The Sensor states it does not have a replaceable battery but when you open the box it has a standard disc or 10p shape battery which you place into the device. No problem there until you realise that there is a special skill required to get the battery into the optimium position. Basically, the battery slot is not designed well enough to ensure the battery keeps a constant connection. Net result is that it is hard to pair and requires constant fiddle with the battery. I would not recommend this product if you siply want a device to work as advertised. Poor.