T520 battery questions

[steveadorjan]

I have a couple of T520s and X230s which are still in great condition. All running on original batteries and I also have a couple of genuine spare batteries (obviously not 100% of the original capacity, but some are still pretty decent).
1. I keep the spare ones in pouches in an ammo box (with the seal removed), just because I am a bit paranoid about old batteries being a fire hazard. Should I also remove the batteries from the laptops that are not in daily use and put them together with the spare ones in the ammo box? And how would that impact the CMOS batteries on the motherboard (i.e. would it deplete the CMOS faster, causing potential booting issues)? If so, is it better to keep the batteries installed in their slot in the laptops during longer periods when the laptop is shut off and stored in a drawer?
2. Could these old Lenovo Li-ion batteries (some are 10+ years old) spontaneously combust and cause a fire? To repeat, they are all 100% guaranteed original batteries, not aftermarket third party ones