Defective battery causing phone to die and not boot up?

I replaced the battery in my iPhone about 7 months ago, purchased thru here, and then recently when the battery level would get down to 50-65%, the phone would die. Plugging the phone in only got you the apple logo screen and would not boot up. I've had to restore the phone numerous times (plus re-jailbreak :-P) Have purchased another battery and it seems to have taken care of the problem. Has anyone else had this issue with replacement batteries? Thanks, Dave