Declining MPG; HSI Bar goes to "PWR" more often

Two thoughts

1) The tire pressures "seem fine". Did you measure them? From your response, it doesn't appear that you did.

2) What has the weather been like that you've been driving in? Raining? Windy or windier than normal? Change in wind direction?

 

I have a 2011 with about 68k miles on it. I drive all highway to work, so a low 12v battery doesn't affect my mileage at all. But just for kicks i checked it over the weekend with my OBD bluetooth dongle and the torque app. My resting battery voltage is 12.2V and when i step on the brake it drops down to 11.7V. According to a battery chart, that means my battery is only at 50% capacity. Since the prius only needs it to run the computer and other electronics(not to crank the engine over, which requires lots of amps), it appears to be transparent to the driver that it's low.

I imagine that if you do any urban or even suburban driving, the engine would kick in sooner and lower your mpg if you had my battery. I'm probably going to order an optima yellowtop this week. It's $150 on amazon, and free 2 day shipping because i'm a prime member.

 

Battery SoC based on voltage is really only accurate when the battery is doing nothing - so OBD is off, the doors are closed (opening doors turns on both the interior lights and a variety of computers and kicks off other actions - like charging the brake system), and the car has been at rest for ~15 minutes or so. Ideally, it should actually be completely disconnected from the car, although that's a bit of a pain. I won't stop you from buying a new battery if you want to (especially if having yours die on you would be catastrophic for some reason), but you may not truly need it.

As an aside, this is why the SoC of the hybrid battery isn't based off of the voltage, but rather coulomb counting. It knows what the voltage was when the car turned on, which was a good time to judge its SoC (and why the battery bars sometimes differ between the time that you turn the car off and when you turn it back on). From that point forward, it counts how many electrons flow into the battery, and how many flow out, and uses those numbers to determine the current SoC. Next time the car is turned off for a while, it can recalibrate the SoC to adjust for any measurement error it may have had.

 

Have you considered the Exide Edge (4 yr replacement warranty), the new Toyota 12V battery (84 month warranty) and the O'Reilly SuperPlatinum? They all have higher AH (capacity) than the battery that came with the car and the Optima. The Exide Edge might be more tolerant of high current charging. IMHO, we need more community experience with these 3 options (Exide, new Toyota and O'Reilly).

 

If the 12V battery is bad, like a shorted cell, it could result in the system constantly trying to charge it. This extra charging would impact gas mileage since the ICE would need to provide that energy to the HV battery system, which charges the 12V battery.
Its common for 201 Prius batteries to go bad. The dealer I bought my Prius from last year replaced before I bought it. I recommend replacing it if you have any reason to question it. It will be easier to replace it before it goes completely dead and you can't get the rear hatch opened....

 

Thank you for posting up numbers, that helps. Just a small quibble, pumping to 38 isn't overinflating, if the max tire pressure (when measured cold) is 44 PSI. It is higher than the factory recommended pressure, but to most people on this forum, it would not be overinflating.

A couple of other thoughts would be a dragging brake or maybe you hit a severe pothole or someone parking the car did a number on the alignment. Have you had the alignment checked? Brake wear? Sorry, we are grasping at straws here......

 

Keep on Keeping on! Warm weather will do wonders for it as well.