HV Battery Disconnect

One press (accessories) or two presses (on) of the start button without the brake applied keeps the hv off using a power relay inside the hv battery module. However with two presses the ac interior blower comes on running on the separate 12v battery. The ac compressor does not run because it has no hv power from the inverter which has no power from the hv battery. At some point in this mode the 12v to the accessories and the ac blower will shutoff unless you manually go to Ready.

On the other hand, when its in Ready, the ac compressor will run to cool the cabin. It can do this without the engine until the hv battery has been significantly depleted. At that point, while in Ready, the engine will automatically start to partially charge the hv battery. This cycle will continue as long as there is gas to periodically run the engine.

There used to be a company that sold a hv inverter that would take the 201vdc battery power and generate 120 vac. This essentially made a Prius a low noise highly efficient standby generator, useful for power failures at the house. Of course the car would have to be left in Ready mode so that the engine could recharge the hv battery.

 

That could happen using the On mode if it did a 12v shutdown and the system was not totally shutoff or put in Ready mode at that time. The computers are still powered up and waiting rather than going into an extreme low power sleep mode.

By the way, there are at least two ways to drain the 12v even when the car is properly shutdown. One is the rear hatch if not completely closed. There is a bulb on the left side of the hatch that will stay on. It is not automatically shutdown like the other lights. I changed it to a led equivalent. The light also has a manual off position. Another problem is the headlight system if you shutdown and then manually turn them on. At that point the headlights can run the 12v down.

As it turns out the 12v battery does not have to supply much current to “start” the car since all it does is wake up the controls. A Prius 12v battery can last for many more years than a conventional car’s battery. But it can and will run down quickly if some lights are left on, even if its new, primarily because it is a low amp hour battery.

As you may know, when the 12v is dead, the hatch won’t work so knowing how to jump it from the engine fuse box is important. Perhaps the best plan is to keep a small lithium powered jump pack in the passenger compartment. I keep one under the passenger seat to ensure it is accessible. Make sure to buy one that can supply power if the car battery is totally flat. Most require a special button push to override protection features. I like the Noco brand. I find their smallest GB40 works fine. Just be sure to add a calendar item to recharge it twice a year.

 

I like the Noco brand. I find their smallest GB40 works fine. Just be sure to add a calendar item to recharge it twice a year.
I just ordered one of these.

 

I have been in your "Parking Lot" waiting situation in the past and my car just shutoff the 12v with no subsequent issues going to Ready. However I know if I did not either shut it down all the way or go into Ready right away, the 12v battery would have continued to drain. I suspect you simply have a worn out 12v battery that really drops fast, faster than mine. So I would certainly get a new12v battery first and foremost. Most auto supply stores can load test it and will usually have a reasonably priced replacement. Many of them will install it for you.

I am not sure what happens in the ACC override mode but my guess is that it forces the car into Ready on an emergency basis even with other significant problems (other than a flat 12v battery).

 

I found one reference that stated the ACC Ready method will bypass the brake actuation switch requirement. So a bad brake switch or relay could be bypassed.

 

I tried ACC Mode with no brake and it went into Ready after ~12 seconds of holding the power button down. I doubt it would work with an excessively low 12 v.

Here is a way to monitor the 12v voltage from the screen.
Prius v Display Audio Service Menu w Battery Check | PriusChat

 

If the 12v battery is original I would shop around and get a new one. Technically its a little different than a typical car battery under a hood. Its smaller, it lives inside, it has a vent hose and its an agm battery. I have seen them for about $150. You might read about Optima batteries but some of us don’t like them because of cheap terminals that break.