I put a 5,700 litre water tank in at home, ironically just before water shortages were announced for Auckland around 6 months ago. Using it to water the garden in summer / backup as emergency drinking water.
For home automation I also run OpenHAB, so am building a small ESP project to report the percentage water level back to OpenHAB via MQTT. It's a pretty straight forward and whilst lab'd up on my desk, it works well. However, as it's outside and away from the house I will need to run it on batteries.
The devices needing power are:
- Wemos D1 mini (takes either 5V or 3.3V feed)
- Control board for the ultrasonic sensor (takes 5V feed)
I have 2 x 18650 batteries (3.7V 3600mA each) powering the project. Currently, these are 2S (so ~8.4V 3600mA of power)
Basically how it works is when the Wemos powers on it connects up to WiFi, takes 2 x measurements one second apart, checks the two readings (to ensure no anomalies) and if ok, sends that % water level back to OpenHAB which the publishes that measurement in the UI (along with a rule that simply publishes the 'last updated' date/time)
I've measured current draw on the Wemos + Sensor board and it's about ~80mA give or take for about 10 seconds. The ESP then disconnects from WiFi and puts itself into deep sleep mode for approx 1 hour. So basically every 1 hour, it comes out of sleep mode takes a measurement sends it off to OpenHAB then goes back to sleep (I also added a MOSFET in as well to logically switch the power on/off to the ultrasonic sensor board once the Wemos powers up / goes to sleep, as I found the board leaked a bit of power when sitting idle so this helps reduce overall power usage)
I have a solar panel for this which I'll run to charge the batteries. Its an 18V 10W PV panel. I also have a small MPPT solar charger that will suit my requirements too (output voltage and powerpoint for this are adjustable) From my calculations, it should easily be able to keep the batteries charged based on mA usage vs average sun etc for my area.
Now for the questions...
1. Am I best to run 2 x 18650's in series (so ~8.4V 3600mA) and then run a voltage regulator to drop voltage down to the 5V required to power both Wemos and ultrasonic sensor board? My concern is the feed to the voltage regulator may take up excess power wasted as heat??
2. Am I best to run 2 x 18650's in parallel (so ~4.2V 7200mA) and then run a DC-DC step up boost converter to 5V to power both Wemos and ultrasonic sensor board?
Or does it really not matter? I also read charging more than one 18650 is preferably done in parallel than series, especially if the batteries are unbalanced. I can run a BMS to assist with series charging if needed, as I have one available to use so not really an issue there.
Thoughts? Or any other tips or ideas that would help with this?