I'm almost embarrassed to ask.
But I've owned a few automotive ACC powered accessories, the type you plug into the round-use to be cigarette lighter-socket.
Most notably a tire inflation machine...and a portable auto vacuum.

Embarrassingly...I've just done both situations for years. Figuring I'd be right, at least about 50% of the time. But I've never really found a definitive answer.
My question?
It always says to plug into the ACC outlet and use the product. Now, I can do this, in accessory mode, with the engine NOT running...or I can do it plugged in, with the engine running.
I have done both.

* NO GARAGE, SO VEHICLE IS PARKED IN OPEN AIR.
With the engine running...especially my tire inflation pump, will actually run "stronger" as it is clearly getting more juice.

My opinion however is either engine on or engine off, doesn't significantly either save the battery or drain it. I can do both. I have done both.
But which is right?
I sometimes wonder if with the engine ON...I'm not actually sending MORE current to the tire pump or vacuum than is needed or that it is designed to take?

I feel I should just know the answer to this question. But to be honest? I've never read any instructions that said plug into ACC socket with vehicle in ACC mode BUT off....or any instructions that said plug into ACC socket and turn on engine.

Which is the correct way?

 

Either. Don't overthink it, my friend.

 

It's only going to draw or take what it needs to run regardless of engine on or off.

 

But there should be a definitive answer. I've been doing "Either" for too long.

In the case of the tire pump, I don't think that's true, it will work with the engine either ON or OFF, but when the engine is ON...it's clearly louder and runs "stronger". It appears to get and take more power when the engine is on.

 

But there isn't always a definitive answer.

Believe me. I'm OCD about a lot of things but sometimes you just have to let go.

 

Since I can't get a definitive answer.
I think I'm defaulting to running the devices in ACC mode, with the engine OFF.
I suppose I'm putting more drain and strain on the battery, but conversely I'm not burning fuel and/or spewing unnecessary exhaust into the air.
ALSO...
You don't want to EVER run the engine in an enclosed space. Because it can kill you.

 

Unless you were to run these accessories for many hours, your battery has more than sufficient amperage to survive their use. If it can spin up an engine every day, it has ample power to run a puny tire pump and vacuum.

 

I'm going with this as the answer. It makes logical sense to me.
Also even though I'm always parked outside in the open air, risk of Carbon Monoxide poisoning, makes me say, the right answer is DO NOT RUN THE ENGINE.