I am new to the Lenovo laptop family. My laptop is a Lenovo ThinkPad T510 and has a 55++ battery in it now, but it does not hold a charge. The previous owner said it probably will need a new battery due to not holding a charge without being plugged in. All the info I've been finding on the battery has been helpful, but the 55++ vs 70++. What is the difference and is it interchangeable?