How to disable low phone battery notification?

That's not really a problem of the watch, but of your phone. You have to disable the notifications there, on the phone. I do not know what version of Android and what brand of phone you own, or what app generates the notifications, but in Android you can slide any notification slightly to the side, to have the access to the properties, and there you can disable those notifications. And in the settings of Garmin Connect, you can also selectively enable or disable notifications from individual applications, or from some system components too, so that might be another way to selectively disable these notifications.

I have the same issue, and sorry, no, it's a problem of the watch, not the phone. 

I get no notification on my phone (Pixel 4a5g, Android 12) that my battery is low, other than my battery icon is red (not even in silent notifications).  Additionally, in Garmin Connect all of my phone notifications are turned off, including both "During an activity" and "Not during an activity," so I can't possibly see how this is coming from my phone.

I'm sure the Instinct is reading the battery status and making its own decision about notifying me about it, and it would be great if I could disable that.

Try this: How to Turn Off the Low Battery Warning Sound & Notification in Android 9.0 Pie « Android :: Gadget Hacks

9537812 said:

Additionally, in Garmin Connect all of my phone notifications are turned off, including both "During an activity" and "Not during an activity," so I can't possibly see how this is coming from my phone.

Are you sure the notifications are about the phone battery, and not about low watch battery?

Yes I agree. I got a baby and was too busy to reply. When this problem happens I have the following options:

switch off my Bluetooth on the phone

Switch on stealth mode on the watch

Disconnect phone from the watch (I have added it to the menu that comes up on a long press of the light button)

Create a custom power mode and switch off phone connection. Then assign it as the default power mode for your activity 

Go to Garmin Connect App - Settings - Notifications - Push Notifications - Scroll to the bottom of the page to Low Battery - toggle off for "Sensors".

Thanks, fixed it for me!