Look at the voltage on the 5V line when the battery is charging:
- what DC voltage do you read?
- with a scope, see if you are getting any DC ripple. If you don't have a scope, connect the 5V supply to Bela's analog input via a resistor divider, e.g.:
5V----+
|
R1
|
+---analog in
|
R2
|
_|_ GND
Using the same value for R1 and R2, in the range between 1k and 100k. Then look at the value on the Bela scope and see if there is some noticeable noise on it and what its amplitude is.