For a application where i need an Arduino + ESP8266 to record a chamber pressure and post to ThingSpeak. I am using the following module which is available on E-Bay for about 1USD.
This combined with a 3.7 to 5V booster completes my power supply. And since its a fixed application, i am powering the module with a 5V wall wart. The Lithium cell backup is required to handle power outages. All well so far. But i have some queries :
I notice that module kind off trickle charges the Lithium cell as i can see the Red led on module come on periodically indicating charging. But mostly the Blue led is lit - indicating full charge. Is this OK for the life of the Lithium cell??
I thought if i leave the 5V power connected, then the Lithium will not discharge at all - but looks like power is drawn from it primarily and the module senses the drop in voltage from 4.2V and charges it once low.
I also plan to use the same set up for a portable instrument … then in that case which is the best place to put the power switch ? In series with the + lead from the cell [or] in series with the output + lead from the charger module ?? If its the later then the on board LEDS of the module will slowly drain the cell…