I have a late 2011 MacBook Pro, I bought it used, looks to be in very good condition. The guy I bought it from gave me a new battery, not OEM for it because it said to replace it now, it was running very sluggish with the old battery. I installed the new battery (off brand) and charged it fully. it is still the same runs very slow, no improvement. However I thought I would try unhooking the battery and boot from the power supply. It runs fine as long as the battery isn't connected. What do you think, is this new battery just junk too? I ran Apple Hardware test and it says 4xxx/6/40000005: P battery ....on the new battery. ??? thanks guys