12v battery questions

12.3 V seems a little low. 12.5 V is a fully charged AGM battery. If you have a multimeter test the voltage at the battery terminals to verify the accuracy of the monitor you are using.

 

The Prius has no alternator, it converts 201v from the HV Battery to about 13.8v (on my Gen 2) whenever the car is Ready.
This is really slow to charge the 12 battery. (My v charges at 14.4v, much faster)

If you have secure place to park it and a full tank of gas, I would leave it in Ready for 12 hours.

 

Get at the least a digital multimeter, test the battery first thing in the morning, with the car off. The car should have had a good drive the previous day.

A brand new AGM battery in 100% health should be reading 12.7 at least. 12.6 would be ok. But reading just the voltage, is kinda like feeling for something in the dark: better is to get it checked out with an electronic load tester. For DIY, Solar BA5 has been around for a while, what I use.

I've got a hunch you were sold a stale battery, but get it tested properly.

 

leaving it in ready mode is another way of charging the battery. same a if you were driving, but easier if you have a secure place to leave it that long.
better to take it back and have it charged, or buy a trickle charger.

a new 12v should test around 13 volts when new and fully charged

but the 12v may not be your problem. with a 12 year old car, you have to get back to basics.

 

12.5 is a minimum, and fine for an older battery. every new battery fully charged will read about 10% more than the rating, before putting a load on it

 

For an AGM battery, brand new, 100% healthy, 12.8~12.9 is the at-rest voltage you'll see, first thing in the morning. If it's just come off a charger, it'll read higher, around 13.0 due to what they call "surface charge". But a day or two later it'll settle back to 12.8~12.9.

All this though is rearranging deck chairs on the Titanic: if you want a definitive diagonsis, get it checked with an electronic load tester.

 

how many miles on her?

 

you may need throttle body work, and other things that might not throw a code.

does she burn any oil?

have you checked/cleaned/changed the air and cabin filters?

 

If you bought the battery from a local retailer, there's a good chance they would have a pro-level electronic battery tester, and will do a gratis test. If it fails, you're at the right place for refund or replacement.

 

yes, an auto parts store should have a load tester and do a free check

 

you should learn how to check your oil and tires pressure, it could save you from disaster

 

MPG is miles driven divided by gallons used (station pump #) and not the MFD "lie-o-meter", that is a GUESStimate at best. But yes, it should not normally change much, unless your driving did.

How long have you owned the car?

As others have posted, the Prii do not have an alternator, the 12v is charged from the High Voltage pack, but the system is not designed to be a 12v battery "charger", but only a maintainer. Therefore, if the 12v becomes discharged for any reason (lights left on etc) it can take nearly forever for the internal system to "recharge" the 12v. That is problematic because 12v batteries (actually most batteries) do not respond well to being in an under-charged state for any length of time (say a day or more), before they start to permanently lose capacity.

In some cases, adding extra 12v loads like high performance stereos, laptop chargers and such can over draw the power available to charge/maintain the 12v, causing it to be continually under-charged.

Also, reading the voltage from the MFD, while a good initial step, really does not tell you about the battery for sure as there are multiple potential electrical connections between the battery and the measurement hardware that can cause a reading lower than at the battery itself; which is all the really matters.

 

Given this information, why are we still fixating on the 12V battery? There are still lots of other possibilities.
Tire pressure
Tire brand and type
Dragging brakes
Driving style and traffic conditions
Air conditioner efficiency and time of usage. (Most of you are not using it now, but we still are here in FL.)
Weather
HV battery condition
Etc.

 

3,500 miles a year is really low, even lower than me. but that shouldn't be a reason for a sudden change in mpg.
without a trouble code it will take patience and a process of elimination.