Eldritch Battery

Eldritch Battery adds the character's base maximum Energy Shield to base maximum Mana and removes that converted Energy Shield from the defensive pool. The converted amount scales with Mana modifiers, not Energy Shield modifiers. Mana Costs are doubled, so a build must compare the larger Mana pool with the much larger cost of every Mana-costing action. Visage of Ayah grants the same Keystone without allocating its tree node.

Base Energy Shield becomes base Mana

Stat conversion happens before percentage modifiers. Flat Energy Shield from equipment and other sources becomes base Mana, then applicable increased and more Mana modifiers scale the converted amount.

Energy Shield scaling no longer scales the converted part

Once converted, the value is Mana rather than Energy Shield. Increased Energy Shield does not scale that converted amount, while increased Mana does.

Mana Costs are doubled

A Skill with a 40 Mana Cost is treated as having an 80 Mana Cost before other applicable cost calculations. The Keystone does not merely display a larger pool; it also creates a substantial sustain requirement.

Spirit reservation is unaffected

The downside names Mana Costs. Spirit reservation is neither Mana nor a Mana Cost, so it is not doubled by Eldritch Battery.

Cannot be Converted blocks the conversion

A source that explicitly says maximum Energy Shield cannot be Converted remains Energy Shield instead of becoming Mana. Geofri's Sanctuary uses this wording and can therefore preserve its maximum Energy Shield while Eldritch Battery applies elsewhere.