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)