Help:15c battery in Esky Sauphin

Hing C hing,

I just bought a 15c 900mAh battery for my Esky dauphin, However I few different with the original 10c 800mah.
For example
1. original battery. very stable.

2. new battery. tail need to readjust and most likely tune to right when tail face to me and some times it move like a second of the watch suddently.

Is it normal and please teach me as I am the new flyer. or i need to buy one original.