ESP8266 HomeKit – Battery Status

Slowly but surely I am looking at the extensions of the “standard” ESP HomeKit accessories, how to implement them. Now I am working on the Battery status. We need some circuit which can effectively measure voltage. Battery monitoring circuit is a traditional voltage divider circuit. I am going to measure 3.7 volt (lithium Polymer) battery.

 

Voltage divider working and calculations

ESP8266 is a tiny device, it works on 3.3 volts. Since it is working on 3.3 volts its pins can source and sunk 3.3 volts only. Voltage greater than 3.3 volt may blow the pin or fry the ESP8266. In our case we want to measure 3.7 volt battery and ESP8266-12 ADC (Analog to Digital Channel) can only accept 1.0 volts.

ESP8266 ADC Resolution

The ADC pin has a 10-bit resolution, which means you’ll get values between 0 and 1024. The ESP8266 ADC pin input voltage range is 0 to 1V if you’re using the bare chip. However, most ESP8266 development boards come with an internal voltage divider, so the input range is 0 to 3.3V. So, in summary:

• ADC Voltage range in ESP8266 development boards: 0 to 3.3V (for example: ESP8266 12-E NodeMCU Kit, WeMos D1 Mini, …)
• ADC Voltage range in ESP8266 chip: 0 to 1V (for example: ESP-07 chip, ESP-12E chip, …)

We need to play smartly here. What we will do is divide the voltage between two resistors and measure only voltage across one resistor and remaining resistor voltage will be calculated mathematically. Typical voltage divider circuit and formula is given below.

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Forumula:

Vout = Vin	Rbottom
	Rbottom + Rtop

When whe fill in our variables that are available, into our formula.

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Forumula:

1.0V = 4.2V	10KΩ
	10KΩ + Rtop

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Forumula:

 

At 4.2 Volts Vbottom is 1.0 volts				At 3.7 Volts Vbottom is 0.88 volts
Vrbottom Ratio =	4.2V	= 4.20		Vrbottom Ratio =	3.7V	= 4.20
	1.0V				0.88V

 

 

After you completed the circuit its time to move on to code. Code is written in C. the most important part is where the measurement takes place.

int low_battery_value;
float battery_value;

homekit_characteristic_t status_low_battery = HOMEKIT_CHARACTERISTIC_(STATUS_LOW_BATTERY, 0);

void battery_low_task(void *_args) {

        while (1) {
                battery_value = sdk_system_adc_read();
                printf ("ADC voltage is %.3f\n", 1.0 / 1024 * sdk_system_adc_read()* 4.2);
                vTaskDelay(3000 / portTICK_PERIOD_MS);
                if (1.0 / 1024 * battery_value * 4.2 < 3.900) {
                printf ("Battery value is low.\n");
                low_battery_value = 1;
                }
                else{
                printf ("Battery value is not low.\n");
                }
                homekit_characteristic_notify(&status_low_battery, HOMEKIT_UINT8(low_battery_value));
                vTaskDelay(3000 / portTICK_PERIOD_MS);
        }
}
void battery_low_init() {
        xTaskCreate(battery_low_task, "Battery Low", 256, NULL, 2, NULL);
}

&status_low_battery,


battery_low_init();

 

DOWNLOAD Eve for HomeKit

 

 

Status when the battery is not low in the HomeKit App.

status when the battery is not low in the EVE App.

Status when the battery is low in the HomeKit App. The Measurements turn red. When we click on the sensor we see that it says Low Battery. Also in the settings of the sensor you will see Battery Low.

Status when the battery is low in the EVE App. You will see that there will appear a Low battery icon in the sensor tile. Also in the settings of the sensor you will see Battery Low icon.