I too have a battery issue on my pi-top [4] DIY. It will only run for a few minutes on battery power. So I have to have it on mains power all the time. The batteries must be generating a lot more heat than they used to as the fan is running at full speed most of the time although the CPU temp is generally between 38-44C.
I messaged tech support last week about getting spare battery packs but no response as yet.
Hummm says charging, but in reality it’s not? I have not done it yet myself. Looks like there’s a temp sensor cutoff and over voltage protection. Nothing else. If it were me, I’d check for 7.4v end to end. Then check the 7.4v on the red and black connectors. No clue if each black is negative and can be used independently or together. Same for red. They are likely pairs connected to the same lead.
The board should have an indication which is positive. Say bottom solder says positive, then the battery with positive on that slot, which means the other side will be negative and connect on the top.
Don’t know wha the white and yellow will do and how they are used, if used, for charging. Did you insulate the batteries? Those disks that were at the end of the batteries?
Thanks for suggestions. I will check the voltage and double check that I soldered it correctly.
Any News you can share?
Unfortunately had no time to work on it.
But I asked my friend for help to test it and check connections - he is electronics engineer. I will share info when I get it back.
Update: after re-soldering of new batteries and checking connections again the result is still the same. Battery is not charging although “charging state: 1”.
Not sure what was the reason… but maybe I fried the voltage protector when was soldering it for the first time or it just died.
For me it’s end of the story 
Already bought “bigger” UPS because at my place power interruptions happens quite often.
Thank you for your support.
BR
How did u get inside the pi-top to access the battery, im trying to open it myself but i dont wanna break it