The APU is normally started from external power via the transformer-rectifiers since you get a faster and more reliable start (greater starter motor assistance). Without external power it's preferred to use one of the engine generators to start the APU, but failing that a battery start can be performed. However, there's no ability to monitor the start until the APU generator comes online and it takes longer to start.
In a dual engine failure scenario attempting a battery start will use about 35% of remaining capacity, so it's delayed until below 25000 feet when start is guaranteed. In a single engine failure you can cross bleed, but APU bleed always preferred because a cross bleed start on the ground requires high engine power. In the air, if an engine fails in flight the FADEC automatically attempts a relight if windmilling speed high enough.
From iPad so apologies for autocorrect problems.
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