Since this new models don't have a removable battery, they've installed a little micro switch under the bottom hatch so if you were to take the cover off to replace ram or HDD the battery gets disconnected for safety reasons. There's a little rubber pad on the bottom cover that presses a little micro switch and if that rubber pad escaped the switch won't be pressed and battery wont work. Also that little switch might get dirty inside from dust and you may try putting a drop of cleaning alcohol on it and tap it a few times.