It's just you. Your firmware should not have any effect on your 3DS's battery life, my boyfriend bought his 3DS 4 years ago and this battery life is the same today as it was 4 years ago. My 3DS is well over 4 years old as well (might be older since it was bought used,) and the battery life is still the same as when I got it.
Like what has already been said before, it's either your battery is having issues, there's a hardware issue, or your paranoid. Have you tested another battery to determine if the battery is completely without fault. If another battery still shits the bed, then it's most likely hardware related, but I can safely say that neither AuReiNAND nor 10.7 are causing your battery problems.