The built-in battery of your MacBook, MacBook Pro or MacBook Air is designed to deliver up to 1000 full charge and discharge cycles before it reaches 80 percent of its original capacity.
Link.
Little differences in capacity are normal and are no indicator for a defect battery. Only contact Apple when your battery performes worse than the design capacity.
The first thing I will look for is dirt in the MagSafe port, there might be something there, but the chances are small.
The second thing I will do is a reset on SMC and PRAM, and any of the hardware stuff. I am not sure if PRAM is related, but hey, we want to rule out any possibility.
You can find how to reset PRAM here, SMC here. (SMC is a more likely problem, do that first)
The third thing I would do is a clean install, but no need to tell you, backup first. Whether a data backup or a Time Machine backup, but data backup will be cleaner.
Here comes the last thing we could do, open it up. Nay, just kidding, send it to Apple, have them figure it out. If it's not out of warranty for long, you could get a free repair, it's about chances.
Best Answer
Most Apple laptops will work safely, but slightly less reliably, with the battery removed. They will behave like a desktop computer that possibly has an inadequate power supply for full load conditions: remove the power, and everything goes poof, which is not ideal, given the ease with which a MagSafe or USB-C connector can disconnect. Repeated 'shocks' of this kind to the circuitry may cause damage to the filesystem. So remember to Shutdown before removing the cable and keep good backups.
Some people say that the Mac may run at a lower clock speed, to reduce power load, though this may depend on model.
However, removal of the battery in this model is not easy. If you're planning on replacing it yourself, then crack on. Otherwise, you might just want to leave it alone until the repair is carried out.