I took my 04 TB into the shop because the battery would go dead if not driven for a couple of days. Prior to this, I was experiencing some performance issues (rough idle, lower fuel mileage, shifting at higher RPM in low gears, etc). I think the battery might have had a bad cell (brand new 6 months ago). The performance problems seemed to have disappeared after the battery was replaced. Any ideas as to what may have been causing these performance issues and why changing the battery would have corrected it (resetting of faulty codes, engine working harder to charge the battery at lower idle, etc.)? I'm just curious for future reference and perhaps others are experiencing similar issues and this could be off assistance.