Dell Precision 7730 laptop not running under battery power. Battery in excellent health and showing in power manager

My Dell Precision 7730 will only work on mains power. It would not boot without the mains power adapter in. With mains power and once booted, if the power cord is removed, the laptop dies. This was sudden, the laptop was working fine yesterday. BIOS and the battery health report say the battery is in "Excellent Health". I use the Dell power manager app to stop the battery charging more than 75% (to prolong the battery life) and there is 74% power in the battery.

I tried rebooting by holding the power button down for 15-20 seconds - no change
I removed the battery, rebooted on mains power only by holding down the power button for 15-20 seconds, and also normal booted, with the battery removed, by just pushing the power button - no change
I have run Dell SupportAssist and updated the bios (to 1.18.2) and four drivers including Dell Precision Optimizer and Power Manager Service - no change.
I don't think the battery is the issue as it appears to be in excellent health and the issue was sudden, ie the battery was not getting worse over time.

I ran Dell SupportAssist as admin. It runs all the options (eg get drivers, true performance etc), Except Scan hardware, does not complete. The Scan Hardware test starts and the elapsed time starts, it says its "Preparing" but the percentage% doesn't move. After the elapsed time reaches 2 minutes, it then stops gives me a small red warning triangle and says that the last time Scan Hardware was run was in March 2021.
It is Dell SupportAssist 3.7, it says there is a new version [3.7.0.148], which it 100% downloads, but for some reason the installation fails. It asks me to reboot to try again. I have tried rebooting and trying to update SupportAssist twice, as admin, but the installation always fails. But as the BIOS and drivers are upto date i don't think this is an issue.

I don't know what else to try.