The Alien 36V system (with the battery in the pannier rack) seems to suffer from battery problems. I've had the identical situation to you, with the voltage reading ok until plugged in, when it drops and won't function. This was in fact a battery issue (although I don't know exactly what).
The other thing to watch out for is the key switch mechanism failing. This has happened to me twice now (am on my third battery in a year!) and manifests itself as power cutting out going over road bumps, which can then be temporarily restored by turning the key off and on again . . . until the next bump. I have now taken the key switch completely out of the circuit.
Anyway, best to get Alien to sort it, as Rog says. They are pretty responsive by email, but the telephone number frustratingly seems to be a closely guarded secret. I had no problem getting them to send replacement batteries.
Best of luck getting it sorted.
Greg
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