10 mpg loss --->SOLVED!

Great info! We have a 2010 pkg v w AT. The city mileage has been steadily dropping since day one. It is now in the low 40's. I went out and ran the vehicle signal check described on this link. The battery voltage in accessory mode was 11.6! The car is 5 months old and has 4600 miles on it. I hooked up the scan gauge and put it in run mode to check the BATT voltage. The engine started and it charged the battery at 13.9v for 3 min and 18 seconds. The battery was at 12.4V when the ICE shut off. Within 10 minutes the voltage dropped to 11.6V. I plan to check it again after it sits overnight and call for a service appointment in the morning. It is due for the 5K oil change anyway. I will be asking the dealer to check the 12V battery. I will post the results.
Bob

Just an update. Talked to service this morning and they said the 11.6 volt reading using the vehicle check function is normal. This evening I took the covers off and checked the 12V battery with a DVM. It was at 12.7, which is a good reading for a lead acid battery. The voltage check was on the nav was reading 11.7. Looks like I need to keep searching for why the MPG keeps dropping.
Bob

 

People who have had bad 12v batteries often complain that the prius will not shut off the engine at stop lights or other situations where we would expect it to shut down. So while the current going to the battery may be minimal, the fact that the engine doesn't shut down when it should could easily drop MPG.

While I cannot quantify the possibility but the current going to the battery could indeed have a negative affect on MPG as well and this is why extreme hypermilers go to great lengths to reduce electrical demand from stereos, headlights etc.

 

Shawn and F8L make an interesting and important point: a small 12V battery drain can cause much larger mileage losses.

In addition to the small amount of power sucked by the failing 12V battery, the ICE must run frequently or continuously to keep charging the failing battery. Likewise, if the 12V battery is really bad, it can drag down the 12V bus, which is used to power all of the ECUs. Without good 12V power, all bets are off on how the car will operate.

Tom

 

Hi All,

Another issue not discused is did PC allow the 12 V supply go to zero volts during the install of the new battery? If not, then it can be said the new battery was the true cause of the low mileage. If he did let the voltage go to zero, then there is a good posibility that the problem was resolved by the ECU's reboot.

 

So, removal of the battery would reset the ECU? There's not a FLASH memory, or something similar, to retain the "learned" behavior?

Since my Prius was purchased used, I asked a similar question at the dealership, and they told me that it couldn't be reset...and that even if it could, it would be counterproductive.

Now that I know better, I may do it anyway...erase the previous owner, and see what my mileage does. (40mpg in town, 47mpg hwy)

Chuck

 

The ECU doesn't do a lot of learning. Certainly there is some feedback from the engine, but the Prius does not adjust to a driver. Usually those sort of adjustments are for shift points. Since the Prius doesn't shift...

Tom

 

Hi PC,

Hmm...Those voltages sound fine to me. They call them 12 V batteries, because they should float at 12 V, or a little higher, 12.6 is a common value.

When I changed my battery it was down under 11 volts. I do not have Smartkey. I did the change as the mean battery life seems to be about 4 years, and that would have been the coldest month of the year here.

I am of the opinion something in the computers had worked itself into poor mileage state. So, when you reset the system by letting it drop to 12 V for more than a few minutes, it got rid of all the "learned" registers. Maybe something in the "Maintenance" mode is retained when the car is set back to normal, without a 12 V drop?

Yea, its bizarre the dealers do not know to turn off the Smart Key system when the the Prius is sitting.

 

Hi ProblemChild,

That was the turn-on drop, sorry to be incomplete and confusing. Its said that if the turn-on drop falls below 10 volts, the Prius wont start. Here is the thread : http://priuschat.com/forums/gen-ii-...leshooting/68589-my-turn-new-12v-battery.html

 

What about a cold battery? Like below zero cold? Mine read 11.1 when I first checked it. Would that be normal for a good battery?

 

Might be. I haven't checked 12V versus temp, but this is likely playing havoc with the Prius control system. It will be charging both the HV and the Aux 12V battery and your mileage will be awful. The HV battery control tries to keep it in a "warm" range and will run your engine to run the battery well into the upper green when it gets really cold.

 

Immediately following a (non-dealer) 90K check two weeks ago I noticed my MPG dropped from 45 to around 40. Especially low when accelerating from stop or going uphill. Service said they checked the battery, load test with 175 amp draw drops to 10.2 volts. Any recommendations?

bsilver386

 

My battery regularly read 8.xV (before it was replaced under warranty) and only once failed to allow the car to start. I don't recommend running around with a battery that sick, but I was waiting for it to be bad enough that it would fail the dealer's test and so be replaced under warranty.

 

I think the problem may be your wife. Does she stay in the car listening to the radio often or for prolonged periods?

 

That battery should be fine. In the prius it should never need to supply 175 Amps, I think it's only about 50 Amps max when booting the computer.

A voltage of 10.2 volts at 175 Amps is not bad at all, it's only about a 2 volt drop which is quite ok for such a large current. Did you know that the rated "cranking amps" of car batteries is measured at only 7.2 volts (approx a 5 to 5.5 volt drop).

 

I tried mine 3 times after sitting cold, then again after driving around a bit. The highest I got was 11.7v, so I took it to the dealer to have the battery checked and they said everything was fine.

I'm getting about 28 mpg with temps right around 0.

 

That voltage seems really low Bill. I know that low temperatures will knock it down a bit but the open-circuit voltage should definitely be higher than 11.7. According to data I have the open circuit voltage should only drop about 0.02 to 0.03 volts per 10F temperature drop. So at 0F it should easily still be 12.2 or 12.3 (no load).

Now if there was some loading during the measurments then I don't know how much the temperature will effect loaded measurements.

You mention that the dealer said it was fine, is the battery under dealer warranty? I'm just wondering if they'd have a vested interest in telling you it's fine.