...I would like to try this but I am concerned about damaging the car, damaging the radio, and damaging the car battery...
You aren't going to damage anything, unless you run the battery down too far.
The connection to the battery can be the same thing as you would use in a mobile installation, or
you can just open the hood and run a pair of FAT clip leads to the terminals. But you probably want
something bolted on to avoid voltage drop at 20A on transmit.
You don't have to leave the car running, but you might want to start it every couple of hours and
recharge the battery to make sure it isn't dead when you're ready to leave.
Some rigs don't do well when the voltage drops. Turning the power down to 50 or 25 watts will
greatly extend the operating time because the voltage won't drop as much on transmit with the
rig drawing less current. (Of course, many of use run 5 to 10 watts and get a lot more operating
Also, I was considering using a sealed lead acid (SLA) battery for HF portable. I was going to use either a (12v 7ah) sla battery or a (12v 12ah) sla battery.
How much power do you plan to run? For how long?
Generally I don't like to draw more current than 1/4 the rated Ah capacity, though I may stretch
that sometimes. While I have a number of 7Ah batteries that I use for temporary testing and
QRP operation, I wouldn't use anything smaller than a 33Ah unit at 100W, and even that is typically
at a low duty cycle on SSB. Trying to draw 20+ amps from a 7Ah battery is NOT a good idea, and
it certainly won't maintain the output voltage for very long. (The internal connections have too
much resistance.) The 7Ah batteries are great for low power rigs that draw a couple amps at