So I've had my SRM just a few months now (bought it unused, but from a fellow STer), and haven't had any issues thus far (except for the occasional time that the Garmin tries to autozero when it shouldn't so my zero offset goes crazy). Never any dropouts, or indications that the battery was getting low or anything. On Saturday, I rode with it just fine, no issues at all. Sunday I rode the MTB so it sat there unused, and then this morning when I went to ride, my Garmin wouldn't pick it up. I tried using the "search" on the garmin, but no luck. Tried again a few times throughout my ride, still no luck.

I'm wondering what could have possibly changed in ~40 hours of not being moved. As I said, battery seems unlikely because it's still quite new, and there have been no indications that it's been getting low. Magnet: maybe I could stack another magnet to get it closer, but that doesn't seem like something that would change overnight. Other than that, I'm not really sure what could have happened while the bike was just sitting around for 40 hours–even if the SRM was on that whole time for some reason, that shouldn't have drained the battery.

I wanted to tinker a bit more (different magnet, etc) when I get home before I contacted you guys.

I would agree that this answer makes the most sense, but I still don't 100% buy it. The crank was unmounted when I bought it, and even though it was probably sitting around for a bit before I bought it, I don't think it was long enough to drain the battery.

The reason I'm skeptical is mostly that I don't want to pay $50 + shipping just to have the battery replaced if it's not at all needed.

I did notice recently that sometimes I would have to spin the crank 5+ times to get the SRM to wakeup–maybe it is a magnet strength issue?

So this afternoon, it began to work again... Sort of. I turned on the Garmin, and waited for it to load as usual. Then went to search for PMs, and spun the SRM. Oddly, I got the yes/no message about whether to calibrate, so I pressed yes. It say for a minute, and then displayed a calibration number of 0. Went back, and the "Calibrate" option was grey (meaning it must have lost contact). Did this about 4 more times, always getting a calibration of 0. So I gave up, and started to ride. This is when I noticed that I was getting power data�at first I expected it to be way off, but it actually seemed right. A bit into the ride, I did a manual calibration, where it showed a normal value for my SRM (402). No dropouts or anything during this ride, it seemed to work well.

So this makes me think it was most likely a weird Gamin issue. The only things changed from morning to afternoon were: temperature, and turning off and back on the Garmin. Everything else should have remained constant. I could imagine temperature could affect the unit, if the battery was quite low, and the temperature was quite low. But it's only about 50F in the morning, and as I said, no dropouts in the afternoon, so the battery seems fine.

I guess I'll see tomorrow morning if I have the same issue...

Maybe this time around, but given that it's been 6 months (of heavy use) since the last occurrence, I doubt that's what it was the first time.