Battery - CR2

The CR2 battery is a 3 volt lithium cell for door sensors and SwitchBot. A short cylindrical cell, 800 mAh, that lasts a great deal longer than an ordinary alkaline battery because it loses very little charge just sitting there.

• Why a CR2 battery suits a door sensor
• What takes one here
• Heat is not a problem
• Buying spares
• The specification

Why a CR2 battery suits a door sensor

Self discharge. A door sensor spends almost all of its life doing nothing at all, waiting for a magnet to move. A cell that quietly drains while it waits is no use however impressive its capacity looks on the label, and that is precisely where ordinary alkaline batteries fail in this job. Lithium loses very little sitting still, so nearly all of the 800 mAh is still there when the sensor finally needs it.

What takes one here

Our WiFi Door Sensor runs on this cell, and so does SwitchBot. Outside the smart home it suits digital cameras, torches and memory backup in older equipment. It is a short cylindrical cell and it is not interchangeable with anything else in the drawer.

Heat is not a problem

It performs at both high and low temperatures, so a sensor on an outside door, on a window in full sun, or in a room nobody shades during the day will not misbehave because of the battery. Store the unused ones at room temperature rather than in the same hot cupboard, which is the one place people habitually put spares.

Buying spares

Lithium cells keep well, so a spare is not money sitting idle. On a house with several door sensors it is worth having one in the drawer, because the sensor that goes flat is invariably the one on the door you care about and invariably at the weekend.

The specification

Three volts, 800 mAh, lithium, cylindrical. Brand varies between deliveries and the specification does not.

One thing that is easy to get wrong: a CR2 is not a CR2032, despite the names looking similar. A CR2 is a short cylinder, roughly the shape of a very stubby AA. A CR2032 is a flat coin. They are entirely different cells and no device takes both. If you are ordering from a photograph rather than from the old battery in your hand, look at the shape rather than the number.

If your sensor takes a coin cell rather than this one, you probably want the CR2032 or the CR2450. Wikipedia explains the differences between lithium battery types.