I believe that's the power supply. I'm sorry, I don't know why the light is dim. If it's not working you can buy a new one.
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Keep in mind the batteries typically only last 3-5yrs. Moreso in the 3yr end. I’ve had to replace both my battery backup’s batteries already.
You can google it and see YouTube videos on how to replace.
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It's a battery thing, commercial UPS are the same. They don't last forever. Different technolgies are better than others, but over time, all batteries wear out.
It's possible to open it & replace the battery yourself. The batteries inside are just standard things they buy from someone else.