Background
I've got ten of these things — First Alert ZCOMBO, Z-Wave smoke and CO combo units, installed back in 2017, about $500 total. All ten have hit end-of-life around the same ten-year mark.
I was ready to serial into one — the board has "RS232" and "serial" printed right next to a port. But before doing that, I bought a new unit instead, the SMICO100, on the idea that if the old Z-Wave daughterboard fits into the new housing, I could skip the reverse-engineering entirely.
Observation
The daughterboard didn't fit. But opening that second unit led to this: across the two units I've now opened — one AA-powered, one 9V-powered — both still had a separate internal battery beyond their primary power source. That didn't add up: why would an externally-powered detector also carry its own internal battery?
Hypothesis
That internal battery isn't there for power — it's there to run a clock, tracking how long the unit has been in operation. That's the actual end-of-life mechanism: not a sensor health check, just an operating-time clock. If that clock's state lives entirely in that battery, disconnecting and reconnecting it should reset the count.
The test
I desoldered the battery, let it sit disconnected for 30 seconds, soldered it back onto the same board, and put the unit back into service in the kitchen to see if the end-of-life chirping starts again.
Result
Still waiting.
Records
These are pictures of the new 9V unit, and the old unit I was working on see what was possible with.
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