Weeks on a Charge: How PlantSense Sleeps

The most common question we get about PlantSense isn't about plants. It's some version of "why don't the readings update every minute?"

The honest answer is that they could — for about two days. Then the battery would be flat. This post explains the schedule the sensor actually runs, why it looks the way it does, and how to read the battery dial so charging is never a surprise.

A sensor that is asleep almost all the time

An ESP32 with its Wi-Fi radio on draws on the order of 80 mA. In deep sleep the same chip draws a few microamps — roughly ten thousand times less. Battery life is therefore decided almost entirely by how often the sensor wakes up, not by how much it does while awake.

So PlantSense spends its life in deep sleep and wakes for a short, fixed routine:

1. Power up and sample every sensor: soil moisture, temperature, humidity, light, battery voltage.
2. Join your 2.4 GHz Wi-Fi network.
3. Send one reading to the PlantSense cloud (and, if you've set it up, to Home Assistant on your local network).
4. Turn the LED off and go back to sleep.

The whole cycle takes seconds. Everything you see in the app — the dials, the charts, the "water in ~2 days" estimate — is built from those short check-ins.

90 minutes, then 3 hours

The default interval is 90 minutes. Soil moisture changes slowly; even a thirsty plant in a small pot dries over hours and days, not minutes, so 16 readings a day capture the curve with room to spare.

Once the sensor has posted three readings in a row without a hiccup, it stretches the interval to 3 hours. Steady Wi-Fi and a healthy plant don't need to be checked as often, and the longer interval is where the "weeks" in "weeks of battery life" comes from. Any failed check-in drops the sensor back to 90 minutes until it has re-earned the longer gap. That's why the site says readings arrive "every 90 minutes to 3 hours": the sensor picks, based on how things are going.

The five-minute exception

There is one time the sensor deliberately stays awake: right after you switch it on. A manual power-on opens a five-minute window in which the sensor doesn't sleep. That's what makes setup feel instant — the app can find it over Bluetooth, hand it your Wi-Fi details, and show the first live reading while you're still standing at the plant. When the window closes, the sensor takes its next reading and drops into the normal schedule.

Reading the battery dial

The battery dial in the app shows charge as a percentage, and under it a small estimate — "Charge in ~9 days", say. The estimate is a trend line drawn through recent battery readings, so it gets more accurate the longer the sensor has been running and can be optimistic for the first day or two after a charge.

Alerts are tiered. You'll get a Warning when the battery crosses below 25%, and a Critical alert below 20%, with one more below 15%. Each fires once as the level is crossed, not every reading, so a slowly draining sensor won't nag you.

The app also works out when you last charged, without you telling it: a sustained rise of more than 5% in the battery history counts as a charge. The same trick is what powers "Last watered 11 days ago" — a jump of 15% or more in soil moisture is a watering.

Charging

Charging is over the USB-C port under the cap with any phone charger. A small charger LED — separate from the status LED — glows red while charging and green when full. A full charge takes a few hours; you don't need to power the sensor off to charge it, and it keeps taking readings while plugged in.

What "weeks" really means

Battery life depends on how far the sensor is from your router (a weak signal means longer, hungrier Wi-Fi sessions), on temperature (lithium cells give up capacity in the cold), and on how many readings end up at the 3-hour interval versus 90 minutes. A sensor two rooms from the router with steady Wi-Fi comfortably lasts several weeks; one on a far garden bed with a marginal signal will want charging sooner. The battery chart in the app will show you which kind of month you're having.

Curious what the Wi-Fi signal dial means for battery? The features overview has the full sensor rundown, and the quickstart covers first-time charging and pairing.