Like DerekS, I have two EarthX batteries in the tail that I connect with an SAE plug located on the aft baggage compartment wall inside the baggage door for easy access. I have a dual pole relay that allows me to use one SAE plug, and either apply power to the main or both batteries for charging via selector button.
I have a 12V/24A ground power unit that I use to keep the system charged, but also for running the avionics in the hangar.
If I'm at somewhere other than my home airport and my main battery is dead, well, that's why I have the aux battery. With the main battery contactor open, I would shut my aux battery contactor and start the airplane. I also carry an SAE pigtail in my "away bag" that can be used with a set of jumper cables to do ground charging if needed.
I have had an experience many years ago where I used 20' jumper cables (ground and then positive to starter contactor post, which was on the firewall), on a dark and stormy day no less, to jump start my '61 Debonair from an airport car. I remember a wave breaking up on to the rocks as the engine fired over. It worked, but wow, dangerous and crazy.
With access to a 12V charger, you can recharge your battery in about an hour - at least sufficient to start the airplane (and if that's not true - you probably need a new one and shouldn't fly with the bad one...), which is my preference in the event of dead battery, vice attempting to jump start the airplane.