I go a 7 function multi tester that i want to use to check car battery. I know it must do that, but I have not idea which setting to use.

Your car battery will probably be around 12V. Pick a DC voltage range a bit more than that, eg 20V range. Do not use a current range, or an AC range or resistance (or buzzer) range, or it could kill the meter.

A well charged battery could show 13.8V If you accidentally short the battery, the wire will explode, I’ve done it. You can test it on a fused supply, eg to a light. If you turn on some high current load, like aircon or screen heater, or full beam lamps, without the engine running, the voltage might drop. If it goes below say 11V it might not be well charged.

DC ranges are often marked with a symbol of two lines - a solid horizontal line above a dashed horizontal line, like this

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