help! about DSO nano v2

I have a DSO nano v2. There is a problem with its power recently.

When I connect it with usb cable to computer, it works well.

But when I disconnect usb, its power is gone. Somtimes it power off immediately.
Sometimes it is still on, but with very weak backgroud light.
It still show USB on the top left corner of the screen when i disconnect the usb cable.

what’s wrong with it?

It could be the battery itself, which would be easiest thing to fix. Or the battery charger controller LTC4054. Or the battery charger switch FDFMA2P853. The fact that you see the USB indicator when USB is disconnected makes me think it is one of the latter two that is broken. If you look at the schematics that would be the only way power could leak back to the USB indicator input.

How to fix it?