Battery disconnecting every few seconds even at 100% charge

Sounds like a hardware issue of some sort. It could be the power adapter, charging cable, or the laptop itself. Here are two Apple articles for troubleshooting charing issues:

If your MagSafe cable or power adapter isn't working - Apple Support

If your USB-C power adapter isn't charging your Mac laptop - Apple Support

If those articles don't help, then have Apple or an Apple Authorized Service Provider to examine the laptop (including the charger & charging cable) to provide you with a repair estimate.

FYI, it is usually best to let us know the exact model of the Mac since there are lots of models....some with their own unique issues. The 2016-2017 USB-C Apple laptops tend to have their USB-C ports wear down with use so the cables no longer fit snugly which can cause this problem. I also find many times that the USB-C ports tend to have liquid damaged.

Make sure the if you are using a USB-C charging cable that you try using another USB-C port on the laptop especially on the other side if possible. If you have an Apple Silicon Mac and are using the Magsafe connector, then try using the USB-C port instead.

Nestor Holynskyj wrote:

I didn't realize that battery replacement was a 24-48 hour affair! I have to dig out an older laptop and equip it to at least last me a couple days-- my laptop is my work life!

Just like you need to have a backup of your data, you should also have a backup computer to use in case something happens to your main computer especially if it is critical you have a working computer to do your job. Laptops can break, can be lost, and can be stolen. Plus, there are a lot of new ways for these newer Macs to fail which may require hours or days to fix even if it is just performing a clean install of macOS & restore from backup. Also, what happens if a part is back ordered and the repair is delayed for weeks or months which has occurred at times even with Apple although Apple is usually pretty good about having a good parts stock.

FYI, I have seen cases where some USB-C Apple laptops may have one bad port, but the other ports are affected as well due to how the power/charging circuitry is designed on them. It also would not surprise me that the Logic Board could be at fault. I see power related issues all the time with these USB-C Apple laptops where the charger must be connected to power on the laptop even though the battery has sufficient charge....to me this indicates the Logic Board believes the battery has been disconnected since it has occurred with brand new & verified working batteries, but that is only a guess on my part. These Apple USB-C laptops are a nightmare for troubleshooting....I should know since I have to support thousands of my organization's Macs....much worse than any other model over the 20+ years I've been supporting Macs.

Good luck.