discipline + eldritch battery

I dont get. Why i should run discipline when i have lower mana with it ?
Discipline increases energy shield but reduces mana ...

It increases your mana regeneration (1.75% of maximum mana), which is not unimportant for MoM builds. However, you should only run it if you can keep ~1/3rd of your maximum life as unreserved mana.

You need to have discipline at a high level linked to a high level reduced mana support gem, also taking reduced reserved mana nodes and increased aura effects nodes will make it worthwhile since it will also let you run multiple auras while keeping a big enough mana pool for your casting needs.