If both "DC FAIL" lights illuminate on the IRS panel it means the battery is dead.
Wrong assumption, Gents.
And that's why you're confused.
Both DC FAIL lights illuminated would mean Battery dead
and Battery charger dead also, which makes the scenario of your TRE wrong.
Even if you loose your battery, for a thermal runaway for example, the Switched Hot Battery Bus will still be powered by the Battery Charger.
The whole DC circuit will still be powered, so actually you won't loose anything, nor the IRS DC backup, nor fire protection, nor clock.
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