12v Battery - Time for replacement?

I'm starting to think that my original 12v battery in my '06 (28K miles) may be getting weak. Below are the voltage readings using the MFD.

12.2 @ 1st press of power button
11.9 @ 2nd press of power button
14.1 with car running

I recently took a 600 mile road trip and averaged 43 MPG, which seems low. I had been averaging 47 - 49. This also seems to coincide with the increased temp and humidity here in South Texas, which is another reason I'm thinking the battery may be tired.

I've noticed that the windows do roll up more slowly in accessory mode, and the dome lamp is dimmer than when the car is on/running. I had never compared before, so I'm not sure if it has always been like that or not.

My car was made in March '06, so the battery is over 4 years old now.

I have never left any lights on to run the battery down. Have never required a jump start.

What do you guys think? Time to replace it? If so, I would be going with the Optima conversion kit.

Thanks for any input!

 

Update: I checked the voltage directly at the battery with a digital multimeter...

Car off: 12.4
Power press 1: 12.2
Power press 2: 12.0
Car on: 13.9

I'm pretty impressed with how accurate the MFD readings are in comparison.

From what I have read here, the 12.4v reading with the car completely off means that the battery is about 50% charged. Does this alone indicate that the battery is nearing the end of its life? The last trip in the car (Friday) was about 250 miles, so you would think it should be fully charged, right?

I recently had my windows tinted, and when I picked up the car, I noticed they had used the radio as it was on a different station, and the climate control had been used to some extent. I now wonder if they did so while the car was not actually on. This was also directly before I noticed a decline in MPG. Maybe that explains the potential diminished battery capacity.

 

You can find out more about the Optima at the following link: 12 Volt (12v) Toyota Prius Auxilary Battery for 2004, 2005, 2006, 2007, 2008, 2009 and 2010 with installation kit

 

I went ahead and ordered the Optima kit. 4 years seems like a reasonable life expectancy for a battery in this climate. I'd rather take care of it before it becomes a real problem.

 

Update: After having run through two tankfuls of fuel since replacing the battery, it certainly looks like the original battery with its reduced capacity did indeed cause the mileage hit. Previous 2 tanks with the old battery only averaged 42 & 43. After replacing the battery, the next two tanks both returned 47 average. So a 4-5 MPG hit from the weak 12v battery.

So the moral of this story is that if you leave your Prius in someone else's hands (like I did with the window tinting shop), warn them not to run power accessories without having the car in ready mode. While my battery was over 4 years old, I was not having any problems with it before then as I never ran it down.

 

I wanted to update this thread with something important that I hadn't previously mentioned. When I replaced the battery, I maintained the 12v to the car with a portable jump-starter battery. So, the immediate MPG increase I observed was not because of any ECU being reset, which could easily result in the MFD displaying a higher average. The new 12v battery clearly made a difference in my case.