What is the minimum battery level required to start Android encryption?
80%

// Minimum battery charge level (in percent) to launch encryption.  If the battery charge is
// lower than this, encryption should not be activated.
private static final int MIN_BATTERY_LEVEL = 80;


Source: https://android.googlesource.com/platform/packages/apps/Settings/+/master/src/com/android/settings/security/CryptKeeperSettings.java#56 (as of android10-release branch)