"Service battery"

Well generally i would say this:

If you see here: https://support.apple.com/en-us/HT201585 - generally speaking the MacBooks have a cycle count "limit" of 1000. At that point they are considered consumed (automatically even if still have enough capacity from what I understand). So I would dare to say that article may lead to the interpretation that that is not expected, but It might also just mean that 1000 is the upper limit of a cycle count.

BUT: If you dont have Problems with how long your Battery lasts I would leave it how it is now. Just to be on the Safe Side I would make a Backup or store the important data somewhere.

If you noticed that the battery does not last as long anymore, then you would have to go ahead and change the battery. But at this point with a 2015 Model Im assuming you don't have Warranty or AppleCare anymore, so you would have to pay, regardless of wether it is expected or not.

To decide what exactly you should do chack out https://support.apple.com/en-us/HT203747

Im guessing you ran a diagnostic, right? (Hold "D" during startup)

Depending on what the code exactly says I would either suggest a replacement of the battery, or just continuing to use.

• "The battery will need to be replaced soon. It is functioning normally, but holds less charge than it did when it was new. " -> If you dont notice too bad of a battery keep using it
• "The battery is not installed properly. Shut down and discontinue use. The computer requires service." -> Stop using it and go to the Apple Store or an Service provider before that thing does not work at all

What Im saying is use your brain on the error message. If it sounds scary, stop using it and get someone to look at it, otherwise use it until you get a new battery or a new MacBook ^^

Sam