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This wikipedia page contains a sentence:

A fifth 906V battery with 155 Ampere-hour capacity was added to support the increased energy consumption from the improved electronics.

906 V seems very high, 90.6 V is believable.

Is there any other information about Soyuz DC system voltage?

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    $\begingroup$ 906V would have its advantages - you could save some launch weight in high current wiring :) $\endgroup$ – rackandboneman Jul 16 '18 at 18:41
  • $\begingroup$ @rackandboneman: You would need less weight for the wiring conductors, but more weight for wiring isolation. But the reliability of so many battery cells in series to get 906 V would be too small. $\endgroup$ – Uwe Jul 16 '18 at 20:26
  • $\begingroup$ Very high voltage batteries of small capacity and low max current, built as stacks of thin metal foil and electrolyte soaked paper (voltaic cells), are reliable and lightweight enough, a good source of high voltage where such is needed. But 155Ah is a very respectable capacity and way more than I'd see in form of a voltaic pile. $\endgroup$ – SF. Jul 16 '18 at 21:50
  • $\begingroup$ Insulation is light, and most any plastic insulation can withstand 1000V if transient free and kept safe from damage. $\endgroup$ – rackandboneman Jul 17 '18 at 7:49
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"Soyuz: A Universal Spacecraft" says the electrical system produces 23-34 volts. I saw a reference to a nominal 28VDC supply to the spacecraft in the Soyuz launcher manual; 28VDC is a global standard for aircraft power supplies.

I think "906V" is a part number, not a voltage; the source for that section of the WP page says (my bolding):

Finally, the spacecraft was upgraded with an extra (fifth) power-storage battery, known as 906V, with a capacity of 155 ampere-hour.

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    $\begingroup$ Confirmed by the Soyuz Crew Operations manual, several batteries are listed as 909A, 906B, etc. $\endgroup$ – Organic Marble Jul 16 '18 at 16:22
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    $\begingroup$ The Soyuz Crew Operations manual also contains the system voltage of 23 to 34 volts. $\endgroup$ – Uwe Jul 16 '18 at 19:26
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    $\begingroup$ ...and Cyrillic 'B' is latin 'V'. $\endgroup$ – SF. Jul 16 '18 at 22:34

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