I just bought a used MacBook. It's 40 months old. The battery has 36cycles. Max capacity was 4600 and it's down to around 3700. Now here's the issue when I unplug it as soon as the charge gets below 90% my capacity drops to only 1260 and I discharge quite fast? But as soon as I plug it back in and it gets above 90% my capacity goes back to normal? Thanks for any help.