ESP32 Deep Sleep Weather Station

Projects General Guidance
1

I'm working on a deep sleep, updating every 12 hrs (or when triggered), weather app, running on ESP LOLIN32 v1.0 MPU, 4.2-inch e-Paper Module for display, Adafruit LC709203 Fuel Gage battery monitor, and LP603449 LiPo battery.

This project will have an on/off switch that, when toggled, will interrupt the deep sleep, go grab the weather data, and then turn back off.

All is working well; however, there is the issue of when to turn off the MCU when power gets too low. As you can see in the image, I monitor the battery voltage and % of charge. This battery has an internal cut-off circuit, but according to the datasheet, that doesn't happen until 2.7v. I believe the worst case for a cut-off should be 3v so as to not destroy the battery and risk of fire, mayhem, and the end of civilization as we know it. Those on the safe side say the cut-off should be 3.2v.

I think I need a cut-off circuit that does not add a substantial current draw that will disconnect the battery. With this board, I'm down to about 7.8 mv/hr, and I plan to use a 3000mAh battery for longer times between charges.

Does anyone have a cutoff design that works with the fuel gauge monitoring?

Weather
Weather1920×1413 402 KB

2

As You monitor the battery, why not cut off using software? A transistor, a resistor and a diode ought to be enough.

3

That’s the plan, but I’m just a hacker. I’m good at taking something and improving it, but not a EE and was hoping someone that has solved this before can show me the “right” way to do it.

4

Why do you believe you know better than the manufacturer of the battery? Why do you believe 3.0V is the more correct cut off? After all....

5

When a battery is nearly flat, it's internal resistance increases.
Meaning, when the battery is about 3.3volt, and you put a load on it, it could drop to below 2.7volt instantly. When you remove the load, it bounces back to 3.3volt.
Leo..

6

When I return home, I'll do some experiments; I'm traveling for work this week. We'll see how well the battery's internal cutoff works.

7

Bad idea. You should already stop using the battery at 3.5volt, and send an email or something. Battery protection is a safety net that you normally should not use. As said, that 2.7volt point only works as intended when the battery is under some load, so it can bounce back to a safe voltage.
Leo..

8

Thanks, Leo. I’ve built many projects, but all used USB power and not batteries.

Data on when to stop draining and recharge is all over the place.

I’ve read 3 - 3.2 when measured under load. First time I’ve heard 3.6v.

I start designing a MOSFET cutt off circuit to disconnect the battery.

9

An Arduino power switch with P-channel mosfet is a frequent topic.
Link to a random page here.
Leo..

10

Battery cut-off voltage really depends on the dropout voltage (under load) of the 3.3volt regulator on that ESP32 board. The moment the 3.3volt supply of the processor starts to drop, it's time to stop waking the processor. Don't know what the sleep current is, and how long it takes you to notice that it's not working any more. That determines the need for a cut-off switch.
Leo..

11

Leo, thanks for the comments. I've designed a circuit while on travel, ordered some parts, and will be testing this weekend. Once I get it ironed out, I will post the results and detailed schematics.

12

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