Every battery-powered sensor ships with a number on the box: two years, three, five. The number is not a lie, but it answers a question the buyer is not asking. Battery life is a function of how many times the radio wakes and how long it stays awake, and both are set by the installation and the platform as much as the hardware. The same model can last fourteen months in one house and four years in the next one on the same street.
Why the Label Says Three Years and Yours Dies in Ten Months
The rated figure is measured under a defined reporting profile: a fixed number of events per day, a defined heartbeat interval, a stated temperature, and a platform that lets the sensor sleep between events. Change any of those and the arithmetic shifts. A sensor on a busy hallway door with a chime automation, a two-minute heartbeat, and a hub that polls for status lives a very different life from the same part number on a guest-room window.
The physics is simple and brutal. A sleeping sensor draws microamps; a transmission draws tens of milliamps. The awake-to-asleep ratio is in the thousands, so every avoidable wake and every retransmission consumes the budget at the rate of weeks of ordinary life.
What Actually Wakes the Radio
Motion events are the obvious wake source, and the one installers think about. The underrated ones do more damage:
Retransmissions after a failed delivery. A sensor at the edge of signal range sends its packet, gets no acknowledgement, and tries again, often several times. Each retry costs a full transmission. A marginal link can triple the energy cost of every event, invisibly, because the event still arrives and the dashboard looks fine.
Polling. Some platforms periodically interrogate devices rather than trusting their reports. Every poll wakes the radio. Sensors on such platforms age faster with no change in user-visible behaviour, which makes the cause hard to diagnose afterwards.
Command traffic. A sensor that participates in automations receives configuration and group messages. A mesh that churns, re-pairs devices, or runs frequent network-wide health checks wakes every sleeping node each time.
False triggers. A PIR that fires on curtains, a contact sensor chattering on a loose frame, a tamper switch marginal on its spring: each false event is a real radio transmission with real energy cost. Reliability tuning is battery tuning.
The Heartbeat Tax
A heartbeat is the periodic "I am still alive" message that lets the platform distinguish a closed window from a dead sensor. Without it, silence is indistinguishable from a fault.
Heartbeat interval is a direct dial on battery life: halving it doubles the heartbeat share of the budget, which on a quiet sensor can be the majority of total consumption. A monitored alarm door sensor can afford a daily heartbeat, while the same hardware reporting every ten minutes pays for that reassurance in months of life.
Datasheets that specify a heartbeat assumption are being honest. Datasheets that quote only "up to X years" without stating the reporting profile are quoting the best case, which is a marketing number, not a planning number.
Cold Weather Shortens the Same Battery
Lithium cells lose usable capacity as temperature drops, steeply near freezing. A sensor on an unheated porch, garage door, or garden gate sees a winter capacity far below its rating, precisely in the season when events matter most.
Cold also shifts the failure mode. A sensor does not usually die at a convenient moment; it degrades, transmitting with weaker margins, retrying more, and accelerating its own decline. The end-of-life curve in a cold installation is a cliff approached at increasing speed, which is why the low-battery warning often arrives later than it should on exterior sensors. Battery type matters here too: lithium primaries tolerate cold far better than alkaline cells. A vendor specifying lithium is making a cold-weather claim, whether or not the copy says so.
Reading the Numbers on a Datasheet
Three figures carry almost all the information: sleep current in microamps sets the floor; transmit current and packet duration set the cost per event; rated capacity in milliamp-hours is the budget. Battery life is that budget divided by the floor plus per-event cost times daily events, plus the heartbeat share. A datasheet providing these inputs can be modelled in a spreadsheet in five minutes, and the model predicts your installation better than the box number.
Also worth reading: what the low-battery report does. A sensor that warns at a defined threshold gives you a replacement window. A sensor that simply goes quiet converts a maintenance task into a security gap you only discover when a door stops reporting.
What to Ask Before You Standardize on a Sensor
The standardisation questions are about the profile, not the battery. What reporting profile the rated life assumes. Whether heartbeat interval is configurable per device. How retransmission behaves at the edge of range, and whether the platform surfaces link quality. What the low-battery threshold is and where the warning appears. Whether the battery is a standard size obtainable anywhere, because a proprietary cell turns a five-minute swap into a procurement item.
A sensor family that answers these is designed for fleets and long deployments. A family that only quotes years on a box is designed for a shelf. The difference is invisible on day one and decides the total cost by month twenty.
See the sensor range: sensors and alarms. Manufacturing background: door window sensor manufacturer. Company: https://xingliaotech.com/. Bulk inquiries: contact.
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