What effect does the 12 volt battery have on the gas mileage? I kind of remember reading that an older 12 volt battery, that can't keep a proper charge, will effect the MPGs.
1) while there is no rule that batteries die at 4 years, it does make a average lifespan, in my experience. This seems true in any car, not just the Prius. (Beware: the plural of anecdote is not data) 2) there are several modes of failure where the 12 volt battery will draw a continual and unreasonable current draw from the HV Battery while in Ready. If your 12 Volt battery has failed in one of these ways, your mileage will be reduced. (In the normal course of use, metals are disolved then re-plated in the battery, if this re-plating goes wrong, it can short out a plate; Plates can shift due to bumps and short out; Corrosion of the terminals can increase the resistance, etc.) 3) We as drivers rarely think of our battery without obvious clues. In 'normal' cars that clue was a slow starter, but the 12 volt battery does not start the ICE in a Prius. It is more likely that you can notice a dying 12 volt battery in a Prius when your Power windows slow down before the car is in Ready. 4) Some one else will swoop in and Post more technical ways to tell your 12 volt battery is dying.
One parameter is the Voltage. If the Voltage is up to 12V or so when the car has been off for a while and the charging Voltage is up to it's normal range 14 to 15 Volts with the car running that's OK. If it isn't it could be due to either the battery, the charging circuit, or something else in the car's electrical system. EDIT: I checked my voltage this morning on a 20 mile trip and it varies between 13.4 and 14.6 Volts and I think I have seen it as high as 14.8. The other is capacity, even if the Voltage checks good the capacity can be low and the battery will go dead with very little use. The only good way to test that is to disconnect the battery and perform a test discharge over 8 hours or so at the correct Amperage. Something most owners won't be able to do and probably most dealers won't either. My way is to replace the battery after 4 or 5 years, especially if I have any reason to suspect it might be bad. Here is a link to a site that sells an Optima battery and a kit to put it in your Prius, I have not used this product but I plan on trying them when I get ready for a new battery. 12 Volt (12v) Toyota Prius Auxilary Battery for 2004, 2005, 2006, 2007, 2008, 2009 and 2010 with installation kit The effect on mileage, as Jimbo said, would be due to a higher than normal charging current if the battery Voltage were low, due to a shorted cell for example. If the capacity were low but the Voltage was still up there would be no effect on mileage.
The obvious clues left for me were that the smart key wouldn't unlock the door, no internal lights, and the car wouldn't boot up.
