Unexplained major drop in city fuel economy

More 12v battery threads:

12V AGM battery life (or end of life) | PriusChat

There's a ton of information buried in these forums about AGM batteries (the 12v type used in the Prius). In general, continual deep discharges, will shorter their life.

 

Somehow, I don't think that one time could have shortened it life that much. But, you never know. I'm sure that our AGM battery experts can chime in and shed some light on this.

Try testing the battery and report back. It's an easy/cheap process...

 

I don't get the sense that the tire pressure is checked all that often, and maybe
not by the OP himself. It should be done at least monthly by the OP and not by
a kid at the garage/gas station. (Vehicle location not specified - New Jersey or
Oregon?)

That said, it is time to consider the engine air intake system, maybe it is partially
blocked or fouled in some way. What to look for:
* Duct work - is it fouled by a piece of road hazard or rodent nest
* Engine air filter - is it clean enough, is it fully and properly seated
* MAF sensor - is it dirty, fouled by an insect carcass, loose connectors
* Throttle body - is it overly dirty

All the above can be set aright by visual inspection and then appropriate cleaning
as needed -- all easy periodic DIY items.

Use the search function to find threads on MAF and throttle body cleaning; two
separate chemical cleaners are used.

My two cents. ;-)

 

web1b,

Thanks for the tire pressure clarification.

By any chance are you running a new set of tires? The MPG reduction you have
noted are consistent with new tires before the 3-5K mile breakin.

And then, there's still the engine air intake system stuff...

 

And you checked them when they were cold, correct? (After they'd sat for MANY hours or overnight.)

Do you have a volt meter so that you can measure the 12 volt's voltage after the car has sat overnight? This is assuming you don't have nav so you can't run the test that we use on Gen 2.

 

One other thing to check... the brakes. A few others have reported pads dragging.

 

I'd trickle charge the 12v for a day and see what the mileage does, if the 12v is always trying to get a charge your ice is going to be giving it.
I'd also get an obd2 reader and see what codes may be found.

 

And how a bad 12V battery will interfere with stopping the engine ??? In my experience engine will not stop if the cooler temp is under a certain temp (depending ECO or Power Mode) or when HV is in the low 2 bar. None of this will be really affected ...

A bad 12V battery will force a higher charging voltage ... instead of 13.5/6 Volt the car will maintain 14.5/6 yes this creates some additional loss load on everything electrical .. but it is just myth that it could drop 10 mpg

None yet put forward anything based on numbers or even a sound theory how this would/could happen. An ageing 12V battery is comparable load to the two low-beam no more ... Of course this would be more noticable in city driving since it is (kind of interestingly) speed dependent.

 

I do not know that but if you can get the voltage you could do something like this. You can leave the car ON after a longish trip in a safe place for some time ... everything is off the HV battery will be charging you 12V battery as long as it gets the rigth voltage. You can check the voltage when the car thinks 12V battery is fully charged it is 13.5V ... if it will not it will be 14.6

Alternatively, check the voltage after a trip at least an hour in my experience that is about long enough to fully charge 12V battery even from a relatively low SOC.

Best way to test is with a multimeter after at least few hours (4-5). But do not open driver side door get in from passanger side to test ODBC or open the hatch and measure the battery directly. The manual has a table about what the voltage should be. Good 12.8-12.6 anything less than 11.4 is trouble.

 

Weird stuff happening? MPGs dropping? Test The Battery | PriusChat should work. I would ONLY check it after the car has sat MANY hours or overnight. Tell us the 3 voltage values.

Do let me know if those steps work or don't work on your Gen 3's nav system. Obviously, on Gen 3s w/o a color LCD touchscreen, there's no way to even follow those steps.

One can get cheap voltmeters (multimeters actually) for under $10.

 

I believe the Gen 2 has that capability. The Gn 3 may not.

You will need a Voltmeter. The other option is Sears or Auto Zone will do the test for free. There hoping you buy a replacement battery from them.

Much of what I read says the battery lasts about 4 years.

 

So, what kind of amp draw would we see from the HV battery while it keeps the 12v at the proper voltage with this aging battery? Have you measured it?

 

I can only see the 12V bus voltage using the SGe, and when my other car was over 3 years I regularly measured the "morning" voltage with a multimeter. In colder days last October after two days of not driving, the battery was 11.8 11.9 which was on the lower end. But my commute is long enough both way to fully charge the battery so weekday mornings it was 12.1-12.2 which is OK. I did not have an aging battery yet. The current car is 5 months old so not an issue.

Measuring the actual current going through the 12V battery would require something external to what ODBC device can provide I do not think there is a way to isolate 12V battery load ... HV battery current (all) probably provided but my SGe do not have Xgauge capacity. It would be really good if someone could provide measurements so this myth/truth can be decided.

But we do know that the 12V DC-DC converter is fused for 100A so that means max 1.4KW is the max for the entire 12V bus (14.5V is measured when there is load either from headlights or A/C or charging the battery).

I am sure someone can calculate how much mpg loss this means at a given average speed, if max 1.4KW is dissipated in the battery.

Comparison the "rated power" in the specs is 98hp ~ 73Kw

 

More importantly than just the current draw's effect on fuel consumption from a kWh standpoint is what is the threshold for keeping the ICE running instead of turning off at stops? When the A/C or heater is running hard the ICE tends to stay running longer than it normally would. Max draw for the A/C that I have observed is approx. 1,800 watts.

FWIW, I'm not 100% certain that a bad (not simply worn) 12v battery can cause this but we've read about anecdotal reports where replacement has made a huge difference in FE. Just because I cannot explain why this would occur doesn't mean that it doesn't. We just need someone with more knowledge to experience the condition. Maybe we can put a hex or voodoo spell on fuzzy's 12v battery.

 

I do not know, the A/C is not on the 12V bus ??? has it's own inverter/converter??

I was researching for my emergency power by prius project it was evident that max power available is 1.4KW (which is the theoretical max because losses at 80-100A on the 12V cables from battery are substantial)

My point was about anecdotal evidence is just not enough ... at least some sound theory would be nice so people with the right tools/equipment could prove it or disprove it.