So much for the DI2 battery datafield on my Garmin. The left hand shifter showed full bars on the datafield during my ride today. By chance I looked at it this evening and it’s down to 1 bar and indicating critical. Anybody else had this sort of problem. I’m not bothered about the life - they’ve given me 12 months and 2000 miles, just the amount of warning.
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On my Wahoo Roam, the DI2 shifters only report as Good or Low. When my RD shifter started reporting low I road another ~10 hours before changing the batteries and they didn’t die on me.
I had kept forgetting to buy batteries for them and was always prepared to use the app to change the shifting on the left shifter to synchronous if the right battery died. 
The only thing I own that gives sufficient notice is my varia. Why the shifters and derailleurs can’t similarly send me notices well in advance of dying (or at all) is beyond me.
I just got the notice to replace one of mine for the first time and while I was looking up which battery model I needed to buy I noticed that the Shimano manual says when you get the low battery warning you should replace ‘within a month,’ so that seems pretty reasonable for a binary indicator that can only tell you ‘good’ or ‘low’.
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I guess I should check mine. I’ve got a year and a half and 14000 or so miles 
Thanks for all the replies. I just think that going from full bats to one in an hour or so doesn’t leave me with much confidence that the one bar will last very long
On another note the main DI2 battery doesn’t seem last anywhere near as long as my old 12 speed one
It’s not designed to transmit a true battery level, only a binary ‘good’ or ‘replace’.
“Full” = 11-100%
That could just be how garmin is choosing to display all battery levels regardless of what kind of data the devices are sending. As far as I can tell the shimano shifters only transmit good vs replace, but I’m also making an assumption there.
Best person to check with to confirm is @terry_bettershifting whose fantastic resource sums up all di2 knowledge. Shifter battery page here: Check Di2 shifter battery level and replace it
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I suspect so, yeah.. I do use a Garmin myself, but haven’t had to replace the batteries on my ST-R8170 shifters yet.
But yeah… the shifters report ‘low battery’ when under a certain voltage. I think Garmin just takes the same status and displays either all bars or one bar. 
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Apple played that game with their buds years ago, and I had a dive computer that played that stupid game too. Well, yeah, you have some battery left, so All’s Good. Then: PFFFTTTTT Oh, the battery died!! The former was annoying, but the later was enraging. It used 2 really odd batteries that sourcing in the Caribbean proved to be nearly impossible and the shelf life of those batteries wasn’t long for some reason. I hate to think of Shimano going with a bizarre size battery to try to ‘fix’ this. There seems to be an infinite number of coin battery sizes and finding replacements for some is difficult to say the least.
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I recently set off on a ride and decided to check the Di2 battery before leaving, knowing I might go do 100+ miles. Full battery bars, I should be good.
At first my unrelated power meter started acting up. Battery indicator on the Garmin said “OK”. It completely quit half way through the ride.
I got a low battery alert on the Di2 maybe 40 miles in. About 70 miles in I got a critical battery alert. I started using my shifters as little as possible.
I made it home, but I was reminded of why I don’t like batteries on required systems. Really not happy with Di2 (ditched AXS on my MTB’s).
My mate now carries spare batteries with him on every ride
Just doesn’t trust them any more
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Many types of Lithium batteries have a fairly flat voltage curves, so it’s hard to have any idea of remaining battery life by only looking at voltage. They basically fall off a cliff with very little warning. And temps can often be a trigger to cause almost immediate failure as they approach end of life. Your battery might show fine on a Friday afternoon riding in 70f temps, then give warning and die in the first 15 minutes of your saturday morning ride with temps in the 40’s. I’ve had my SRAM shifters go from good to warning to dead in less than an hour of riding. Now, I just make a habit of replacing batteries at least once a year and before any events that matter. And I always keep an extra on the bike. Coin cells are cheap. I also have wireless blips on my race bikes, so always have redundancy on shifting just in case.
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This. Coin cells will often be fine right up until they’re not, with no warning.
They’re coin cells, they take up next to no room.
Carry extras if you think you’ll be away from civilization for a month.
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