Where is CMOS battery

It's buried on these... You need to remove the bottom cover and the lower trim to get to it. https://www.parts-people.com/blog/2018/0...

These use the more common 2 wire Dell battery, whereas the 6220 used the infamous E6X00/10 3 wire batteries which are a lot harder to find. I'm not going to provide links but you can find them on eBay or Amazon readily. Dell uses the same battery today in their modern Latitudes so you might even be able to find an OEM one for a newer model and as long as the polarity is right, you're good to use it. It's the common one Dell has used for a long time: C8KT2