Actually temperature (heat) depleats the HV battery quicker.

I park for long periods while I am away to work out of state.

I have the SOC read-out on my scan-gauge.

In sub-freezing winter, my HV battery loses about 6% charge per month.
In summer, it is about twice that much. Still nominal in my opinion.

As a safe-gaurd, I always run it for 5 minutes in the driveway before leaving on a long trip to get it to the maximum 53% SOC.

 

Happened again today... well, almost.

I was finishing up another outside project today and once again listening to the radio (> 1.5 hrs) with the switch in the ACC position. Radio started to go out so I ran to the car (<3 seconds) and went from ACC to ON. ICE fired up and the radio started playing fine again. Must have caught it with that last little bit of power before it dropped below the point of no return.

I'm pretty sure the 12V battery is weaker than it should be and will fail sooner rather than later. I just can't bring myself to change it out before that happens. My wife knows how to jump-start a car...

 

Yes I've found in 3-4 weeks of storage the vehicle can go in to a no-start condition with both HV and 12V batteries dead. Perhaps making sure the batteries are topped off before storage would extend the period.
I understand all modern vehicles have these issues with long term storage, since they run alarms, computer memory, radio presets, clocks, etc. while they're off.

 

Chris-

Now that you know what you are doing wrong, STOP DOING IT!
Most people's stock underhood battery lasts 5 years. 6 year in mild climates.

If you turn your key to "On" without starting the car, your HV battery will downconvert 330v to 14v to run all your low voltage stuff. This will work for 45 min to 1 hour. Then, your HV battery will shut down at 35% charge as a safety... to hold enough cranking power in reserve to start the car. After this "click" (you can hear the relays to the big battery open) only your 12v battery is powering your electronics... and you get about another hour, to 2 hours as you describe. THIS IS HARMFUL to your 12v battery. DON"T DO IT!

Lead Acid batteries, like the one under your hood don't like to be deep cycled.
Run them down once or twice, and you need to buy a new one.

The solution is to keep the key to "run", not ACC.
And really "run" the car for 5 minutes per hour is all you need to do to keep everything charged indefinately.

HTH,
-John

 

It is possible to drop your HV battery below 35%SOC but it is not recommended to do so.

 

If the HV battery "clicks off" at ~35%, you can turn it back on via cycling the key, but this is not recommended.

I've seen mine go down to ~22% and it still started ok.

The purpose of the button under the driver foot well is a bit mis-leading.
It has been discovered that this button uses 12v energy to heat the HV battery pack. The warmer the pack, the more amps it can put out, so may be "just enough" to push a borderline HV pack over the edge to start the car.
No amount of pushing the button can bring back a deeply discharged HV pack.
It is really NOT a 12v to 330v charger.

Before you get any ideas about pushing this button to warm a cold battery before you start your day, think again. Two conditions must be met for the button to even activate. The battery must be A) cold and B) below a certain SOC which is lower than 22%. At 22% the button on my car would not activate, because the car determined it was not needed.

Dang engineers thought of everything!
I speak only for the 2005 which I own and have thouroghly tested.

 

Even that translates to more than 6 months of storage for the HV battery.

My understanding is that the 12v runs the electronics trickle charging for the electronics, so what is the HV battery being used for that it would discharge?

 

All batteries self-discharge just sitting there. Even Duracell AA batteries have a shelf life and expiration date. Those are in years.

However, your FEH has 250 "D" batteries welded in serries.
NiMH self-discharges quicker than alkaline.
Plus there are all those taps and welds and sensors that may be sensing at a few milliamps continuously.
But NiMH is rechargeable, while alkaline is not. Hence a trade off.

Lets make up a realistic number, and say 20% charge is needed to start the FEH. Some people park at 40%. Some like myself, make a point to park at 53% before a long trip. (53% is as high as the car will go at idle in the driveway)

At 6% loss per month in sub-freezing, and 12% loss per month in summer, you can re-do the math.

53% -12, -12, -12 gets you to 17% in 3 months of being parked. Car may not start.

If you park at 40% (the lower end the car allows while driving it) you see you probably won't start after 2 months (40% - 12, -12 = 16% ).