Which battery is discharging.

In accessory mode, you are discharging the 12 volt battery.

In READY mode you can't discharge anything significantly, until after you have run out of gas.

 

About the only thing I'll do in ACC or ON is roll the windows up or down. Anything more than that, I put it in READY.

To add to what Chap said, be sure it is in Park and not neutral. Neutral will run down the traction battery if you leave it there long enough (besides being a generally bad idea in any car). See the manual for a fuller explanation.

 

Yep, there's a story I read 5 yrs ago - whether true or an urban myth, not sure - of someone who parked and absent-mindedly left it <READY> and in "P" - at the airport while he went tripping. Supposedly returned after a week to a lovely cool car and only ¼ a tank of petrol left. Details in my mind a bit sketchy now.

 

I would be doing this in READY mode and Park. You also said you had a tender hooked up to the 12V, so I would disconnect that before going to READY. I would not touch the Power button in the car with the tender still hooked up.

 

That sounds very familiar. I'm pretty sure I read the same account. It's certainly plausible.

 

It shouldn't hurt anything to start the car with any kind of charger connected to the 12V battery. If it did, there'd be a lot of blown up chargers around, including all the ones that I've owned.

 

This is an over-simplification, but:

There's a reason that we say a certain electrical thing (refrigerator, motor, TV, etc.) "draws" a certain number of amps or watts. It's because the power source is not pushing current or power into that device; the device is drawing it. The supply will provide that much current or power if it is able to. That's why plugging 0.6A LED lamp into a 15 amp outlet doesn't blow up the bulb. The 15A breaker will allow 15A if needed by the load, but it won't force 15A to the load if the load doesn't demand it.

If a car battery is very low, it'll draw more current from the charger (up to the charger's limit) than a nearly full one due to the big voltage difference between the charger & battery. On top of that, newer chargers have some smarts built in to allow them to vary their output voltage so as to better maintain the battery.

 

It's not the charger that I would worry about. We all know that the Gen 4 Prius has some sort of charge monitor on the negative terminal that controls the 12V charge rate/voltage when in READY mode. I would not want to tempt fate and screw up any of the car's electronics.

 

Nothing wrong with "better safe than sorry." My schooling, training, and career in electricity make me confident that there is no risk. Even though I express my confidence, I'm not denigrating those who exercise caution when dealing with stuff in which they don't have schooling. I do the same with stuff I don't fully understand.

 

I’ve had our 2010 fully on (aka “ready” in Toyota speak), and moved it forward or back a foot or two (to facilitate floor jack use, for example), with a smart charger (securely) connected. No issues.

@Ozark Man : with a smart charger connected you should be ok to keep the car just “on” (not “ready”), say for protracted time while adjusting settings.

 

If it's a concern, put the charger negative clamp on the car body rather than the battery negative post, and then the battery state sensor sees exactly what's going on.