2010 Prius with 180,000 - MPG going down....

1. Yes
2. Not, necessarily. Which battery?
3. When you can drive it forward or back, or ICE doesn’t come on.

 

Yes cooler weather can affect mileage but for me in NC the winter mileage is only about 4mpg lower than summer mileage. And we get a lot colder than 45-55 (tonight will be 22).
If your hybrid battery is getting weaker you will notice the range of discharge and recharge will begin to increase rather than staying closer to the indicated "normal" level on the battery icon. Even so, it could run like that for quite a while but yes your gas mileage will suffer.
If you're talking the hybrid battery dying, it gives a warning on the instrument panel but by then it's pretty much over and as Grit says, the car won't run at all.

Even 43-44 seems low to me unless you drive it pretty hard.....mine averages 52, higher in summer, lower in winter....but I do try a little.

 

Colder weather means your tire pressure is lower, unless you've check it and filled them back up.
Also, colder air provides more drag. Colder means everything takes longer to warmup (oil tranmission fluid).
More rain = lower mpg. More darkness = more head lights.
Colder means heater fan running and maybe seat heaters.
All these little things add up.
You are just noticing it because the Prius gives you the data. It happens in non-hybrid cars too.

Your battery is probably fine. If it drops in the summer I'd investigate.

Mike

 

I also live in the Los Angeles area and I have also noticed reduced fuel economy. One factor seems to be that the ICE shuts off less often, even when warmed up, when the HV battery temperature is below 80 degrees F. Normally the battery is around 100F. This happens fairly often when cruising down the freeway, leaving the battery with little to do. Normally in sunny SoCal, the greenhouse effect on the car warms the interior enough even on cold days, so that after the engine is warmed up, the interior keeps the battery warm enough but it has been cloudy the last few weeks. I have been tinkering with turning on the heater to 70F even when I do not personally need it to keep the battery above 80F. It seems to help a little. In situations where the battery is being more heavily utilized and cycled, it seems to heat itself up and does not need the cabin warmth.

 

1.) Yes...but not usually by 10 percent in areas where people describe 45 to 55 degrees as "unusually cold" in January...above the equator.

2.) No. Not at all.

3.) Depends on which battery you're talking about, Traction or Auxiliary. If you're not sure which is which then the result will be pretty much the same in that your car will become a stationary object until corrective action is taken.
There will not be much in the way of warning signs in either case except in some cases the Aux (12v) battery may cause some unusual codes.
I've replaced these three times on 2 (work) G3s and they all pretty much failed completely with no warning signs.
YMMV.
It might not hurt to add an inexpensive jump pack or a set of jumper cables to your JIC bag if you do not already have one of these items stashed on-board.

If your car is ten years old and has 180,000 miles on the odometer....in a climate like California, and there is a sudden 10-percent drop in observed mileage I'd be looking under the hood for the problem and reviewing your maintenance practices.
If your idea of vehicle maintenance is throwing the key fob at a mechanic semi-occasionally and hoping that the bill isn't too much, then I would strongly suggest that you review some of the threads in this forum dealing with head gasket and oil use problems.
It is possible that a combination of reduced tire pressures and battery efficiency might be part of the root cause, but I'm still betting against it.

Good Luck!

 

winter gas formulation

 

I have to ask: ever cleaned the Exhaust Gas Recirculation (EGR) circuit, or intake manifold?

 

Try setting the heat on max and then turning it off for several minutes when it gets too hot. Try timing it so your heat is on and at max when you're accelerating hard enough for the ice to be on and turn off the heat completely when you're about to start coasting or about to be on ev power so the ice is not running unnecessarily. Also it's extremely important to only use the front windshield defroster only when absolutely necessary. Using the front windshield defroster turns the ac on even the heat is on to dry the air to help defogger and defrost better.