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Here's a paper about the batteries they used, but I don't see any mention of temperature testing below 0°C:

https://www.jstage.jst.go.jp/article/tjsass/66/6/66_T-22-48/...

JAXA said that the mission design for the lander was one lunar day, which generally doesn't involve dropping much below 0C (https://space.stackexchange.com/a/37374)

So ya, they probably didn't do any significant testing below 0C.

They seem to reference a paper at "19)"[0] which goes into much further detail. It's odd though reading where they reference it, they talk about nothing below 0 C at all.

[0] https://ieeexplore.ieee.org/document/1145732

Is that because that's the limit of the sensor or an actual reading. It's something that happened with dosimeters at Chernobyl where people didn't realize the levels were so bad until someone pointed out that the sensors were maxed out because the levels were higher than the range the units were designed to display. I would hope that's not the reason, but it has happened in the past

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