ESP32 drains LIFePO4 3.2V battery, any suggestions?

Question

I'm trying to build a smart scale using this ESP32, which is connected to a HX-711 amplifier + load-cell. It's also connected to an HC-12 RF communication sensor.

Here are pictures of my setup: https://i.sstatic.net/jrxtf.jpg

The purpose of this device is to measure weight changes and transmit the current weight to a distance device using the RF sensor.

No need for Wifi or Bluetooth on this board.

The problem is that this device should be wireless, and the batteries are getting drained in a couple of days at best, and I'm kind of helpless at the moment.

I went down to take weight measurements every 8 hours, and during those 8 hours I'm putting my ESP32 into a hibernation mode which should be the best one for battery consumption saving, but that also doesn't help.

I started to think that the ESP32 is not meant for this assignment and that maybe there's a better alternative board that might do the job and keep the battery alive for a few weeks/months instead of a couple of days.

Can anybody recommend a different board, or maybe suggest what else can I do to make this circuit run without draining in a couple of days?

As mentioned, I do not need the Wifi and Bluetooth, and mainly using the ESP32 due to its deep-sleep/hibernation mode.

I'll also add the main parts of my code, maybe the problem is there:

#include <WiFi.h>
#include "esp_sleep.h"
#include <Arduino.h>
#include "HX711.h"

const int LOADCELL_DOUT_PIN = 22;
const int LOADCELL_SCK_PIN = 23;

HX711 scale;

int currentWeight = 0;
int previousWeight = -100;

uint64_t TIME_TO_SLEEP = 60*60*8;
#define uS_TO_S_FACTOR 1000000ull

void setup() {
  WiFi.mode(WIFI_OFF);
  WiFi.disconnect();
  btStop();
  rfSetup();
  weightSetup();
  delay(1000);
}

void loop() {
  weightLoop();
  delay(2000);
  esp_sleep_pd_config(ESP_PD_DOMAIN_RTC_PERIPH,   ESP_PD_OPTION_OFF);
  esp_sleep_pd_config(ESP_PD_DOMAIN_RTC_SLOW_MEM, ESP_PD_OPTION_OFF);
  esp_sleep_pd_config(ESP_PD_DOMAIN_RTC_FAST_MEM, ESP_PD_OPTION_OFF);
  esp_sleep_pd_config(ESP_PD_DOMAIN_XTAL,         ESP_PD_OPTION_OFF);

  esp_sleep_enable_timer_wakeup(TIME_TO_SLEEP * uS_TO_S_FACTOR);
  esp_deep_sleep_start();
  delay(1000);
}

void weightSetup() {
  scale.begin(LOADCELL_DOUT_PIN, LOADCELL_SCK_PIN);
  scale.set_scale(173.90393);
}

void weightLoop() {
  scale.power_up();
  currentWeight = (scale.get_units());
  delay(1000);
  scale.power_down();
  sendWeightToHub(currentWeight);
}

void rfSetup() {
  HC12.begin(BUED);
}

void sendWeightToHub(int newWeight) {
  HC12.write(String(newWeight).c_str());
}

Last thing - When I check the current in the multimeter the current - I get 0.01 (out of 20mA) when sleeping, and up to 0.05 (again, out of 20mA) when awake. That doesn't add up to draining the battery in 2 days though, so I guess I did something wrong there.

Any suggestions?

Answer 1

First, 18650 is not LIFePO4 battery as you mentioned in your post title, it is Li-ion battery, with nominal voltage of 3.7V, and max charge-up voltage of 4.2v, cut-off voltage at about 2.5V.

Secondly, your board is not designed for battery operation, if the product image you show matched your actual board, the AMS1117 regulator on the board has a very high Quiescent Current of 5mA, and 1.1V dropout voltage. The board also has power LED that is constant On (and likely consumed 1.5-2mA constantly) even during sleep mode, so the current consumption at the deep sleep is likely 6.5-7mA. The high dropout voltage also means that even if your battery is fully charged at 4.2V, with high dropout voltage of 1.1V across the regulator, it will only barely able to output 4.2-1.1=3.1V instead of regulated 3.3V, the ESP32 will probably brown-out at about 3.0V, plus the high leakage current and PWR LED, and a questionable battery, it simply barely work for a while.

Get an ESP32 board that is designed for battery operation, both Adafruit and SparkFun have boards designed for battery operation, so is China's DFRobot.

Answer 2

Besides what hcheung said,

I do not need the Wifi and Bluetooth, and mainly using the esp32 due to its deep-sleep/Hibernation mode.

Then why use ESP32? There are many other microcontrollers that can do what you need, for example STM32 or one of the Arduinos. One of the problems with ESP32 is that the sleep mode with RAM retention is quite power-hungry compared to other micros, and the deep sleep loses RAM so you have to reboot it, which also uses quite a lot of energy.

You would need to be careful about picking a micro and read the fine print on sleep modes if you were running it on a coin cell. But for a 18650, pretty much any micro with a RAM retention sleep mode will do the trick. Ironically, the ESP32 is one of the few that won't work well, because its light sleep mode with RAM retention uses 800µA, whereas most micros will use a couple µA or less.

I was about to recommend Raspberry Pi Pico because it has a buck boost converter which is nice to have when running on battery, but its sleep current is 180µA which is quite high too.

If you want to run from a 18650 and not bother with a buck-boost converter, using a 3V3 LDO will give 3V3 when it is fully charged, dropping to a bit lower than 3V when it is close to discharged. So...

HX711 is specified to run on 2.7V minimum. For micros, most STM32 M0 will run down to 2V, 8-bit AVR runs down to less than 2V if you run it at 4MHz, so it's fine. You don't need a high power micro like ESP32 to measure a weight and transmit it.

The RF module will probably need more, but perhaps you can replace the LDO with a lower dropout one.

So I'd suggest changing the micro, and if possible using parts that can run on the battery voltage directly, or using a 3V3 LDO and parts that will tolerate voltage drop down to 3V.