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According to Wikipedia, when Salyut 7's solar-array-pointing system failed in 1985, its batteries were rendered unable to charge, and the station almost immediately went completely dead:

The first order of business was to restore electric power. Two of the eight batteries were destroyed, the rest fully discharged. Dzhanibekov determined that a sensor in the solar array pointing system had failed, preventing the batteries from recharging. A telemetry radio problem prevented the TsUP (mission control center) from detecting the problem. Salyut 7 had quickly run down its batteries, shutting down all its systems and accounting for the break in radio contact. The cosmonauts set about recharging the batteries and used Soyuz T-13 to turn the station in order to point its solar arrays to the sun.

But solar arrays produce power whenever they're illuminated, meaning that, even with inoperable solar-array pointers, Salyut 7 should still have had power at least intermittently, whenever it was outside the Earth's shadow and happened to have one or more solar arrays in sunlight. This should have happened fairly frequently, as Salyut 7 was not frozen (no pun intended) into a single, unchanging orientation; instead, to quote Wikipedia again,

[...] As the crew approached the inert station, they saw that its solar arrays were pointing randomly as it rolled slowly about its long axis. [...]

Thus, the station's rotation should have periodically brought at least some of its solar arrays into sunlight; even if the illuminated arrays were suboptimally oriented for generating electricity from the light absorbed thereby, some power should still have been available, at least momentarily, which should have caused Salyut 7 to intermittently start to wake and send (fragments of) telemetry from time to time.

Instead, Salyut 7 went completely and uninterruptedly dead until Dzhanibekov and Savinykh arrived to repair it.

Why?

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Your assumption would be true for a simple, dumb system that is directly powered from its solar panels, but in a complex system like a space station there are several complications.

First, we don't have evidence that the solar panels actually pointed towards Sun for any extended amount of time. For all we know, the station rotated along its long axis, so it might have pointed its solar panels towards Sun for some amount of time during each rotation or might not - e.g. if the axis of rotation pointed directly towards Sun. In the latter case there would be almost no electric energy produced at all.

Second, let's assume the solar panels produce a decent amount of power every now and then. We know that ground control wasn't aware of the problem, so no countermeasures were taken. The batteries were still connected to the electric system and likely now energy saving procedures were done. Empty batteries will have a low voltage at their terminals and draw a lot of current when connected to a charger. It is very likely that this power draw is large compared to the power delivered by the non-ideally oriented solar panels. This will cause the voltage of the power bus connecting all subsystems to stay at a low value, likely below the threshold for switching them on. Even if they switched on, the increased power draw would immediately reduce the voltage on the bus and switch them off again.

If ground control had known about the issue, they could have switched off less important systems and maybe have kept the main controls and transmitters online to investigate or maybe solve the situation. But this last point is pure speculation.

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  • $\begingroup$ If 2 batteries were dead (possibly because of over discharge) the other six batteries were likely discharged so much that they needed to be "reanimated" instead of recharged which the unaligned solar panels for sure were not able to achieve. Taken self-discharge in account in regard of already empty batteries means that they maybe also have taken permanent damage. $\endgroup$ – GittingGud May 21 at 6:32

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