Samsung Galaxy S21+ battery drains too quickly

The checks in more detail

A weak 5G signal makes the radio work hard, so use LTE in poor areas. Put rarely used apps to sleep, and check Settings, Battery, Background usage limits. After the final software update, wiping the cache partition clears leftovers. Adaptive battery and the Auto optimize daily setting restrict background activity. If the phone loses ten percent overnight with Wi-Fi on, a process is running; test in Safe mode. A battery that falls from forty percent to shutdown has worn out and Samsung can replace it.

Heat and fast charging at night speed wear. The S21+ owner's best habit is to charge to about eighty percent when possible, since lithium cells age faster at full charge and heat; Samsung's Protect battery option does this automatically, and combined with turning off Always On Display, it adds weeks to the battery's life. If the phone drains while idle on Wi-Fi, test in Safe mode for a day. Check also the Galaxy Store and Play Store for automatic app updates that run in the background, since a day of many updates drains the battery;

set updates to Wi-Fi only and to run at night while charging. Samsung's Adaptive power saving mode adjusts performance to your habits and is worth a try on an older battery. The S21+ has a feature called Auto optimize daily that restarts the phone at set times, and some owners found battery life steadier after turning on Auto restart at night. Check Settings, Battery and device care, Automation for Auto restart, and consider enabling it so that the phone refreshes itself while you sleep.