I purchased several Zigbee PIR motion sensors from AliExpress for my home automation setup. These sensors are powered by a single CR2450 coin cell battery, which is rated for 3V. Upon receiving the sensors, I had problems paring some of them with Zigbee2MQTT. On a hunch, I directly powered them with 3V from a bench power supply, and they paired immediately, implying that the batteries were already dead. Rather than replacing the batteries, I wanted to try modifying the sensors to use two common AA batteries, allowing me to recharge them with my existing battery charger. In theory, this should also increase the battery life of the sensors, given the higher capacity of the AA batteries. I do see new lithium CR2450 batteries boast a 10-year life span, which is impressive. Nevertheless, I wanted to try this mod out.
I started off buying a couple of dual AA battery holders from Amazon, wanting to skip the 1-3 week AliExpress ship time. I chose a model with a built-in on/off switch, which would allow me to easily power cycle the sensors.
Since this mod may not work in the long run, I wanted to make sure that the change was reversible. I cut a small square out of the main sensor body to allow for wires to pass through, still allowing the battery cover to be reattached.
I stretched out the battery pack leads and trimmed them down to about 1/2 longer than I needed to mount everything in place. I lightly applied flux and pre-tinned the battery contacts and the battery holder leads. I then soldered the battery holder leads to the battery contacts, ensuring correct polarity. (look at the coin-cell battery)
Lining up the battery holder with the sensor body, I stuck the sensor down with double-sided tape, pushed the excess wire into the sensor body, and covered the exposed wires with black electrical tape. This is good enough for now.
With the batteries in and the switch on, the sensor powered up and paired with Zigbee2MQTT immediately. Surprisingly, the battery was showing only a ~40% charge, but I’m not too worried about it at the moment, as the batteries weren’t fresh off the charger. This could also be related to the sensor’s battery monitoring circuitry, which may not be calibrated for AA batteries. Time will tell.