Summary

This is a low-power environmental (ambient temperature / relative humidity / barometric pressure) Home Assistant-integrated WiFi sensor. It has a lifespan of over 2 years while powered by 2xAA LiFePo batteries. It runs on a ESP32-C3 based module. It remains in deep sleep mode most of the time and wakes up every 30 minutes to send environmental data to Home Assistant. It uses the low-power BME280 environmental sensor.

The board is powered by a pair of 1.5V AA LiFePO lithium batteries (non-rechargeable). The decision to use these batteries was based on:

• Their ability to discharge almost to capacity while maintainng a steady voltage (unlike alcaline batteries). This eliminates the need for voltage regulation
• This allows for both saving on components count - and the additional power draw
• Batteries very low self-discharge current (ie, shelf life of > 10 years)

The board is using standard ESPHome firmware with custom YAML configuration.

If you choose ot manufacture your own PCB, the corresponding Gerber files are included.

Hardware

     

Battery Capacity

The total battery capacity $Q_\text{battery}$ can be calculated as:

$$ Q_\text{battery} = Q_\text{self\ discharge} + Q_\text{board quiescent charge} + Q_\text{useful capacity} $$

Where:

• $Q_\text{self\ discharge}$: Capacity consumed due to self-discharge
• $Q_\text{board quiescent\ charge}$: Capacity consumed by the quiescent current of the circuit
• $Q_\text{useful capacity}$: Capacity consumed during measurements

Calculate available battery capacity and longevity

To convert mAh to Coulombs:

$Q (\text{C}) = I (\text{mAh}) \times 3.6$

Where:

• $(Q)$: Charge in coulombs (C)
• $(I)$: Current capacity in milliampere-hours (mAh)

For Energizer Ultimate Energizer L91 AA lithium battery:

$Q_\text{battery} = 3,000 mAh * 3.6 = 10,800\ Coulombs$

Quiescent Charge

The board consumes < $6\text{uA}$ current while in deep sleep:

Charge used by the board in deep sleep in 1 year: $Q_\text{board quiescent\ charge} = 0.006mA * 3.6 * 24h * 365days = 378\ Coulombs$

Self discharge

The L91 AA lithium battery boasts a shelf life of 15 to 25 years at 21°C (70°F), indicating a self-discharge rate of less than 1% per year. Calculating loss of charge due to self-discharge for a 2-year cycle:

$Q_\text{self\ discharge} = 10,800\ Coulombs * 0.02 = 216\ Coulombs$

The remaining useful capacity of the battery after 2 years:

$Q_\text{useful capacity} = 10,800 - 2 * 378 - 216 = 9,828\ Coulombs$

Battery lifespan

Single environmental measurement and subsequent communication with HA consumes between 200mC and 250mC of charge:

Assuming worst case, the total number of possible measurements is:

$N_\text{measurements} = 9,828 / 0.250 = ~~ 39,000\ measurements$

Considering 30 mins sleep between measurements, the sensor lifespan on a single set of fresh batteries would be:

$N_\text{days} = 39,000\ measurements / (2\ measurements\ per\ hour * 24 hours) = $ 812 days or 2.2 years