Why the label and the real world disagree
A battery-powered sensor is often advertised with a lifetime measured in years. Those figures usually come from lab conditions: moderate temperature, strong signal, a gentle reporting schedule. In the field, a few ordinary factors can cut that number sharply. Knowing the usual culprits helps you predict the life of a device before you deploy it and diagnose it when it dies early.
Think in averages, not peaks
Battery life comes down to simple arithmetic: capacity divided by average current. Capacity is typically given in milliamp-hours (mAh), and average current in milliamps (mA).
Here is a hypothetical example. A cell with 2,000 mAh of usable capacity, feeding a device that averages 0.1 mA, would last 2,000 / 0.1 = 20,000 hours. That is about 833 days, or a little over 2.3 years. Double the average current to 0.2 mA and the life halves to roughly 1.1 years.
These numbers are illustrative only. Real capacity depends on the cell, the temperature and how fast it is drained. The useful lesson is that small changes in average current matter a great deal, and the average is dominated by whatever the device does most expensively.
The radio is usually the biggest cost
For most sensors, transmitting and listening cost far more than sensing. A temperature reading may take a tiny amount of energy; sending it over a radio can take thousands of times more for the moment it is active. A few related points follow:
- Choice of network. Some wireless protocols are built for short, rare messages, and others for high data rates. Low-power networks and Bluetooth Low Energy are designed to keep radio time short. Cellular IoT connections can also be tuned for low power, but joining a network and holding a connection can cost more than a short local link.
- Listening. A radio that waits for incoming messages draws power the whole time it is listening. Devices that only send, and never need to be reached, can keep the receiver off almost all the time.
- Weak signal. When the signal is poor, the device may transmit at higher power, repeat messages or spend longer connecting. A sensor placed at the edge of coverage can use noticeably more energy than the same sensor in a good spot, so sensor placement matters for battery as well as for data.
- Retries. If a message is not acknowledged, the device usually tries again. In a congested or unreliable network, retries can multiply the radio’s energy use.
- Joining and reconnecting. Re-establishing a Wi-Fi connection, for example, can take far more time and energy than sending one short packet on an existing link.
Reporting rate
How often the device wakes up and sends data is the next big lever. Reporting every minute rather than every ten minutes means ten times as many radio events. For many uses, such as a room temperature that changes slowly, frequent reports add little value but cost a lot of life.
Two common tactics help. One is reporting only when something changes beyond a threshold, with a slower periodic heartbeat to confirm the device is alive. The other is batching several readings and sending them together, since much of the radio cost is in setting up each transmission. The sleep behavior that makes this possible is explained in our article on duty cycling.
Sensors themselves
Some sensors are cheap to run. Others are not. Gas and air quality sensors often include a heated element that draws steady power, and some need a warm-up period before each reading. Cameras and image sensors are demanding. Motion detectors that must stay alert all the time draw a small continuous current that adds up over months. Reading the sensor specifications for current draw and warm-up time is worth the effort before choosing a part.
Cold weather and heat
Temperature changes what a battery can deliver. In the cold, chemical reactions slow down, internal resistance rises, and the cell can deliver less of its rated capacity and struggle with sudden current bursts such as a radio transmission. Some chemistries tolerate cold much better than others; that is covered in the article on choosing battery chemistry. Rechargeable lithium-ion cells in particular often cannot be charged safely below freezing, which matters for outdoor solar devices.
Heat has a different effect: it tends to speed up aging and self-discharge, so a cell sitting in a hot enclosure may lose capacity sooner.
Leakage, self-discharge and the quiet drains
Even a device that does nothing slowly loses energy.
- Self-discharge. Every cell loses some charge over time. Some types lose very little per year; others lose much more. For a device meant to run for many years, this can be a meaningful share of the budget.
- Sleep current. The device’s own idle draw, from the processor, a voltage regulator or a sensor that is not fully switched off, runs continuously. A part with a poor sleep current can dominate an otherwise careful design.
- Pull-up resistors and leakage paths. Small design details, such as a resistor holding a line high, can leak current constantly.
- Moisture and dirt. Damp can create tiny conductive paths across a circuit board.
Firmware behavior
Software can waste energy too. Examples include a loop that keeps the processor awake instead of sleeping, a failed connection that is retried forever, logging that writes to storage too often, or an update check that runs more often than necessary. Over-the-air updates are valuable, but a large download uses a lot of radio time, so many battery designs schedule them carefully.
How to find what is draining a device
- Measure the device’s current in its different states: deep sleep, sensing, transmitting, and joining a network.
- Estimate how long it spends in each state per day.
- Multiply and add to get an average, then divide the battery capacity by it, leaving a safety margin for aging, cold and self-discharge.
- If the real life is shorter than the estimate, check retries and signal quality first, since they are the most common surprises.
For a fleet, battery level reporting through device management tools lets you spot units that are draining faster than their peers, which often points to a placement or signal problem rather than a faulty battery.