The brief notifications of low battery on the watch (at 20% and 10% charge) are easily missed. This results in the user being unpleasantly surprised when the battery (suddenly) dies.
There have been other posts about this, proposing a notification on the phone, which would be helpful.
As a quick and easy alternative, please just display a (solid or blinking) RED DOT at the corner of the watch’s display once the charge dips below 20%!