Effect of 12-volt battery on mpg: 6mpg difference? Why?

Well, it's not going to have anything to do with "low-voltage spikes in the 12-volt battery voltage readouts" because in READY mode, that voltage is under strict control of the DC/DC converter and it doesn't change much.

Now, it could have to do with the battery accepting a constant, unusually large charging current at that converter-controlled voltage. It would take somebody driving around with a Hall-effect clamp meter on the 12 volt battery lead and recording the current going into it. Multiply that by about 14 and you've got watts. Divide by 746 and you've got horsepower. It's simple physics and straightforward math, but it requires that current measurement with the bad battery. We can measure our good batteries all day long and we'll only see what the normal charging current is. The only measurement that will tell the story that needs to be told here is of the current into the bad battery of someone having reduced mileage.

That, unfortunately, is the measurement that I have so far never, ever seen anyone make in connection with this mileage observation, and as you've turned in your bad battery, you won't be supplying it either.

-Chap

 

To replace the 12 volt battery the old one was disconnected losing all custom settings. Perhaps the computer is relearning your car.
After a few tanks it may settle back down.

 

That's a good idea, but maybe too good to be true? I've completely disconnected a couple of times in recent months, once for brakes, once to install a charger quick connect.

Subsequent to each episode I did notice some apparent recalibration behaviour: principally the engine doing odd revving up, just the first drive or two. Mpg seemed about the same though.

 

interesting that you were getting morning readings that fluctuated by as much as .6 volts.

 

Just before changing mine I got around 45 mpg on a 340 miles highway trip. After I replaced the 12v battery on the same route I got 55 mpg. All these numbers are calculated when topped at the gas station, not from the trip computer. Both trips were on cruise control set to 70mph, with AC on and in "ECO" mode.

When replacing the battery, I was too lazy to set all the settings on the navigation again so I swapped them as fast as I could and it didn't reset anything.

 

I believe that depends on state of charge and temperature.

 

That might be part of it. I see 14.4 or so the first few minutes after every cold start, even in warm weather, but for a shorter time.

 

There are failure modes of the 12 volt battery that result in huge current draw and therefore power usage. More power is less MPG. There are 6 cells in a 12 volt battery, if one short circuits, the battery can only charge to 10 volts, but the DC to DC converter will try for 12 forever.

 

While I don't question the existence of such failure modes, I still think it would benefit the PriusChat community if, one day, some member or members who believe that to be the problem would also take the current measurement that would confirm or disconfirm it. So far in all my years of PriusChat lurking, I haven't seen that step taken.

-Chap

 

It turns out at least one person has made such a measurement, three years before I posted that, even; I just hadn't stumbled on it until now.

Measured 60 to 70 amps initially into a 60-to-70-percent discharged battery (the post doesn't explain how that determination of percent discharged was made). Slightly under a kilowatt (at around 14 volts), or about a horsepower and a third.

Now, with anything like a sustained current of that magnitude into a battery (such as with a shorted cell), I'd expect substantial and noticeable amounts of heat and outgassing. That's nearly like having a regular household space heater tucked in the corner of the trunk.

-Chap

 

My 2016 Prius II has about 23,000 miles. I live in south Florida and the car has always been driven within a 100 miles of home so no cold weather driving. My driving is always within our community or nearby roads with almost no interstate driving. Since we bought the car new in July 2016 I had been getting 52 mpg as verified when we filled up the tank...until about March of this year when the mileage dropped to about 48 mpg. The original equipment tires had to be replaced in mid-April so I switched to Continental True Contact tires that are supposed to be excellent for mileage. After that my mileage dropped to 42 mpg. Having had a 2010 Prius I know changing tires can make a mileage difference. The dealer supposedly scanned for error codes when the car was taken in for an oil change on June 29 and nothing was found..so they are at a loss for the drop in mileage. Can it be the 3 year-old 12 volt battery is weak and is bringing down my mileage? The dealer doesn't seem to put much credence in that theory. He talks about cleaning the fuel injection, etc. Any advice is greatly appreciated.

 

The 12v might be lower if you don't drive long enough to put a full charge on it.
But probably not low enough to effect fuel mileage. You could buy an AGM battery charger and
charge the battery overnight, then you'll know it is fully charged.

As tires wear out, fuel mileage will drop, which seems to have happened with you because you
said they needed to be replaced just after the mileage dropped.
I have the same tires on my 2010. They are a little noisey, but are great tires! At least for me!
I average 47mpg but have gotten 55mpg for mostly highway driving. I run 36psi front and rear.
My average city driving is usually about 40-43mpg. There are a lot of stop signs and red lights.
And other drivers so I can "ease" off the line using only electric very often. But it's a lot better
than the 16mpg I get in my van!
They really gripped on the drive across the state in pouring rain and flooded roads!!!
According to others here, their mpg is lower than other tires, which cost twice as much, and don't
last as long. I have just over 55,000 miles on the tires and they are just showing signs of wear.

I use Marvel Mystery Oil in my fuel tank every 10 fill ups or so. It does a great job, and I've used
it on other vehicle. You can also buy a bottle of fuel injector cleaner and put it in the fuel tank the
next time you get fuel. What kind of fuel are you using? 7-11? Cumberline Farms???? I've been using
Sunoco in all my vehicles and have never had a problem. I try to fill up at the same pump each time.


Have your driving habits changed? Even slightly will effect fuel mileage. With only 23,000 miles, nothing
should need to be cleaned. Maybe throw a bottle of Marvel Mystery Oil or Injector cleaner in it the next time
you fill up and drive it on the high for a while. But the car a chance to fully charge the 12v battery and clean
out the injectors, if they are dirty.

 

As tires wear, their rolling resistance decreases and fuel economy will usually improve. That's why replacing tires often comes with a hit to MPG.

 

Less tread, less grip, worst on wet roads. More energy needed....

 

I've had tires changes that made no difference. Then again, having just put on new Michelin Primacy MXM4 (17 inchers) this spring on our 2010, there was a solid 10% hit. Very depressing. Each tank it's picked up a bit; this latest tank is somewhat promising.

Bottom line: it could just be the tires; I'd give it time.

 

I will be buying the Continental True Contact again! Mileage is good, wear length is great.
Dry road grip is great, wet road grip is great. Price was great!
The radio is loud enough to drown out any tire noise, so I'm happy!

 

csk47,

Try rolling down the windows without being in Ready. (Don't press brake while hitting Power) time them then go to Ready and time them again. If there is a marked difference, you may have a weak battery.

Get a Multimeter and get numbers at the battery.
Off:
Acc:
Ready:

www.amazon.com/AstroAI-Digital-Multimeter-Voltage-Tester/dp/B01ISAMUA6