The icon that you are referring to is not a red battery.
You are instead referring to the disconnected icon, which is a red phone symbol with a line through it:
The disconnected icon is shown at the top of your watch face whenever your Apple Watch has lost its Bluetooth connected with your iPhone and also has not connected to Wi-Fi as a fallback.
Bluetooth disconnections can be normal, happening whenever the paired iPhone becomes unavailable (for example, if it is turned off or too far away).
Your watch and iPhone should reconnect automatically a short while after being brought close together again.
If they do not reconnect automatically, keep your iPhone and Apple Watch close by each other and follow these steps until they do reconnect:
- Check that both Bluetooth and Wi-Fi are enabled on your iPhone:It may help to toggle Bluetooth off and on again on your iPhone (Settings> Bluetooth).
[*]Check that Airplane Mode is not enabled on your watch:
When viewing the time on your watch, swipe up on the watch face> swipe left / right to the Settings Glance.If the Settings Glance already states Connected and the symbol at the upper-right corner is a green phone symbol, then your watch is already connected to your iPhone via Bluetooth.
[*]If your watch is not connected and Airplane Mode is turned on, tap the button to turn it off.[*]It may otherwise help to toggle Airplane Mode on and off again.[*]Restart both devices, turning both off together, then restarting your iPhone first.
For the best performance from your watch, keep both Bluetooth and Wi-Fi permanently enabled on your iPhone.
About Bluetooth and Wi-Fi on Apple Watch - Apple Support