Laptop Inverter Draining Car

I do not understand why laptops have such small batteries can run a very long time.
Yet once you plug in your laptop to your car through your inverter the car's battery instantly dies if its not running off the alternator.
It doesnt happen in 18 wheelers with 4 batteries but I'm sure it does drain it faster than even all the lights!
It feels like 12v power is just being thrown away, not efficiently being converted to 120v.

I've always believed that electronics hooked up but not on, dont have a power drain.
(like power supplies plugged into 120v but nothing connected)
This leads me to believe otherwise.
Thanks

You could put an ammeter on the power lead to the inverter and see what the difference is with nothing plugged in and with the power supply.

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My ex once told me I have a perfect face for radio.

An inverter has a switching power supply. Even the small ones have at least an amp of current draw with no load.

So what would that be? 1amp an hour draw? Batteries have like 800amps?

One amp is a lot of draw for a vehicle that's just sitting. While your battery might be able to provide 800 amps at best, the amp-hour rating is much lower (maybe around 70). This will deplete a battery not in a matter of weeks, but days.

I don't even think most modern cars draw at tenth of an amp once the computers go to sleep (maybe 20-30 minutes after you shut off the engine).

The only way to know for sure how much current your power supply is pulling is to use an ammeter.

One other option would be to use a relay to supply power only when the ignition is on. Your laptop battery should hold up while it's not being used. Then when you get in and start up the car, it'll recharge the laptop.

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My ex once told me I have a perfect face for radio.