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?

 

I would like to use a TP4065 Charging Module

I think you mean a TP4056 charging module.

but when the cell isn't charging will the battery kick in automatically?

It should once the output from the buck converter drops below around 4 volts.
As far as the heat can you insulate or shield the battery from the sun?
Using the TP4056 module you will need to change a resistor on the circuit board to drop the charging current.
Looking at the photo of this board it's resistor labeled R3.
Changing to 3.3K should set the current at about 360ma which is .1C on a 3600ma 18650 cell.
It will take longer to charge but within the output current of the solar panel.

 

You want to use something that optimizes the panel. BQ24210 Or something similar

 

I don't know if I'm capable of removing and soldering on a new resistor but can try, as far as the battery heat it will be in a enclosed box but the box will be in the sunlight so figure it would get pretty hot in there, I was planning on some vent holes of some kind so maybe it will be fine, I was hoping to be able to just do it this way but as I said I know about nothing about this stuff, just trying out a idea.
The specs say input is 5volts so I can drop the solar panel with the buck converter to that so input to the TP4056 is 5volts and it's charging the battery and feeding the transmitter at the same time also it says it shuts the charge off at 4.2volts so when the battery is charged it would stop and the solar cell would power the transmitter untill there is no sun and the battery should kick in, am I close on this? and also do I need a Diode on the solar cell to keep it from discharging the battery r does the TP4056 do that?

 

am I close on this?

Correct.

do I need a Diode on the solar cell

No diode required.