, but when you think about it, you can buy a knock off pro-mini for the same price as an ESP-12 so it’s not going to break the bank.