Today's Crappy MPG

Sounds like you could try to see if this correlates with a hot battery. Does it also happen in the morning before the car has sat in the hot sun for hours, or if you've parked in shade or with the windows open? Is it happening only during warmup, or after the car has been driven a while? Hearing the fan run is a sign the battery may be hot. But, I can't tell you if this will affect your mileage by this much, I have a Gen III, and am only going into my first summer, and haven't experienced a hot battery yet that I've noticed. If you do suspect a hot battery, you might see what you can do to keep the interior cooler, like a window shade, or even window tint. JimboPalmer is right, if the hybrid battery overheats, the car stops using it to prevent further overheating, and runs everything off the ICE. This gives you less power and probably higher fuel consumption.

 

Since the OP said this:

" but the first time I test drove the car ( before buying it - the 12v was dead and needed a charge ).. but after a 1 day trickle charge - everything seems ok with the 12.6 in the mornings."

I'd still suspect the 12v. If the battery WAS dead and needed a trickle charge, even if you are getting acceptable readings? I'd change it.

12v or the work of Satan. You pick it....

 

What brand and octane of gas?
The tires, what brand/model and what pressures are you running? bald or new?

It can't hurt to keep an eye on the 12v Aux battery.

My first/worst tank was 39.5 mpg...now average 49.5 mpg (summer) and 46.5 mpg (winter).

 

Hi Spcalan1. You definitely should check out the cooling of the traction battery, it seems to be having a serious issue with the heat.

I'd like to second Jimbo's recommendation. Take a look at this thread and especially take a look at the photo's I added near the end of the thread : http://priuschat.com/forums/gen-ii-...-fur-dirt-causing-major-battery-problems.html

 

That's great, but it isn't just how you drive. The other questions about trip length, city vs. highway, etc. are important because where you drive makes a big difference. I'll give you an example:

On the highway I don't mind running the A/C because all the energy, which doesn't seem to be much, is supplied by the ICE. No big deal. In the city or when stuck in traffic I really try to avoid running the A/C because, while I'm crawling along, the engine isn't running and the HV battery is being sucked dry by the load. Five minutes of that can be enough to drain the battery to 2 bars, and then the engine has to run constantly again to recharge it. That's an inefficient way to burn gasoline.

Not only that, but I paid close attention to battery temperatures during my 3-year stint in Columbia, SC. I saw evidence that running the A/C hard in traffic ("hard" would mean cooling down a car that has been parked in the sun for a while) may have done more to hurt battery temperatures than help.

I'm not saying that this is necessarily the thing that's hurting you. I'm just showing you one example how driving environment can impact fuel economy.

 

How long are your trips?

Short trips (for all varieties of autos) are by far the #1 reason for relatively poor fuel economy.

For example, I moved and my mileage went from averaging over 55 mpg to averaging over 45 mpg. My commute changed from 40 minutes of back roads to 15 minutes of highway. I never try to drive it easy - I just drive it like any other scoot I've owned.

The first 5 minutes are the killer (warm up time) and that part is now 1/3 of my commute instead of around 12%.

I'm still VERY happy knowing that I'm getting much better fuel economy (gas) for my commute than with any other auto sold today. I'm also burning less fuel overall given my move closer to work.

-Brad

 

Definitely reasonable to check the 12v, and the traction battery cooling as possible things to fix. Beyond that, focus on decreasing AC demands. As Jimbo said, you are not gaining an advantage by letting the cabin swelter, but you can take steps to decrease heat gain. Suggestions:

1. Tint windows
2. Leave the windows cracked open for ventilation when the car is parked
3. Look for shaded parking
4. Buy a car cover. I have seen generic covers at Costco or Walmart for about $50.
4a. Buy a nice fit window shade for the front windshield, and park facing south. Or even better, southeast for a short morning park, and southwest if you are coming back to the car in the afternoon.
5. Start a drive with windows open for a couple of minutes to ventilate the car before you turn on the AC if the cabin starts out really hot.

You should also realize that heavy AC use is going to take a bigger chunk out of Prius MPG than a gas guzzler. As an example, consider a guzzler that consumes 4 gallons an hour vs a Prius that is consuming 1 gallon an hour for the same driving. Now add on a 0.2 gallon/hour AC demand to each car: the guzzler's MPG drops by about 5%, while the Prius MPG drops by 20%. Since the Prius starts out as a gas sipper, additional loads are readily apparent. Or put another way, the awful baseline fuel consumption of the truck hides the AC use cost: the 11 mpg drops 5% and is now 10.45 mpg, while the Prius MPG drops 20% from 45 to 36.

 

Also, it should be noted that there is a difference between battery voltage and current.

A bad battery can make 12.6 volts with no load on it and the voltage will drop as soon as current is drawn from the bad battery. But you can't measure the battery under load because the alternator is charging it. What to do?

The dealer can check the battery for you.

And running the car in ECO will cut the AC usage but keep the car somewhat cool.

 

Hi Alan,

Yes, hybrid battery and traction battery are synonyms. I posted earlier about checking that the ventilation to the battery is not obstructed. I think earlier posts provided links for you how to reach the vent and fan. Since the battery fan is coming on, the battery *is* overheating, but that is not necessarily a sign of malfunction.

