so its been a while sence i got my esp ex-401 with active emg 81/85, and decided to try and switch the battery's, i had a battery i bought when i got the guitar and put it in after struggling to get out the original, this battery was bigger but i managed to both fit it in.. they were both 9v battery's so i assumed it wouldn't matter, i plugged in the guitar and it sounded like !@#$, so i through the old one back in and it went back to its normal sound. it was even harder to get this battery out.

i dont know what i should do :/ maybe a dumb question to many of you but i have no clue
What brand battery is it? Maybe try and get the same brand that came with the guitar?
Agile PS900 Blue Flame: SD Blackouts, TonePros TOM, Grover Locking Mini-Rotomatic Tuners.
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Peavey Bandit 112
Do you have a multimeter? Check the voltage level on one of these. It should be running above 8.8 volts to get the best sound. The lower it gets the more difference your gonna hear.
you have to insulate the battery. You also should use alkaline batteries, not heavy duty.
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alright. and by the way i took the dan electro battery outa my wah and it sounded the same as the stock battery, so is it just the 9v i bought?