MPG and the 12 volt battery

I hope that I am now reporting the findings from a long journey that has come to an end. Starting out I knew very little about the OBD system within my little Prius. However, now thanks in large part to this community I know a great deal more than I did in the beginning.

I hope that my experience and what follows will assist others.

Sometime ago I posted a thread voicing my concerns about my lack of fuel economy.

My troubleshooting was a process of learning and elimination. After satisfying myself that the engine was running as good as could be expected and eliminating other small things, like error in calculating my fuel economy, tires, etcetera I was led to the 12 Volt battery. The battery voltage was consistently a “little” off. That is because I was only reading the battery voltage with the engine running. The defect only showed itself when the vehicle was in the (IGN) mode and the battery was under a load. Not much of a load. I thought it was a long shot because at the time I could not figure out how a defective 12 Volt battery could affect fuel economy. Even now I only have a theory on how this might happen, but I am here to tell you a defective battery did in fact affect my fuel economy. MPG when from ≈ 32 mpg to 42 mpg HWY instantly and all that I did was change the battery. The ECU is still learning and perhaps it will even get better. See the attached file for details of the data.

Here is my theory. The system saw the defective but not dead battery as needing to be charged and attempted to do so on a continual basis requiring the engine to run longer.

The battery that I replaced was a Toyota battery with an 80-month prorated warranty. Supposedly, I could exchange it for a new battery for only 50% of the MSRP. When I went to our only Toyota dealership here in Yuma to replace the battery. They tested it and said it was good. I showed them the facts and figures, but according to their little machine it was good and there was nothing that they could do to help me out. To prove my point, I purchased a new Toyota battery and had them install it. Without any further alterations or changes the system began to preform properly. They told me that my diagnosis was “most probably” true but that they were incapable of exchanging the battery unless their little machine said it was bad. Go figure

Here are some facts and figures that you may find useful.
Toyota battery listed as the replacement in a 2007 Prius is #0054-21171-325
Notice I said “THE” replacement. There were no other listed options.
It is a conventional FCLA battery not an AGM.
It is vented and may be valve regulated
It comes with a 84 month prorated warranty.
$165 to ≈ $185
Battery size S46B24R CCA 325 RC 67 20 HR 45Ah ADF 325A
The TrueStart Toyota battery is made by Johnson Control, same battery as an Interstate and are delivered to Toyota on the Interstate truck.
They say hindsight is 20/20 and in hindsight I would have never gone to the dealership. I was enticed by the prospect of saving a little money. There are several better aftermarket batteries.