I have an old mp3 player which originally ran from a 3.7V LiPo battery. It could be charged from USB using a mini-usb connector. The device goes into charging mode when power is present at the usb charging port, but will not operate in playback mode without voltage at the battery terminals. The battery is long dead and I removed it and soldered a connector to the battery input of the circuit. Using an adjustable regulated supply I can run the device by supplying 4.25V at where the battery used to be (the battery indicator shows "full" at 4.2V and "low" at 3.6V) or from 3 AA batteries, however I would need a smaller wall-plugged power source for practical (stationary) use. I tested the device using a smartphone charger and ran it briefly (2 minutes) with no apparent problems. The measuered voltage was 4.85V. I would need some advice on the following:

* What (if any) problems could arise from running this device at 4.8-5V? The device expects around 4.2-4.3V (voltage of a fully charged LiPo), so this would be 0.6-0.8V higher. Short test operation (about 2 min) went fine, could there be problems if playing back music for hours?
* Is there any way to diagnose such problems? Perhaps by measuring temperature on the surface of the IC?
* What are the possible consequences? Besides device failure, could this also cause a fire hazard?

Here is an image of the player's internals: 

[![mp3 player circuit](https://i.stack.imgur.com/a9vCj.jpg)](https://i.stack.imgur.com/a9vCj.jpg)

The square IC is marked M7103A-A1, however I could not find a datasheet for it. The rectangular IC's markings are unreadable. The cable in the lower right corner is where the battery used to be. The charging port is on the lower left.