My mechanic has told me 12 volts are being drained from my battery even while my car is shut off, also whenever I go to brake for a light and shift my car to nuetral my car starts to tremble and turns off, not every time but sometime, my car headlights also flicker whenn I tap the brake pedal, and I got my alternator tested and it was fine but the woman told me my battery isn't producing enough power, idk if I just need a new battery or what??? Please help with any advice