Battery charging issue.
 

I am using the stock charger for the UD818A drone but wanted to do some modification. I got a 2500mah battery that the drone can carry easily and soldered on the correct JST connector, I will assume I connected it right only because it seems to work in that regard. The battery that I got, like the stock battery, works with a 2p connector only. The problem is that the 2500mah battery, when charging in parallel with the 500mah stock battery, will be cut off by the charger at the same time as the 500mah battery. The battery does work with the drone but works for about the same time using each. Is it possible that the stock charger is stopping the charging at 500mah on purpose? If so, how can I solve that?