I've an UPS but it's not working fine. Disassembled it a bit and tested components that I could without unsoldering, everything seems good. Then the problem is :
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Normally I should be able to connect the batteries and the UPS would just be in "sleep" mode, the screen would be fully light up but the UPS should be off
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The problem here is that firstly when I plug batteries the fan turns on, it should not and only turned on when on batteries but when the UPS is on
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The second problem is that the screen doesn't work as intended. This is a screen that has prewritten zone that just have to be lighted up (don't know the name of this kind of screens, the type like in digital clock), here only the backlight is lighted up (normally all the possible drawing are lighted up too), but quickly it begins to drop in lightness to turn black at the end (the fan continues running)
Check the batteries they are 2 x 12V at 12.6V here
Thanks for your answer
EDIT : on the photo the two blue and brown cable at right are the power plant electricity going to the UPS
If the fans are running at full blast, maybe you accidentally disconnected a temperature sensor?
I don't think there is any temperature sensor because the fan is simply turning on when on battery. Update the post with a picture of the pcb