10 mpg loss --->SOLVED!

Congratulations! :cheer2:

Even though I follow Gen II 12V battery issues and FE/MPG maximizing
threads very closely, unless there is a complaint of some weird
electrical condition, I wouldn't list a bad battery as a typical cause for
the very low MPGs that you were seeing.

This won't help you, but I offer it for others who may be in your
former condition -- the very first piece of non OEM equipment you buy
for a Prius should be a ScanGauge... even before floor mats or a
Sharkfin antenna... maybe even before the first gas fill-up.

Its use in monitoring ICE temps when you put grill blocking in is well
documented. So is its use for monitoring ICE conditions when you're
going for max FE/MPGs. More on-point, with the SG, you can check the
condition of the 12V battery any time you want, even if the car is
turned off.

Case in point: When my '08 was new, the 12V battery read 12.7-12.8V
after the car sat overnight. Now, some 2 years later, it shows 12.5. It
has lost some capacity and it will continue to do so. When it gets down
to 12.2-12.3 I will replace it with the Yellow Top.

Get thee a ScanGauge; about $160 at the PRIUSchat Shop, or on
the 'net. :rockon:

 

OK Problemchild Please correct the upside down American Flag! You may get 55.6 mpg. or change your logo

 

The upside down American Flag is a signal of distress.

 

huh? Why is an upside down flag a problem?

Sorry about OT question.

edit:- I didn't see the last post.

 

Darn it Pat, here we go again. I know the flag looks okay to you, but that means it is upside down for us. Sheesh.

Tom

 

I'm skeptical. What is the explanation of MPG hit with a poorly charged 12V battery? I follow MPG for ages in my cars and many of them went thru 12V battery failures and I don't ever recall getting a MPG hit during those occasions…

 

The theory is that if you have a bad 12v that wont fully charge the car will think "why is the battery low? I'll try charging it." and it will continue to try charging it. Sending all this extra energy to the 12v instead of moving the car or putting it into the high voltage battery. Send this energy away from the wheels will lower your MPG.

 

Interesing and I understand the theory, but to lose that much (approx 15% - 20%) mpg I think that the battery would need to be sucking (and wasting) a heck of a lot of power. A quick calculation gives me a figure around 1000 Watts, could that be right?

 

You can't do that quick calculation with the supplied data. You have to know how much power the car is using, which varies with speed and other factors. The bad battery is essentially a fixed load, where the power for the car is a variable.

Certainly the DC to DC converter is able to deliver that sort of power, but the 12V battery would get very hot if it dissipated that much power. I would worry about a fire at those levels.

Tom

 

Wow- I would think a battery should last at least 5 years. I wonder how many have had failures before then?

 

Many. It's not a big battery, and the Prius continues to draw power even when shut down. Leaving a door open overnight is enough to do a lot of damage to a small battery like that.

Tom

 

That's interesting...

Does anybody with a scangauge knows what the critical charge level for the Prius is (gen 2 and 3)? In other words, at what charge and bellow do we start to get a MPG hit?

 

Ok, the smaller size of the Prius battery will make it hard to do apple to apple comparisons, but...

In a 'normal' car a weak 12v battery may make it hard to start, even I would notice that, and replace it.
In a Prius, the HV battery runs the 'starter' so the 12v battery need only fire up the computers, I might not notice it was weak and keep driving it.

 

Hmm...alright engineers and brilliant Prius experts explain to me how a failing accessory battery could impact mileage?

I thought the traction battery and the 12volt were separate systems?

Really, I'm not being combative, I'd just like to know if a failing 12volt could have that much impact on mileage and why. Thanks.

 

The 12V battery is charged by the high voltage system. It takes gasoline to generate the high voltage power used to charge the 12V battery. A failing 12V battery will continue to absorb electricity without ever getting charged - think of it as an energy sink. In practice what happens is the lost electricity heats the 12V battery, so you have a little electric heater under the hatch floor, and it runs any time the car is powered up.

Tom

 

If less than 18 months old, should it have been covered under the warranty? Or was it not "bad enough" to be considered for replacement by Toyota?

When is bad "bad enough"?

 

Since his signature is problemchild, clearly he is in distress and needs help.

On the other side, if you are are not in distress and putting a national flag upside down , it show that you are not respectful to the flag or to the country of the flag.

 

It's a common known problem with the Prius, although it is often forgotten. All sorts of strange things can happen if the 12V is weak. A gas mileage hit is one of those things. Tom pointed out one reason for that.

There has been some concern about weak 12V from shipping and sitting on dealer's lots among other things.