Why is my soundbox turning off after 20 minutes?

Why is my soundbox turning off after 20 minutes?

I have had my soundbox for a year now with no problems. Recently it started turning off after 20 minutes. I thought it was because I had not charged it but I have charged it fully (the light is green) and I am still having the same problem. I have also tried using it while it is plugged in and it still turns off after 20 minutes.

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