Dell 7212 slow when second battery low

I have this strange issue with my Dell 7212 rugged tablet which has 2 batteries. When one battery is very low (sub 20%) it slows to a complete crawl even though the other battery is full. I normally resolve this by simply removing the low battery then it springs to life again. It's not the battery, I have 6 and the same behavior happens with all of them.

I'm assuming it must be some sort of power management issue but I can't see anything in the bios that would help, or in windows (same in Windows 10 and 11). If anyone has any suggestions I would be grateful to hear.

Thanks.