I cannot imagine that sticking a battery pack in a stuffed toy is going to do anything good for the battery.
A li-ion battery's lifespan is heavily dependant on its temperature. Higher temperature=loses capacity faster.
I'd be surprised if that battery lasted more than a year before it become basically useless due to capacity loss.
Samsung Galaxy S3 LTE (i747)- Rooted, stock firmware, safely ensconced in my Lifeproof Fre
HTC Incredible S - S-off (rev hboot) - returned to stock ROM and handed down.
HTC Wildfire S - BL unlocked, unbranded, gathering dust.
Archos 32 - SDE flashed, rooted, gathering dust.