Found a weird battery drain issue in an old motion sensor last week

I was doing a routine system check at a client's house, one of those older residential setups. The central panel was showing a low battery alert for a single hallway motion sensor. I pulled the unit down, and the new battery I put in just six months ago was already dead. That made no sense. I started checking the wiring in the ceiling box, nothing looked frayed or loose. On a hunch, I got out my meter and found a tiny, constant draw even when the sensor was supposedly in sleep mode. The client said they never had this problem until recently. I'm thinking the sensor's internal board might be failing and pulling power when it shouldn't. Has anyone else run into a gremlin like this with older gear? I swapped the unit out to be safe, but I'm curious what actually causes it.

4 comments

4 Comments

joseph_kim1mo ago

Hate when old sensors start ghosting power like that. Dealt with a similar drain last month that turned out to be a voltage spike from a bad transformer.

cole_baker1mo ago

My buddy's work rig had that same phantom drain all last winter. Turned out to be a worn out regulator on the alternator, it was letting just enough juice back through to confuse the sensors every time the engine was cold. Took them ages to find it.

piper_kim1mo ago

I mean, could it be something simple like dust or moisture getting inside the unit over time? I've seen that cause a slow, hidden short on a circuit board that acts like a constant trickle drain. Maybe a seal failed and it's just barely enough to kill a battery way faster than normal. It would explain why it just started recently if the client had some humidity change or even just more dust from construction nearby. Always a pain to track down those tiny paths when the board looks fine at a glance.

the_wesley12d ago

Had a customer's car last year with a similar slow drain. We found a tiny crack in the housing for the cabin light control module. It let in just enough moisture from car washes to form a conductive path on the board. The thing would pull 50 milliamps, just enough to kill the battery in three days. You're right, you have to check every seal and grommet, not just the board itself.