Hardware Old 3ds battery replacement

[lbrt]

My og 3ds used to last around 4 hrs of playing pkmn s/m, bravely second etc. on 2 brightness, everything off. Now it's less than 2,5 h. I've got 2 questions - where can i buy og new batteries/ custom batteries (EU), and could it be possible, that switching from a9lh to b9s shortened the battery life?