I'm trying to build a small transmitter that is powered by a 3.7 volt battery and also a solar cell to run the electronics in daytime and charge the battery. I have almost no experiance in this so am looking for some advice, I have a 6volt 3.5watt 6oomah cell that gives out power even on a cloudy day, my transmitter outputs 100mw so not much but enough power to drain the battery in 2 days, right now I have a Lithium Ion battery 3.7 volt and am dropping the power with a small buck convertor from the solar cell to the battery, no charge controller now, question I have is I would like to use a TP4065 Charging Module, as I see it I can drop the voltage on the Solar cell down to 5volts with the buck convertor and run to the module and charge the battery and power the transmitter at the same time but when the cell isn't charging will the battery kick in automatically? and also I'm worried about heat and the Lithium battery having issues in the sun outside and this will be outdoors, would a Lifepo4 be a better option?