If you use the "cigarette lighter" it's should be limited to 120W, which is 10A @12V. Like someone else mentioned there is loss in any inverter, so it's more like 100-110W AC to your charger.
I've got a 4runner with a 300W inverter built in. It's 300W max under all circumstances. What most people don't realize is that there is start up current and that is higher. If you pull 10A normally, it could be 11-12A on start up. Also there is parasitic loss in the cable, connectors, systems, etc... Just make sure you leave some margin at least 10-15%.
I just clip it to the battery in the daily Jeep and it can charge up to a 6S.
Unless you have very low current you have two options.
1. What he said. A lot of chargers have a 12V input. Just run that straight to the car battery. This is the simplest and cheapest option that I can think of.
2. A solar generator (Ecoflow, Jackery, Bluetti, Ankar, etc). The run about $1/watt hr and you can use it camping or around the house too. But, again make sure the wattage (peak and sustained) will work for your charger.
I have an EcoFlow that took a dump and need to see if I can send it in for warranty, so I can't recommend it at this time. Never used it myself, but I do have buddy that is in love with Bluetti.
It's very cool technology and more useful than one would think.