[asshanson]

Nice call on going to Lithium, I really hope that's big enough for the car. All specs point to it having enough power, I had a 4.5lb Shorai that was about 8x8x3. Price and specs sound about right, just over double the volume and weight, and price was around $200. Definitely take it inside for the winter though, and if your car draws any power while off, keep it on a tender (keep in mind some Lithium batteries require a special charger, not the normal chargers) if you don't start the car more than once every few weeks. Once they die, no guarantee they're coming back.

I had a cutoff switch on the battery itself so I didn't have to worry if I planned to take a 3 week trip or had the car down for extended maintenance or something, just turn the switch and didn't have to unhook the terminals or remove the battery from the car. Highly recommend.

EDIT - couple more thoughts: Those Lithiums can crank out some serious power for their size, but not for an extended period of time like the lead acid or AGM. Just less overall capacity, even though max output is the same. That's why you have to be extra careful about leaving it sit for extended periods of time. A higher capacity battery can sit for a month with no issues, and CCA has nothing to do with that. Amps just means power delivery output, Watts is the total energy storage capacity which unfortunately isn't a highly advertised number.

Say your battery at 2.5 lb has 290 CCA which is amazing for being so light. I haven't done research on numbers but let's say it has .3 kWh total energy, it can expel that energy very quickly to start a motor, and being fully charged from driving, after shutdown can maintain the car's parasitic draw for 2 weeks.
And a 5 lb lithium can maybe do 330 CCA, not much more max output, but double the weight. However, with total capacity of .6 kWh (double the mass and stored energy capacity), it could last a month with parasitic draw.
Of course a Lithium will have many times more energy capacity per weight vs AGM or Lead Acid, but going so small will really reduce the number of days you can let the car sit if you don't have a battery cutoff switch for the car, solely because of the constant draw that cars have.