Soul Balm generates 5 EP on use. You do not lose any excess EP that would go over your maximum EP; instead, you will temporarily have more EP than your maximum. EP will resume regenerating as normal once your EP drops below your maximum.
Previously, in addition to restoring 5 EP, during Unite and Fight events, Soul Balm could be used to grant a crew-wide 1% ATK or HP buff that stacks up to 30%. However, as of the November 2017 Unite and Fight event, this was replaced by Officer Skills.
Soul Balms are rather rare, only regularly obtainable with Coop MP, which takes a very long time to reset. Events may have 1-2 of them as rewards, but overall it is very hard to accumulate many Soul Balms.