My macbook pro turns off when unplugged from charger ?

Hello everyone. I have a Mid 2010 macbook pro. It was working fine until last September when the battery kept dying. So I bought a new battery (apple) and installed it myself. A week or 2 goes by and it constantly shuts down as soon as I disconnect the charger, or even when it's disconnected and has a charge of 87%. I have done an SMC which has not helped at all. By any chance do you guys what may be the problem ?