Watch shuts down during exercise

To me, it sounds like the most likely cause is battery degradation. As batteries get older, they sometimes no longer discharge in a graceful way. In a “normal” battery discharge, the battery voltage declines gradually as the battery is being discharged. When a Li ion batteries get older, they sometimes experience sudden, large, unexpected voltage drops during discharge, especially if the discharge rate is high. The watch electronics need a certain minimum voltage to operate properly. When there is a sudden, unpredictable drop in the battery voltage below the minimum level, the watch cannot react fast enough to handle it gracefully, provide warnings, etc. It just dies or attempts to reboot as many people have described. The various solutions that people have offered such as turning off WiFi, background apps, etc. are all actions that reduce the load on the battery, i.e. the discharge rate. Under low/normal load conditions, the battery voltage may behavior ok and decline gradually, but under the higher load conditions, the sudden voltage drop may occur. When the watch is in exercise mode, it is continuously active, and also measuring heart rate, both of which add a lot to the rate of battery discharge, thereby setting up the conditions for the voltage crash and the watch shutdown. You can employ the “workaround” strategies of reducing load by turning off WiFi, etc., but this is likely only a stopgap as the battery performance will continue to decline and, eventually, even the reduced load will be too much. My suggestion is that you consider having the battery replaced. I think Apple will do it for $79, or you can take it to a variety of repair shops. I suppose there are kits to do-it-yourself, but I don’t recommend it, especially if you care about the water resistance.

I have a Watch 2 that is experiencing this issue. Turning off WiFi has helped, so I will use it this way for awhile, especially since it is an old watch that I only use for exercising. (I have a newer one for other times.). But I think the battery’s days are numbered...

I see that some people have sent their watches into Apple, and Apple has returned them saying the diagnostics check out ok. But unless their diagnostics test the watch under high discharge conditions, they may not detect the problem. The rate of battery discharge and voltage decline under low load conditions may look ok in their diagnostic test, so they may think that the battery still has adequate life left.

Could it be that your Watch battery is old enough that it is no longer able to provide the maximum power when the Watch needs it? If that is the case, it is conceivable that the Watch, when drawing a larger current than the battery can supply, would turn off.

The cost of replacing a battery of an otherwise perfectly working Watch costs about $69 at Apple. Cost will be different if Apple finds that there is something else wrong with the Watch. See this link and scroll down to the Battery Section: https://support.apple.com/watch/repair/service/pricing

I have been having the same issue with my series 4 Apple Watch for the past several months. My battery health is 85%. Whenever I go for my morning walk and activate a workout app (EITHER StepsApp or Apple Workout) my watch crashes and reboots into the 10% low power mode. By trial and error I found that if I reboot the watch once or twice I am able to see the correct power level. In addition, the watch recharges to 100% in the normal time so it is clear that the low-power mode message is an error. The problem has been occurring at least once a day during my exercise for the past two months. Re-pairing did not help but I found that turning off the cellular connection appears to “fix” the problem.  After turning cellular off, I went for 10 days without a single crash. I then turned cellular back on today and the problem reoccurred. Given that the issue has been reported across multiple models, multiple workout apps and multiple system versions, I find the argument that it is a problem with peak power load somewhat convincing. As I understand it, in outdoor workout mode the watch is constantly monitoring heart rate, the GPS is active and, if you don't have your phone with you, the cellular connection is drawing power. In my case, the cell signal is weak along my route so this possibly compounds the load. 

Turning the cellular connection off is not a satisfactory solution as the main reason I purchased the cellular version of my watch was so that I would be contactable without my iPhone in case of emergency. I can understand Apple being a bit cautious, given the past issues with peak power load on their iPhones, but this is clearly a problem that needs to be addressed.

Ok here is an update to my escalation on this matter Apple.

The good news is that I found the support really quick and efficient, they phoned back and always very polite.

They took reports/logs of the iwatch via my phone

The BAD news. The engineer acknowledged it was a software/battery fault but as I have an Iwatch 2 they no longer support the model. Despite it being their fault and not mine they would not offer any compensation just £30 off a new watch in part exchange.

So really **** result, some of you with more newer models may have a chance of getting it fixed.

Apple if you monitor these things, if its your fault you should help and not wash your hands of it.

The engineer was very sympathetic but has no power to offer anything......he agreed it was rubbish but could not do anything.

