Newbie is skeptical about 12v battery/mileage relationship

With 5 Mi commute each way you are not going to get much more than 46 MPG.
Have you been on longer trips? How much did you get on those?
Pumping air up to 40 Psi could add about 2 MPG. IMO Toyota recommended pressure is on lower side. Treadwear was much higher on the sides when I bought my 2005 with 73000 Mi on it.

With my 42 Mi one way commute and driving the car at about 70 MPH I am getting 50 MPG in summer and 44 MPG in winter in Suburban New York.

First 4-5 Miles of my commute average around 30 MPG.

 

The discussion of 12v batteries causing MPG loss is when a cell gets shorted. The Prius charging system provides a constant voltage (about 14v) which in a good 12v battery limits the charging current to below 2 amps decreasing to a trickle if fully charged. A shorted cell reduces the battery voltage and significantly increases the charging current. Since the charging system can provide up to 100 amps (which, at 12v is a loss of almost 2 horsepower), it can reduce your MPG noticeably (up to 10% when cruising).

JeffD

 

Anything (constant charging due to a defective battery) that causes the gas motor come on more than needed for forward motion "wastes" gas...ie decreases mpg. Another good example is the AC running more during very hot weather. Small percentages of 50 MPG are much bigger numbers and more noticable than the same percentages of lesser MPG readings in other vehicles. The prius uses most of the tricks in the book to wring every ounce of energy from the gas. In total these strategies equal the best hybrid mpg around.
:welcome:

 

Relative to the power it takes to move an automobile, charging a battery that is mostly charged already (it had enough juice to start the car after all) should be trivial.

Also just to add to what has already been said, just because the 12v started the car, it does not mean it is healthy. In a regular car your 12v takes a beating when it turns the starter over. Therefore if the car starts, your battery is in pretty good shape. Otherwise you click click click. However in the prius, the 12v flips a couple relays and that's it. If the brake has no pressure, then it draws a bit of power to pump them up and then flip 2 relays to connect the HV battery. The HV battery is what is responisble for spinning the motor that starts the engine.

Also, have you ever hand spun a brushed motor generator? With no load it spins really easily. Add a load, even a little bulb, and you have to work for it. Now have you ever pushed a car in neutral? It takes a lot of energy to get it moving, however once it is moving it takes almost no energy whatsoever to keep it going. Good 'ol physics, it is acceleration.

So everytime you glide, you can be drawing almost no power or you can draw that + wasted charging current.

I think the other guys have taken care of the rest. Hopefully you see the light.

 

There is no generator.
If the 12V battery has a shorted cell, it will be noticeably warm after the car has been run for awhile. That's easy to detect.
The battery is in the rear passenger side of the hatch, under the floor. Easy to get to if you lift out the hatch floor, then the cargo container.

-MOST- people who get poor mileage in a Prius do so because of short trips and/or "zippy" driving. Once the car is warmed up mileage is phenomenal. It takes 10 min or so to -fully- warm up a Prius. A little longer in winter.
You can test this using the MFD. Once you have driven for 10 min reset the MFD (second screen bottom right). Then watch the calculated mileage. It will be much higher than what you will see doing this when it's cold.

 

Don't worry about the aux battery giving you / NOT giving you a mileage hit. Your 2005's aux battery is roughly 5 years old. Get rid of it. They're prone to die on you when nearing 5 years. Switch to an optima. Same size, better price, more power:

http://priuschat.com/forums/gen-iii...shooting/68837-12-volt-battery-elearnaid.html

There are tons of threads here regarding increasing your mileage. Following their advise you WILL get a charge. The tricks & traps of better mileage I've learned here over the years has helped me a TON:



My next goal is to hit 80mpg on one tank. Several folks here already have done it. And it DOES give you a charge ... OCD speaking


.

 

Please report to us when you reach 80 :rockon:
Charlie

 

Try rebooting, search andyprius, do the speedlimit, drive more than 5 miles OR live with 46mpg.