You can maximize battery life by turning on Low Power Mode or preventing apps from refreshing in the background.
Low Power Mode turns off certain features, such as Always On Display, background heart rate and blood oxygen measurements, and heart rate notifications. Other notifications may be delayed, emergency alerts may not arrive, and certain cellular and Wi-Fi connections are limited. Cellular is turned off until you require it—when you stream music or send a message, for example.
For more information about the effects of Low Power Mode, see the Apple Support article Check battery health and usage on Apple WatchMaximizing Battery Life and Lifespan