Managed account
SharePoint Central Administration—Security—General Security—Configure Managed Accounts
Register Managed Account
Service accounts are used by various farm components to operate. The account password can be set to automatically change on a schedule and before any scheduled Active Directory enforced password -change event.

The administrator will not know the automatic updated password.

If you change the password for AAA\SP_Admin by Active Directory Users and Computers, you must manually update the password by Service.msc console. If you don't, the SharePoint 2010 Timer service does not start.

However, from SharePoint Central Administration--Security--General Security--Configure Managed Accounts

When you change the password for SP_Admin from SharePoint Central Administration, SharePoint automatically updates the logon information for associated components. You don't need to manually update the password for SharePoint 2010 Timer service in Services.msc console.