Internal battery not detected, can only power on with charger

Well, at this point, especially if you were planning on replacing it, you're pretty much done. On the other hand, if you wish to continue using this notebook, then I would suggest that you make a Genius Bar appointment at your local Apple Store or Apple Authorized Service Provider to have them take a look and provide you with your options. Alternatively, you can contact Apple Support directly, by using the "Get Support" link in the upper right-hand side of this forum's web page.

I suggest starting with the following Support article: Check the condition of your Mac notebook’s battery - Apple Support

Is it reporting "Normal" or "Service recommended?"

This article may prove helpful, as well: What is the Battery Status menu on Mac? - Apple Support

One excellent third-party app you may want to consider using is coconutBattery. It provides a significant amount of detail on battery health.

It sounds like an intermittent problem. But it also sounds like a significant battery issue. If you battery capacity is at 80% or below, Apple advises it's time to replace it. But even if it's above 80%, you're having either battery issues or charging circuitry problems. Since both are hardware issues, Apples needs to do a hands-on evaluation.