Door and window sensors die two ways. The boring way is a flat battery, which announces itself as a low-battery push and is fixed in a minute. The quiet way is corrosion: the battery leaks, the electrolyte creeps onto the contact spring, and the sensor starts lying - reporting low battery on fresh cells, dropping off the network, or just vanishing. Fleet operators who track failures find corrosion, not capacity, at the top of the list.
The chemistry of a slow leak
Alkaline cells leak for two predictable reasons. A deeply discharged cell builds internal gas pressure and cracks its own seal, so a sensor that let a cell run to zero is a sensor holding a leaking cell. And cells parked at full charge in a warm device age their seals - the spare set in a hot drawer is on the same clock. The electrolyte that escapes is alkaline and mildly conductive; it wicks along the spring, corrodes the plating, and keeps working long after the visible crust has been wiped away.
What corrosion actually does
Corroded contacts do not fail open; they fail resistive. The elevated contact resistance drops voltage under load, so the sensor reads a fresh cell as depleted - the classic complaint of a low-battery alert that returns a week after new cells went in. Worse, the resistance fluctuates with temperature and vibration, which looks like intermittent connectivity and sends troubleshooting in exactly the wrong direction, toward the radio and the hub. By the time the pattern is recognized, the spring is usually pitted and cleaning is temporary.
Lithium does not leak, but it oxidizes
Lithium primary cells remove the leak mechanism, which is why they are the right default for a device you plan to forget for two years. They are not maintenance-free: terminals still oxidize over long dwell times, and mixing chemistries inside one device is its own failure mode. The honest rule is one chemistry per device, one brand per batch, and a scheduled replacement date rather than a wait-for-the-warning policy.
What good hardware and good discipline look like
On the hardware side, the questions are contact plating - gold-plated springs resist corrosion far better than tin - plus a sealed battery compartment that keeps electrolyte off the board, and a battery door that opens without tools, because a maintenance step that needs a screwdriver gets skipped. On the fleet side: replace on schedule, in batches, one chemistry and one brand per device, and log replacement dates per unit so a future intermittent fault has a suspect list. Corrosion is not an accident; it is a schedule nobody kept. We design battery compartments for exactly this failure mode across our sensor and alarm range - details on the door and window sensor manufacturing page, with the factory overview and contact page.
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