I have old mp3 player iRever H340 40G. it's battery is dead and it is not available in the local market. i found that it can replace with iphone battery, the thinness and the voltage but it's smaller then original iRever, So it can put in side iRever easily.
After install it work perfectly. the fist time of full charge , i let it play mp3 to 6 hours, and Video to 2:30 hours.