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When can you get it: During the main story quest The Womb of the Mountain, or, much later during or after the main story mission The Heart of the Nora.
The next Power Cell can be found in the All-Mother Temple in Mother’s Watch during the quest The Womb of the Mountain, right after you complete The Proving.
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After finding your equipment and speaking to Matriarch Teersa, cross the hall to the locked door before following her out of the stronghold.
Next to the locked door is an open ventilation duct - move through it to enter the room beyond the door and collect a Power Cell.
Please note, after this quest concludes, the gates to the All-Mother Temple will close temporarily. If you come back after The Womb of the Mountain, you may find the doors locked. Don't worry, this is just temporary. Mother's Watch will re-open in the course of the main storyline. Since you won't be able to acquire all five Power Cells before The Mountain That Fell anyway, there is no rush to get back in -- but there is a way to break through the wall boundaries at the back of the temple when mounted if you're feeling adv1enturous and don't want to do it the old-fashioned way.
To get back into the All-Mother Temple, simply return as part of the The Heart of the Nora mission. Note that you cannot fast-travel directly to the temple (for reasons you'll soon see), so pick the nearest-by campfire and travel on foot.
Note: Some users have reported that the door to the temple doesn't reliably open when you fast-travel to Mother's Watch after The Heart of the Nora mission is completed. To make it open, walk away from the town for a bit (across the bridge at least) or fast-travel to another campfire a bit farther away, then walk back into town.
