Question: I am thinking of getting a portable air compressor (for filling tires) that clips directly to the battery because it can draw up to 20A. (As I understand it, the usual max in many vehicles for the lighter socket is 15A, so that is why this air pump has the direct connection.)

That would work fine in my conventional car, but would it still work in the future with a Prius or other hybrid? My concerns are two:

1) From some online photos, it seems that there is not easy access to the 12v auxiliary battery in some hybrids; what disassembly (if any) is required to get access?

2) From some online discussions, I get the idea that the 12v battery in hybrids is not as powerful as a standard 12v. auto battery. Is this true, and would drawing 20A be a problem?

Answer: 
That coolant has oxidised over time.

It needs testing to see if it will still protect your cooling system down to the required temperature. If it does not, then it needs replacing. Just about all good mechanics or good garages have that measuring device.
Dyes.............................