Significant MPG Loss after Replacing 12v Battery

Has there been a recent cold snap or change in driving routine (new route)? If the new 12v is bad out of the box (rare), the ICE could be running more trying to keep it charged.

Have you completed an entire tank?

 

Check your tire pressures too; especially if you took it for service. They are notorious for lowering them.

 

Welcome. I'm pretty new here, too. Sitting a week might be a clue. The traction battery doesn't like to sit very long, but a week shouldn't hurt a healthy battery. It will be interesting to see if mileage improves once you're back to your regular habits.

 

1. When you refuel, do you normally clear the mpg counter on the MFD? If you do not do so, then the mpg counter keeps a running average of mpg since the last time it was reset. Hence, (prior to the 12V battery replacement) the displayed mpg may benefit from thousands of miles driven in warmer weather when the Prius will produce better fuel economy. However, the mpg counter memory is lost when the 12V battery is disconnected.
2. How many miles have you driven since the 12V battery was replaced?
3. Are you aware that typically for the first 5 minutes, mpg will often be in the low 20's regardless of ambient air temp because the engine is running to heat up the catalytic converter, thus reducing exhaust emissions?
4. As previously suggested, see what the mpg is when you take reasonably long drives. If your weather is turning cold, do not expect the same mpg that the car achieved in moderate weather.

 

Changing the 12v supposedly resets the ECUs and everything is cleared, so the ECU has to remap/relearn your driving habits. Fuel mixture etc could be affected, so it's conceivable that an initial change in mpg is possible.

 

It's likely a combination of things.

1. I notice I've logged a little worse fuel economy on my scanguage for the first 50 miles or so after disconnecting the battery for work on my stereo system/fog lights. This is likely due to the ECU remapping engine fuel/air/timing as xliderider pointed out.
2. However a bigger hit occurs when the car has sat for a long period. The Prius has an insulated tank for storing coolant, which it then pumps back into the engine on startup. It can keep some coolant fairly warm for about a day, but after that it's basically back to ambient temperature. The engine will burn a lot more fuel at startup this way, and especially if you only take the car on a short trip, it won't have time to warm fully and you'll see not-so-great fuel economy.
3. The Prius hybrid battery self discharges several percent the first day, and then will discharge a few percent each day for about a week, slowly falling off.
4. If the 12V battery that was installed wasn't full, the Prius had to pull energy from the hybrid battery to charge it. The Prius is notorious for not getting good fuel economy when this battery is low
5. The cooler weather this time of year will reduce fuel economy due to a number of factors.

However all of these effects will disappear once you return to normal driving habits for a couple of days. Report back after a week or so and see if the car is behaving normally.

 

Whenever your concerned about mileage issues you should have a baseline to refer to in the future that way at least you can determine if whatever you had done to the car affected its mileage:

Create one:

Make sure your tires are where you want them pressure wise. Find a nice 2-3 mile uncrowded smooth level road. Turn off the ac or heat.
Clear the mpg icon Pat's referring too. Get up to 60 mpg's and set the Cruise control. Don't touch the brakes or gas from here on out. Drive on CC for a few miles until the mpg's average out usually 2 miles. Note the mpg reading after a few miles on CC. Thats your baseline. It should read like 60 mpg's or whatever.

That little test will kinda reset the mileage meter too. Sometimes if you just creep around town alot for days your mileage tanks and it will take a nice long road trip to make the mileage re-appear again.

Btw, now that you have a new battery take care of it. Once a month put it on a charger using the front jump points. Will make your new battery last alot longer. And you'll get an excellent sense of the battery's health how it reacts to the charger. I have been doing this for 40 years and have never had a 12 volt battery leave me stranded. Always been able to catch it before as all of a sudden it will not improve on the charger. It happens quickly when the 12 goes south. With no voltmeter on the Prius dash it can be a disaster.

Also if you want to improve your mileage replace the Inverter Coolant. The Inverter is the heart & soul of a Prius. Make it happy and it will make you happy.

 

If the Multi Function Display is showing the battery voltage while the car's on, that can be misleading. Try checking with a digital volt meter when the car's off, after sitting overnight is best, to dissipate surface charge.

 

I think Ed means 60 MPH.