How long until battery dies when parked?

I park my (2012 - owned since 11/11) vee for 3 weeks every year in July. I've never had any issues with starting it after the 3 week sit. I do disable the SKS. In the interest of research, I would be glad to provide voltage (12V battery) before and after in a few weeks. Surprisingly, the hybrid battery loses very little charge during the 3 week sit (2-3% maximum).

 

absolutely. make yourself a permanent harness.

 

Here are the voltage and SOC before and after sitting for 3 weeks:

Note: The voltage for the 12v was taken from the terminal on the fuse box. To take the "after" measurement, I accessed the hood release from the passenger door to avoid having the brake pump lower the voltage.

12v before 12.38, after 12.27.
Hybrid before 58.0%, after 57.6%.

 

My understanding is that the 12V battery is there just to start up the electronic system not to crank the starter motor. And even with the SKS is on, it shouldn't deplete the battery in a few weeks. Before reading this thread, I would assume that several months will still cause no problems at all.
And if the battery goes dead, you can jump from under the hood and since I have several radio control 3S Lipo packs (12.6V), a pair of alligators clipped wires should save the day.

BTW, a member said, "The SKS system will shut down by itself after some time. It's in the owners manual."

 

That depends.....on the type of battery in use, the health of the battery in general and the SOC right before the car is parked.

For instance, these numbers are a little disturbing because it tends to indicate that the 12V wasn't anywhere near "full" at the time it was parked.
A conventional lead acid should be 12.6 with no load and an AGM about 12.8.
Apply a little load to that 12.27 and it is dangerously near the "low" point.

 

To keep it simple, I would have said to leave the car 3.5 weeks .... and see what happens. Might start, all is good. And if it doesn't, jump it.

However, there appears to be some additional detriment to letting battery run dead in a hybrid? First choice is to NOT run it down, but WHAT exactly would be damaged if it did? I probably have much to learn, as I want to keep this car long time.

 

Nothing special about the 12v batt in a Prius, compared to any car. Except its a very small and wimpy battery that never really sees any deep cycling. Running it down is bad for ANY battery, and should be avoided. If its already aging, running it down can kill it.

In any other car, the 12v batt is used to crank a cold engine, which gives it some "exercise". That seems to actually benefit a battery as long as its not extreme. IMO, the 12v batt in a Prius never gets any exercise which can actually shorten its life. People around here report 12v batt life from 3 yrs to 7-8 yrs, so nothing is certain. But I suspect the main variable is how its being used.

 

What he said. ^^^^^

 

As far as I'm concerned, one dead-battery event justifies the extra cost of installing a deep-cycle battery.

 

Not really. 1st, the Prius never deep cycles the battery, so such a battery might even see less life.
What it does justify is replacing the battery on spec after 3-4 years. The replacement batteries available are superior to the OE battery.

 

In my experience, a dead battery can result in costs and inconveniences far beyond the extra cost of installing a deep cycle battery in place of the conventional one.

A dead aux battery results in the car being dead in the water as I have experienced. This requires a boost, which may not be readily available. Sometimes when that happens, the battery will not take a charge, and the car won't run until the battery is replaced. All of this can result in loss of valuable time and/or money.

I've had a good measure of peace of mind since I installed a deep-cycle battery in my other vehicle. it cost me $200 and after 6 years, it's still working perfectly. It's already paid for itself in countless ways.

 

Your experience is just that; ONE experience.
It proves NOTHING, necessarily.

The point IS that you probably would have had just as good an experience if you had just gotten a name brand AGM battery of the same or slightly higher capacity and could have done that for about $135 probably.

The only reason to pay extra for a "deep cycle" is if you are going to purposely deep cycle it often.......which is not a good idea really with any battery in your car.

 

Not really. Before the change, I was looking at going through another Canadian winter with a stock battery, which is a recipe for much distress. I purchased their top-of-the line lead-acid battery which combines the best features of deep-cycling with cold-cranking amps (not your run-of-the-mill AGM battery). Not only does this battery have deep-cycle capacity, which protects it from deep-discharge damage, but it's expected to outlast the stock battery by many years (which it has). Also, if I do find it dead, I don't have the worry of damaging the battery beyond repair by boosting it.

It proved itself to me, and whether it proves anything to you is not much of a concern to me.

 

Then why do you keep "advertising" your theory like you DO care ??
NO, never mind. Rhetorical question.

The facts are:
A good AGM will tolerate a chronic low charge condition much better than a conventional wet cell type. It also will not be critically damaged by a single deep discharge event.
Also cold weather alone is not dangerous to a battery, especially not one that does the engine cranking.

And to repeat, if you will NOT be having deep discharge events repeatedly, a deep discharge battery in a car is just a waste of money.

And I don't care if you believe that or not.
I do think you should not be preaching it to others however.

 

It's no theory; it's a fact as far as I'm concerned.

 

Deep cycling and cold cranking ratings have exactly zero relevance here for battery performance. Nor do they have any relevance for predicting the life of the battery in a Prius.

 

You're entitled to your opinion, but I think it's very relevant to the topic. The Prius battery seems to be a point of contention for many users, and one solution to the constant pain of having it go dead and being replaced is the solution I described.

 

I've yet to see any evidence of such "constant pain of having it go dead". If anything, the consensus around here is that it's not much of a problem. Given the number of Prius drivers here, and on the road, reports of battery issues are pretty rare.

We can all agree that the OE batteries are crap, as they are in ANY car. Whether the Prius calls for something more than an average quality replacement battery is the question, and it does not. Any decent quality replacement will outperform and outlast the OEM.

 

I could agree fully, if I never left the city, or I didn't have to contend with -40 temperatures, but in my case, a reliable battery is a must. The definition of "reliable" must surely change with geographical location.

 

When you are NOT using the battery to spit out a 100 amp surge of current to turn the engine over.............the geographic location and ambient temperature has little to do with it. "Normal" cold does ***NOT*** damage a battery in any way.