[Edited by Moderator]

I don’t think it’s that complex. If I toggle off both mobile data and Wi-Fi before training I don’t have a problem and battery uses maybe 5% in an hour of Workout tracking (with GPS). If I leave Mobile Data on I get a restart and the battery is drained 25%+.

I think it’s a battery management algorithm issue. It’s confusing rapid battery drain with low battery levels and going into panic mode!

I’ve had zero restarts when turning off mobile data and WiFi.

It’s not good that this needs to be done either.

Updated to 7.03. About 45 min fast run, no issues. Everything on. Streamed music the whole time via Apple AirPods. Even more hopeful it's fixed now.

For reference, at 60% battery on return after run mid-day.

I had the same issue and I tried reset, unpair and re-pair and set as new. These did fix the issue temporarily but random restart and slow charging issue came back. Worked with apple support and tried phone reset and watch reset together and it fixed the issue. It didn't immediately fixed the issue after 2-3 days the random watch restart during workout stopped happening and now its ben working well for more than a week. So give t some time after the resets.

I had upgraded my phone from Xr to 12 so maybe that had something to do with the watch issues along with various software updates.

The battery is a chemical reactor, not an electronic device. When the materials in battery degrade, they can lose their ability to support high current flows. Under these circumstances, the electron flow can be suddenly interrupted, even though there is still plenty of energy stored in the battery. It is like throwing a switch into the "off" position. The battery voltage then drops precipitously to zero, or something close to it. The watch electronics have no time to react gracefully to this sudden, unanticipated voltage drop, and the watch simply crashes.

I do not know specifically how the algorithm that measures the battery percentage in the Apple watch works, but, in general, these algorithms operate with some level of assumptions based on the known specifications/characteristics of the battery. A battery which has been fully charged is assumed to be full. The decline in the battery percentage is a calculated number, usually based on the cumulative amount of energy used, or on the rate of decline of the battery voltage, or a combination of both, perhaps further modified to take into account the cumulative number of charge-discharge cycles (the age) of the battery. The algorithm assumes the battery is healthy and that it will discharge gradually in a predictable manner, with a gradual decline in voltage.

When the watch has crashed in this manner and it reboots, it then comes back up in a relatively passive mode, with much lower current draw than when it was being heavily used and crashed. The battery voltage looks ok again, and the battery percentage algorithm then picks up where it left off, assuming that the last calculated value before the crash for the amount of remaining energy is correct--which, in fact, it may be, because the problem is not the amount of energy remaining in the battery, but rather the battery's ability to support high current flows when lots of things are running at the same time. This is why you can have these crashes even while showing a high percentage remaining.

Mine has also been shutting down since April. Start an outdoor walk/run/ride...shuts down within a few seconds. But it doesn’t shut down if I’m on the indoor bike. So I’ve tried everything. Update, unpair/new set up, bring phone with me/leave phone at home, etc. I’m now convinced it’s WiFi related. If I turn off WiFi before I leave my house and start an outdoor activity, I’ve had no issues with it shutting down.

My friend who is able to complete open swim workout present and past sent me her screen shot settings. I have mirrored them and will test today. It’s counter intuitive but “turn off power saving mode” in workout app. BCBach correct that the low battery warning is false and triggers watch off. Friend does NOT take phone with her on open water swim nor turns off WiFi. I’m cautiously optimistic.

This is what I believe is happening per Apple Support: WiFi on the watch is capturing WiFi router signals and “crowd-sourced WiFi hotspots and cell towers location to determine approx location” the 2 watch updates allow contact tracing apps.

The work around for those experiencing shut downs is actually turning off WiFi on the watch face; not the access panel but the settings icon on the Apple Watch: toggle WiFi OFF. This has allowed me to record 1 hour + open water swim in the workout app.

It seems several Apple Watch Series experiencing this shutdown.

This is indeed a battery issue. I caved and got a Series 6 on the 12th. Before I traded my Series 2, I asked them to run diagnostics on it. They did, and said the battery did not pass. I do believe it is an issue of not being able to support "peak performance", as its defined on the iPhone and iPad.

What burns me though is the Apple Store genius pulled the first diagnostic report from February, when this first started happening my watch failed to record data for 3 days and the battery had actually failed diagnostic checks then too, but the AC tech did not tell me that on the phone. Had I known that it was failing, I would have upgraded then and saved myself ~9 months of frustration. 😒