In the Woodside Ramble 50 km my Forerunner 610 was powered up and recording for almost exactly 5 hours before it powered down, 4:47 into the race and 32 minutes before my finish. This was, needless to say, a downer.
The watch is rated for 8 hours, not 5. So what went wrong? Battery fatigue? False marketing? Accelerated draw due to a challenging GPS environment? A personal curse that I shall always get sucky batteries?
Well, this last option has proven likely with various cell phone and laptop batteries in my personal history. But in this case, I stumbled across the simpler explanation when reviewing DCRainmaker's "in depth review" of the Forerunner 610.
One-second mode. I had the unit on one-second mode to give better time-resolution on Strava segments. I looked at the Smart Sampling mode for the Forerunner 610 here. Sampling times blew out to as long as 7 seconds in smart sampling mode (a few longer, but those intervals may be due to signal loss), while 1-second sampling is quite simply 1-second sampling. I looked at Strava segment timing reliability here.
DCRainmaker has an outstanding GPS product comparison page. Forerunner 610 is on the low end of the battery life spectrum with 8 hours nominal. The Forerunner 220 and Forerunner 620, for example, are both a remarkable 30 grams lighter (DCRainmaker weighed his 620 @ 44 grams, and the 220 came in at 41 grams). Note newer 610s replaced the metal backplate with plastic, so are around 10 grams lighter than mine, at some loss in the advantages of metal backplates. But both the 220 and 620 are rated to 10 hours rather than 8. This should certainly be enough if I were to do a 50 miler. 8: unlikely.
But it's a mistake to say "my longest race is x hours, so this is how much battery I want." It's really nice to have a buffer. You're not going to risk turning on your GPS right at the start line: too much going on; too easy to forget. And you may well forget to shut it off when unplugging it before heading off to your race (the Forerunner tends to be power-up, in non-GPS mode, coming off the charger, and this mode alone results in significant power drain).
This is why seasoned ultra runners I've spoken with prefer the XT series of triathlon watches. These are heavier and bulkier, but have batteries designed to last the ironman distance. I'm a weight weenie so don't want to be carrying that sort of bulk around on my wrist. But I still have the option of simply carrying my Edge 500 cycling computer in my pocket. I'd lose some GPS accuracy that way (the 610 does better) but 14-hour battery life wouldn't be a problem (I used it most recently for that long in my 13:47 Devil Mountain Double Century last year).
Another ultra option is the Fenix series, the most recent the Fenix2. Here's the DCRainmaker review. It comes in at 85 grams. Battery life has its cost.
For now, I'll go back to smart sampling mode on the Garmin Forerunner. There's some loss in Strava segment timing (at least until they go to inerpolated segment timing, such as I use for timing Low-Key Hillclimb "self-ride" events), but the advantage in potentially improved battery life is worth it.
added: There are different opinions about whether 1-second mode should affect battery drain. Indeed, my initial feeling was that it shouldn't. I suppose I'll need to experiment.
added 2: According to Garmin Support (see this post on Garmin Forum), Smart Recording has no benefit to battery life. So my unit is just cooked.