BBB Power Down

Hi.

I bought BBB Rev.c 2 month ago.

Two device is problem with power.

I used 5v DC USB power to BBB.

Nothing connected sensors, Nothing run programs, only POWER UP.

But, My BBB powered down a few hours or day,

And, I didn’t use to my BBB.

5v DC USB Power used BBB is Computer USB Port with BBB, smartphone chager with another BBB.