Bad MPG - Do I need a new battery?

Wow! 15-20 mpg in the city!!! You may have some big issues if that really is the case. Even if there was no battery, the gas engine alone should easily get you around 30 mpg at least.

How many miles on the car? You may have a bad cell or two in the traction battery. I am guessing a lot of your engine power is being drawn to charge the bad cells in the main battery. Does the graphic display show the battery being charged the whole time?

Anyhow, an aux battery (12V) swap should be done as the first step. If nothing else, to rule it out as a cause. However, 12.4V cold is not concerning. I am just wondering if you measured the battery under load (acc mode).

Bottom line, that's the worst mpg number I have seen for a Prius. It is likely that you have some other problem than the 12V if you are really getting 20 mpg. Make sure you measure it at the pump just to rule out the computer error.

 

i wouldn't pay them a cent until you've tried all the advice you get here.

 

If you measured this in Accessory mode (on the MFD) or Off (with a separate voltmeter), this sounds great, better than my nearly new car. If measured in Ready mode, where is should be 13.5 or higher, then it would be quite bad.

Get is measured, pronto!

I think they are right in that something else is going wrong, as bad 12V batteries don't normally hurt anywhere near that much.

When you are sitting at a stop light with a fully warmed engine, does the engine shut off?

Normally I would point to this, but you seem to have answered nearly everything. But what are your normal city and highway trip distances? Fuel economy complaints/queries? Please copy, paste & answer these questions, esp. if you're new

 

Questions: What percentage of the time does the ICE run? Can you get EV mode? How does the HV battery gauge on the MFD respond, i.e. does it go from all green to low purple and back again in short intervals, or does it hang close to 5-6 blue bars? Has the inverter coolant pump been replaced? Can you see turbulence in the inverter coolant reservoir when the car is in Ready mode?

 

To the OP: you arent looking at the instant gas mileage are you? You really need to fill up the tank, reset the consumption and your trip ODO and then see what you are averaging over a full tank of fuel. My instantaneous mpg's sometimes read 12-15mpg too when I accelerate but once I get the car on plane, they go up to 40-70mpg and it all averages out. You can see my fuelly banner below but right now Im getting 45mpg and my running average is around 47 I think.

 

This is outstandingly bad mileage. Congratulations to the OP on what may be the worst Prius mileage ever. My first thought is that the OP must be misreading the mileage, such as looking at the instantaneous mileage instead of the average. If it is indeed the average, I suspect a failing HV battery or completely frozen brakes. Maybe the OP is driving around with the parking brake engaged, unless the OP's trips are only a quarter-mile long.

Tom

 

Have you changed your spark plugs??? i don't think is talked about much and I only signed into this forum and priuschat to let owners know. I have a 2007 and I just hit 120,000 miles. I purchased it with 18,000 miles on it and the maint manual says to change the plugs at 120,000. When I first purchased the car I was getting 36-39 mpg and I thought it was normal because people were reporting a wide range depending on how they drove the car.

Well after 100,000 more miles I changed the plugs today with Bosch Iridium ($36 total) with $16 mail in rebate. I went from 40.2 mpg (recently) --- for some reason my mpg's got better over the years to 50.6 mpg on my 36 mile commute home. This is over 10 mpg difference with no change in how I drive. Not only is the car running smoother but you can tell the engines are working in better unison. Wish I had changed them on day one at 18,000 just to know they were new and quality plugs. This is also very easy to do yourself - google some directions and go for it.

 

Around DC it is either a parking lot or Indy Qualifying Trials. Sitting in crawling traffic is going to chew up the FE, especially when running the heater or AC. If the average is 15mph when moving then the actual average is going to be much less when all the stopped time is added.

Do all the free & cheap stuff first. If the voltage on the 12v is low then swap it out. If the condition persists then check the HV battery last.