I suppose that an aging traction battery might be more obvious when driven in a hot and humid environment, but I would not head down that path until I had eliminated easier (and cheaper) answers. One symptom of an aging battery are wide swings in the SOC bars over short periods on the MFD from green to purple not explained by driving terrain or use. This car is new to you, so it would be hard for you to know what is normal.

In your shoes, I would
1. Check that the ventilation path to the battery is clear, and the 12v is not on its last legs;
2. Decrease heat load;
3. and expect 45 - 50 mpg on > 20 mile drives at 65 mph on a freeway in hot humid weather with the AC on.

 

Yes it seems that your traction battery overheating is the problem. All the other things that people have mentioned can cause decrease in MPG but not from 48MPG down to 12 MPG. No way that is normal, that is a problem. Please read my previous post and follow the link to checking the hybrid battery cooling. I'm certain that is the problem. Get the battery ventilation issue checked out ASAP.

BTW. The fan in the back runs most of the time but it's variable speed and usually too slow (quiet) to hear. When it's loud enough to hear then you generally have a problem.

 

I was going to suggest checking the battery fan. It takes a solid 10 minutes of removing interior pieces. When I checked my '06, it was 99% clean. The time I spent on that gave me piece of mind.

If I'm by myself, I'll set the A/C to 76F recirculating (68F during the winter). If the fiance is in the car, it drops to 72F (70F during the winter). Moderation when heating and cooling the car will save gas during any season.

Tinting your windows definitely won't hurt. Mine are 35% in front and 20% in back, except the little window under the spoiler is not tinted.

Did we want to mention what tires/gas you have/use?

The Prius loves top-tier fuel. 87 octane, of course. If I put ARCO or Safeway (yes, the grocery store) gas in my mileage drops like 10%. And it doesn't like Chevron, which is still a surprise.

 

Perhaps I misread, I thought he sees < 10 mpg in spurts, but about 40 mpg overall.

 

That's exactly what he said. He wants to know why it would show 10 mpg when he thinks it should be much-much higher.

Drove the wife's 2008 and saw 1.4 mph instantaneous mpg at 10 mph going up a steep hill. That did get my attention!

Average 43 mph with short in city trips. It's down to 39 after the last oil change. My guess is it's badly over filled. (since Speedy also billed her for 5 qts).

 

1) how long in time and in miles is your average route?
--------------------------------------------
For the first few minutes (depending on how much
effort it takes to warm up the ICE) the Prius fuel
efficiency is low. As the Prius is driven longer the
initial cost of heating up the coolant/ice becomes
less important. If you drive the Prius only 15 minutes
per trip and/or less than 5 miles in one driving session
then your MPG will take a hit.

2) What is your tire pressure?
---------------------------
The Prius fuel efficiency is very sensitive to the tire's
rolling resistance. Tire pressure may vary due to
the outside operating temperature. Older tires may
have slow minute leaks as well. The higher the tire
pressure the lower the tire's rolling resistance. When
the tire pressure is set higher than the maximum
sidewall pressure setting (usually 44 psi), tire tread
traction may suffer depending on the tire's condition,
the tire's design, and external road surface conditions.
Low tire pressure can significantly reduce any vehicle's
fuel efficiency / MPGs.


3) if you have the AC on
-------------------------
IIRC The optimum speed for a car with the
AC on and the windows close is slightly higher
than without the AC on -- about 50 mph
to 60 mph.


4) Did you check the Prius engine oil level?
-----------------------------------------
if Prius engine oil levels have dropped it would
lower the ICE fuel efficiency.


5) if your HV battery is fan is on all the time
your HV battery maybe overheating and you
might be running solely using the ICE. If this
is the case than you need to run the AC to
cool the passenger cabin air which is drawn
to cool down the HV battery. If this is constantly
happening you need to get the HV battery system
checked - it maybe a sign that one of the cells
are dead.

Even if the HV battery is having
problems a hypermiling technique called a
pulse and glide should help bump up your FE.

6) is your foot feeling heavy today?
---------------------------------
The Prius FE drops when the driver demands
too much torque so to keep the Prius FE up requires
accelerating gently on level roads or accelerating
while the Prius moving downhill.


Walter Lee, neophyte hypermiler
2010 Toyota Prius III, OEM floormats
Blue Ribbon/Dark Grey
Yokohama Avid S33 ( 48 psi front, 44 psi rear)
ScangaugeII (AVG, RPM, SoC, GPH)
no grill blocking
DC/MD/VA area urban/suburban driving
odeo +12700 miles, 59 mpg overall
best tank 66 mpg, worst tank 52 mpg

 

It's worthless to have the dealer check the 12 volt battery for you. Just check it via method that the OP is already aware of.

We've had a # of cases where dealers have supposedly tested the 12 volt and it "passed" but yet the 12 volt turned out to be a problem.

 

" ... normal trip to work - 8 miles ..."

That's probably it in a nutshell. How long does it take in minutes to cover that 8 miles?

Understand that in about the 1st 5 minutes you are ALWAYS going to have very poor FE (all other things being equal) relative to those afterwards. If your average commute is around 15 minutes, your mileage is very good considering that a full 1/3 of your drive is within this timeframe.

-Brad

 

Fan cleaning threads:

http://priuschat.com/forums/gen-ii-...oubleshooting/82092-battery-fan-cleaning.html entry #8

http://priuschat.com/forums/gen-ii-...ur-dirt-causing-major-battery-problems-4.html entry #31 & 32