Buy another battery in the car, and you can output 220V AC inverter in the car when you travel. I want to know how to use the car's own circuit to charge the battery.

The battery of the vehicle itself is the starting battery, which can adapt to the short-term high current output, and the terminal voltage will drop below 10V at the moment of starting.

The batteries of UPS and inverter shall be power batteries. Their internal structures are somewhat different.

So strictly speaking, even if the voltage is the same, don't connect it in parallel with the original car battery. Another reason is that the inverter will work abnormally when the car starts.

Solution: The negative pole of inverter battery is directly connected with the negative pole of automobile battery, and a diode is connected in series between the positive pole of automobile battery and the positive pole of inverter. This can not only charge, but also prevent the interference to the inverter when the car is started before. The diode can be 10-20A/50V.