Cold cranking amps are not related to amp hours rating.450 amps for 1 minute = 7.5 amp hours
As a side note, I have always disconnected my house batteries over the winter but to exercise the generator, I simply started the engine, waited 20 seconds and then started the generator using only the engine battery (since the house batteries were physically disconnected). I never had a problem doing that.
Edit: It is interesting and useful info that you can start the generator with the house batteries totally disconnected. This would cover the harder starts during the winter monthly exercising the batteries.
The two SOK 206ah batteries installed in my modified battery compartment on our 2018 2552 start the 4K generator even when down to under 20% SOC.They also run my AC or microwave running through a Victron inverter.
I assume you have separated the generator starting current requirements into the brief "inrush" demand, and the actual cranking current needed for longer duration. The CCA requirement specified by Onan (450 amps or whatever) relates to inrush--which on the Onan 4000 has been measured at about 350 amps but exists for less than 500 milliseconds. Actual cranking current depends on battery voltage, but should run about 70-75 amps for whatever duration until the generator starts.These may be some of the reasons why, in actual experience, folks have no problems with their lithium setup handling the generator demand, even with lower state-of-charge.Mike