Isn't it Nice that they picked a battery chemistry which could bear 'deep discharge' (maybe unto 0.000x V.. ?) for a prolonged period. Presumably too, any built-in "heaters" simultaneously lost their energy source: what else could one conclude re that solar isolation? Bizarre things happen at these low Kelvin temps, when one thinks of those semiconductor junctions trying to function ... as a few mWatta ... trickle in.

So even this minimal self-recovery earns a passel of kudos. No?
Clever Lads. Whether or not it makes it back to full functionality, at all.