Unfortunately, that did not solve my problem. But it is nice, that I can limit the battery charge to 85 % now Also, powertop now reports both discharge rate of battery and power consumption by system.
Fortunately, that is not necessary!
On a random hunch, I thought - what if my system is actually using more power while lid is closed, hence needing to cool more? So I ran powertop and got a snapshot just as I opened to lid, and sure enough - CPU at 100 %! I did it once more and figured out that systemd-logind and journald is taking up all the resources… Looking in journalctl I saw that systemd-logind tried to suspend when the lid was closed, but did not have permission to do so. OF COURSE systemd is the problem and a nuissance! I totally forgot that all my test OS’ on this laptop used systemd, and I always disable suspend. systemd just does not get suspend disabled, when, you know, you disable it, it requires arcane knowledge about editing specific files in /etc/. So, my problem is fixed! Finally I can close the lid when the machine is running!