Issue: battery not detected; computer continually booting into diagnostic mode. 

Solution: Fully plug in internal battery cable. Long story short: internal battery cable was not fully plugged in. It was plugged in enough to power the computer but not enough to allow the battery to be detected. 

I hope this is an appropriate use of the forum. I don't have a question but a (preemptive?) solution. I had a recent issue where I worked inside the laptop and of course pulled the battery ribbon first. I finished the work, connected the battery ribbon, replaced the cover. When I powered on (without adapter, battery only) the laptop booted into diagnostic boot. This kept happening. I allowed diagnostic to complete each time and lost overall about 2 hours with this problem. The diagnostics returned a result of no problems each time. Every time I pulled the AC adapter cable however, the computer abruptly powered off. Oddly it would start and run on battery power. At one point I booted into BIOS setup and as I was scrolling through the menu items I came to the battery section which showed a "battery not installed" message. I found that very odd since I had already used the laptop without the power cable! Aslo, Windows 10 did not detect a battery when I booted into the OS. I opened the cover and checked the battery, even unscrewed the mounting screws, took it entirely out, screwed it back in, and reconnected the ribbon. I still had the same problem. At one point I read something on another forum which involved powering off, removing power cable, removing the battery, booting with only the power cable and no battery installed, then powering off, installing the battery, then powering back on. The solution did not help. I tried it again, but noticed when plugging in the ribbon this time that I could still see some metal showing between the pins and the battery cable! It turns out that where the notch to guide in the battery ribbon was placed made it look as if the cable was fully inserted. All that and I did not have the ribbon fully connected! When I depressed the ribbon in fully, all worked fine again.

It sounds dumb but what confused me to no end was that the power was actually getting to the computer. In other words it ran off battery only power! Yet it showed no battery installed, even when I was running it directly off battery power. This of course kept triggering the diagnostic boot  mode.  I was afraid there was something more deeply wrong with the computer (motherboard, etc.) and was actually ready to take it to be repaired $$$! 

I have never had this happen but have only had laptops with external batteries that just click into place. My guess is that the ribbon was plugged in enough to power the system but not enough to be detected by the system. 

Of course I searched all the forums and did not find an answer so I wanted to post this in case anyone else had the issue.