How many days the battery last without driving the car?
 

Assuming it's all stock and no mods performed, do you think we can leave the car for 3 weeks without driving it and still have enough juice in the battery to start the car?
How many days maximum from your personal experience that you have left the car the longest before starting it fine? Or any bad experience at all?

Reason I'm asking is because I am planning to go away for around 18 days this year and I just want to make sure that the car will start being undriven for 18 days... Thanks

I'd say a week at most but it varies from battery to battery, weather conditions etc. I'll go out on a limb and say that when you return from your trip you won't be able to start your car

I've gone several months in other cars. I would think you'll be fine. I've certainly been gone more than 18 days without my car being started. Worst case scenario, jump or charge it and drive around for a while.

This or unplug the battery it should last more than 18 days.

No issues with two weeks. Started up fine. Time of year might impact if really cold.

If I unplug it, that means I'll have to reset everything, time, sat nav settings, etc..
I'm from Aus, and it's not winter here. So I'm hoping it'll last at least 18 days...

I've done 2 weeks before.

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Mine was left in Eastern Canada for months without being started and it fired up in -20C.

Folks... If you're worried of such things I suggest buying a battery tendor. They're cheap and you'll use it on other vehicles, bikes etc down the road.

You can put the tender on a few hours before you want to start up.

Power is consumed by the immobilized when the car is sitting.

Buy a trickle charger from Amazon like @OICU812 mentioned. Only uses electricity when needed, so it's not like it's consistently draining power from the outlet.

There are some funny ideas in this thread. And by funny, I mean strange.

If the battery is in good condition, a week, 18 days, a month, even 2-3 months is not even worth worrying about. Although at the 1+ month mark, disconnecting the battery or buying a charger is probably something you should consider, just to save potential hassles.

FWIW, I used to put my MX5 away for the winter. After the first year, I never went through the hassles of disconnecting the battery again. It was sitting in storage for 3-4 months every year and started right up, year after year, every time, without fail. Likewise, my parents are snow birds and spend the winter months down south. They leave one car up North and I'll usually make the point of driving it during a few day stretch of good weather, at least once or twice during the winter. But there have been times when I didn't. Last week, I just started my father's 9 year old 5 series BMW (original battery) after it had been sitting in the garage since the end of October. Fired right up, first turn of the key. (I'm more concerned about the gas sitting in the tank for that length of time than I am about the battery). Although at 9 years old, it's probably time to replace the battery anyway. He probably dodged a bullet on that one.

And I put my own FRS away for the winter. Although from late November, I have driven it twice, with the longest shut-down period being about 2 & 1/2 months. No problems starting it up.

And actually, come to think of it, I once owned a 1991 VW GTI. I once had that car put away in storage for almost 9 months. And yes, you guessed it, no dead battery. Although, had I known at the time it was going to be away THAT long, I would have disconnected it and I'm probably lucky in that case. But back in those days, there wasn't much electronic taking a hit on the battery when the car was shut off.

All that said, for 18 days, you will NOT have a problem.

Funny and strange indeed.
I have no idea what you mean by that, but I disagree anyway.

My dad's crappy 10 year old Hyundai, with the original battery, sits in the barn for 6 months every winter, and starts with no problems every spring.
If a car won't start after being parked for a few months I would consider it a problem.
If the battery is very old I'd change it first, otherwise I'd check the car for electrical problems.
Who worries about letting their car stand still for a couple of weeks?

Just got back from a 31 day vacation and had ZERO problems starting my FR-S or my gf's kia.