Electrical experts, what in the heck draws so much power in IG-ON mode?

What you describe sounds pretty normal to me.

Lots of pro tint shops and detailers use trickle chargers- the good ones are pretty used to it, for lots of cars these days.

You can use the repair manual to verify that the accessory mode current drain is normal.

Either way, the cheapest, simplest, toughest fix is going to be a bigger battery.

 

Pressing the power button twice with your foot off the brake puts the car in IG-ON mode. In this mode, the inverter cooling water pump runs. It's likely that electric pump is the source of your drain.

 

Right, the thread title is a little misleading, because it talks about "accessory mode" (one button push with no brake), but the car was actually in IG ON (two button pushes, no brake). That makes sense for the OP wanting to use the power windows, which are only powered in IG ON. There are only a few items, like the entertainment system, powered in accessory.

In IG ON, pretty much the entire car is powered up; the only difference from READY is it hasn't connected the traction battery or started the engine.

 

Nice, I hadn't caught that- our Prius uses a key instead of a pushbutton, so I didn't pay attention to the press count.

 

Was the cabin blower on at all? I would think probably yes since most people don't set that fan to OFF before turning the car off. So in IG-ON it would have started back up at whatever the fan speed was.

That blower can use a lot of power. Anyway, by my measurements the current out of the auxiliary battery is ~2.3A for ACC, anywhere from 9A to well over 20A in IG-ON, depending on what is running. On my car it is almost exactly 20A in IG-ON with all lights off, all doors shut, AC off, cabin fan at 3rd highest setting (one above the middle). That nice round number can be used to load test the battery - let it sit like that for 15 minutes and it pulls very close to 5 Ah out of the auxiliary. But don't measure the voltage in that mode, the car has to have been off for at least 5 minutes, and if it was running before the first voltage measurement, bleed off the surface charge with 30s of headlights. (Or maybe more if your battery is in better shape than mine.)

Back to your specific case, you indicated the car was in IG-ON for 45 minutes. Using a round number of 15 A current (blower somewhere near the middle of the range) that would have been 11.25 Ah from the battery. If it is brand new it has 46 Ah and that wouldn't make much difference. However on my car, with just 12.5 Ah capacity left, it would have been almost completely depleted. Note that being down to 11.4V while the load is applied is worse than it looks. The load drags down the battery voltage. The voltage you care about is the open circuit battery voltage (no load) without surface charge and after the voltage has rebounded from a load. Rebound can take a couple of minutes.

For camper use, or when just sitting in the car with the radio or AC on, the car should be in READY and the transmission in park. That way you cannot run down the 12V, and the motor will start up to recharge the traction battery as needed.

 

One way to do this right without the hybrid system running with several engine starts per hour, use a second, high capacity, marine or agm battery. It runs your camping loads connected through a low loss battery isolator. Typically used in rvs with the same requirements.

One of the better solutions


Standard RV Store Solution

 

Anyone who powers their vehicle off or turns the key off in a regular car with the stereo full blast the air conditioner running full tilt etc etc etc headlights on high beam deserves whatever they get electrically that's coming to them That's all I have to say about that look how long it takes to boot up an airplane to take off all the switches and electrical stuff and what have you Large equipment machinery generators all of that and you just hop in your car hop out of your car and like nothing when you treat things like that electrical gremlins arise sooner than later generally speaking.

 

An RV places all of those batteries outside the cabin. Where would one put a 2nd 12V battery in a Prius so that it could vent properly? The only method I can think of is to mount some abnormally short AGM batteries in the space below the hatch area floor, above the tools. If those have vents one could in theory splice their vent tubes into the tube for the standard auxiliary battery. Not that I have ever seen a battery shaped like that, but cannot think of any reason one couldn't be built in that geometry.