The sensor that stays awake
A camera’s image sensor, processor and Wi-Fi radio together draw far too much power to run all day on a battery. Battery cameras therefore keep them switched off and leave one inexpensive component awake: a passive infrared sensor, or PIR. It watches for changes in infrared radiation, which is to say heat, across a set of zones in front of the camera. A person, an animal or a warm car crossing those zones produces the change that wakes everything else.
PIR sensors respond best to movement across their view and least to movement straight toward them, because a body walking directly at the sensor stays within the same zones for longer. They can be set off by things that are warm and moving but uninteresting, such as car engines, sunlit branches or heating vents, and they detect nothing through ordinary window glass, which blocks the far-infrared radiation they sense.
Plug-in cameras, with power to spare, usually detect motion differently: they compare successive video frames and flag the areas that change. Many then classify what moved, as a person, a vehicle, an animal or a package, either on the camera or on the maker’s servers. Some newer models add radar, which measures distance and direction and can draw a more precise boundary for alerts.
From movement to recording
Waking takes time. Between the moment heat crosses the sensor and the moment video starts, the processor has to start up, the camera has to re-establish its link to the router or hub, and recording has to begin. The sequence is short, but it explains a familiar complaint: a clip that opens with a visitor already at the door, or a car already halfway past.
Cameras with constant power can close the gap by keeping a rolling buffer of the last few seconds and attaching it to the start of each clip, a feature makers call pre-roll or pre-buffering. Battery cameras generally do without a buffer, and some models enable one only while connected to constant power.
What drains a battery
Battery-life figures in specifications assume a modest number of events a day. Real life depends on how often the camera wakes and how long it stays awake each time. Every motion event, every second of clip and every minute of live viewing costs charge, so a camera facing a busy street can exhaust in weeks a battery that would last months facing a quiet yard.
| Factor | Why it costs charge |
|---|---|
| Motion events | Each one wakes the camera and records a clip |
| Clip length | Longer clips keep the processor and radio running |
| Live viewing | The stream runs for as long as the view is open |
| Weak signal | The radio transmits harder and repeats lost data |
| Cold weather | Lithium cells deliver less of their capacity |
Signal strength matters more than it appears to. A camera with a weak link to its router or hub transmits at higher power and resends data that was lost, and that extra radio work drains the battery even when the number of events is unchanged. Cold is the other common factor: lithium cells give up less of their capacity at low temperatures, so winter battery life is typically shorter, and makers publish an operating temperature range for each model.
Makers use two kinds of battery. Some cameras run on disposable lithium AA cells, as Blink’s battery cameras do; others carry rechargeable packs, as Arlo’s and Ring’s battery models do. Solar panels sold as accessories keep rechargeable models topped up, and many battery cameras can also run from a wired power supply.
Height, angle and the shape of the view
Placement guidance from makers converges on a few principles. Outdoor cameras are commonly mounted around 7 to 10 feet (about 2 to 3 meters) above the ground: high enough to be out of easy reach, low enough to see faces rather than the tops of heads. Doorbells sit much lower, at roughly chest height beside the door, where a visitor’s face fills the frame.
Angle matters as much as height. Because a PIR sensor responds best to movement across its view, a camera aimed so that visitors cross the frame, rather than walk straight at it, detects them sooner and more reliably. A slight downward tilt keeps the sky, with its glare and passing warmth, out of the picture, and angling away from a public road cuts alerts from traffic.
Detection range is finite. Many models detect people reliably at about 20 to 30 feet (6 to 9 meters), and makers list a range for each one; beyond it, a camera may still show a person on video without its sensor ever registering them.
Glass, night vision and weather
At night most cameras switch to infrared illumination: small LEDs around the lens give off light that the image sensor can see and people largely cannot, apart from a faint red glow. Behind a window that light reflects straight back off the glass and washes out the picture, and a PIR sensor behind glass is blind in any case. Indoor cameras pointed outside through a window therefore perform poorly at night, which is one reason outdoor models exist.
Some cameras add spotlights or floodlights, which give color video at night and act as a deterrent. Outdoor models carry an ingress protection rating, such as IP65, that states their resistance to dust and water; the second digit refers to water, and a higher number means more resistance. Indoor-only models carry no such rating and are not designed for damp or freezing conditions.
Other people’s property
A camera’s view rarely stops at the property line. Maker apps provide privacy zones, which black out parts of the image, and motion zones, which limit where movement triggers alerts; both are commonly used to exclude a neighbor’s windows, a shared hallway or the public sidewalk.
Recording sound is treated differently from recording pictures in many places, and some jurisdictions require the consent of the people being recorded. Many maker apps let audio recording be switched off separately for that reason. Local law, rather than the camera’s settings, decides what is permitted.