Suspected Battery Issues, but ...

You do not give us any clue what car you own, let just pretend it is a Prius of some sort, you are on PriusChat.

A 12 volt battery can fail in many ways, SOME of those failures can cause the charging subsystem to attempt to recharge the 12 volt battery whenever you are in Ready. This can cause a dip in MPG such as you see. Perhaps the simplest test for a bad 12 volt battery is to run the power windows with the car NOT in Ready. If they are very slow, you want a new battery.

We have no idea how old your car or battery is but on most cars, after 4 years, your battery is on borrowed time. The Prius is actually kind to batteries and if you never left lights on you may still have the original 12 volt battery.

I recommend the Optima Yellowtop over the Toyota OEM battery.
If you Prius is 2004 or newer, this is a good choice.
Optima DS46B24R Optima battery direct fit replacement for Prius 2004 +
For 2003 and before
Optima battery and installation kit for Prius 2001 to 2003

 

If the 12V battery is about to die it can cause odd symptoms. If yours is the original 6+ years old battery it is already elderly. Prius doesn't monitor the 12V battery at all, and a dealer diagnosis can easily give a false positive for it.

The good news is that you can check it yourself in a few minutes with no tools (skip the first few paragraphs of verbiage down to "DO-IT-YOURSELF TEST PROCEDURE"):
http://priuschat.com/forums/newbie-forum/73400-weird-stuff-happening-mpgs-dropping-test-battery.html

The high-voltage traction battery is closely monitored by the car. If it is about to die you will get warning messages and trouble codes that should allow a dealer to diagnose it very quickly, so I wouldn't worry much about that possibility yet.

 

We clearly both wrote at the same time, but Richard did a better job than me. At least we don't disagree.

 

Please post your location as well, as this has a major impact on the advice that can be dispensed (temperature, terrain, warranty, independent shops, etc).

If your car is more than 4 years old, then replace your 12V battery out of hand.

If you park an older car in the heat of the day with a fully charged battery, the battery may overheat. In such a case, there is a significant change in the amount boost and regen that the battery will supply/receive, making MPG suffer. If you hear the battery fan running in the back, particularly upon startup, then you know there's a problem but there's not much you can do about it, until the car actually sets a code for a bad cell.

 

Four years is a little early for a 12 volt battery, but some have to be less than average for average to work. Richard linked to a nice test for first thing in the morning.

 

HTML:

OK, so it seems that most think that the 12V may have something to do with it. 

Hi Iria, I completely agree with the above advisors. The Prius 12v aux battery is an odd entity, once its weakened or 'elderly' it requires more frequent charging, more ICE running and then it makes the ecu act senile. Since you have scheduled maintenance I'd grab a new 12v battery and then you can relax for another 4-6 years. Around $300.

I just replaced my 12v batt last week with an Optima yellow top. Its an easy and clean DIY job if you are inclined. You can order at elearnaid or Advance Auto Parts ($200) and have it delivered free to your door within a week. Make sure you print out the discount coupons.

 

If you are handy, this Optima fits just perfectly
Optima DS46B24R, available on line for about $190 and a $25 rebate.

 

Short trips will cause this (both low MPG and dropping of SOC). Has your typical journey changed at all recently?

You should check the 12V battery voltage after sitting overnight. Check both the no-load voltage and when running the headlights. This will give us a good idea of the health (or otherwise) of your battery.