Should the battery fill completely?

I have thought about asking this question or looking for it... In 2 months of ownership of my 2010 Prius I can count on the fingers of one hand the number of times the battery indicated completely full. And that for only a few fleeting seconds. Far more often it's between 4 and 7 bars, and sometimes it drops down to 3 or even 2. This seems to make sense; if the battery is full, then the engine probably isn't using it very efficiently - or you have just descended from a mountain pass.

 

Mine has never been full.

 

This has been explained a number of times in other threads, but considering all of the new owners it deserves another short explanation:

The answer to this question comes from looking at how the Prius uses the HV (traction) battery. With the Prius, the HV battery is used like a flywheel. It stores extra energy when available, and provides extra power when needed to supplement the ICE (Internal Combustion Engine, or gas engine). Contrast this to an EV, where all of the energy is stored in the battery. With the Prius, most of the energy is stored in the gas tank. The energy stored in the HV battery is equivalent to about two tablespoons of gasoline, which is nothing when compared to the 10+ gallons of gas in the tank. The Prius is a gasoline powered car that uses electronics to boost mileage. It is not an electrically powered car.

As stated above, the HV battery has two tasks: 1) Store extra energy, and 2) Provide supplemental power to boost the ICE. These two goals are in opposition: to provide room for storing excess energy the battery should be kept empty, but to provide boost power the battery should be kept full. The Prius compromises and tries to keep the HV battery at about half full, allowing it to serve both of its functions.

In practice the HV battery is normally kept a bit above half full. This is because there is normally more need for boost power than for extended regenerative braking. External influences will sometimes cause the Prius HV battery to run at higher or lower levels. For example, descending a long hill will force the HV battery SOC (State Of Charge) up to high levels due to the extended regeneration, while hill climbing will draw down the battery due to extended use of the electric motor. Cold weather will also tend to force the HV battery to a high SOC. This is because the ICE runs a lot to make heat, so extra energy is available for charging the battery. Also, a cold HV battery cannot hold as much charge, so it fills more quickly.

A last comment deals with the high and low limits of the HV battery. Rechargeable batteries last a long time if they are not completely discharged. The Prius babies its HV battery by charging and discharging through only a small portion of its total capacity. This makes the battery last a long time. When your battery display shows a full battery, the HV battery is not really full. It has reached the high limit and the Prius will charge it no more, but it is still well below the actual physical maximum of the battery. Likewise with the low limit. This is why there is some confusion over people posting things like "My battery was completely empty..."

Tom

 

Had my car for 9 months. The battery icon has only been full once, and that was just briefly while coasting downhill on a drive from the Lake Tahoe region to Sacramento - in this case from 6600 feet elevation to 50 feet elevation. In 'normal' driving in my area, mostly flat terrain with occasional trips along some gentle rolling hills, the battery has never reached full capacity. Incidentally, the car estimated that I got 37mpg on the trip up to Lake Tahoe (50 ft elevation to 6600 feet elevation over 90 miles). Not bad considering I got about 37 mpg with my previous car (Corolla).

 

37 mpg is about right for that distance and climb, presuming you can get 50 mpg on flats at the same speed.

The same guideline says you should achieve about 78 mpg on the way back down. Did you?

 

What was your MPG with the Corolla?

 

My battery as been as low as 2 bars, and all the way full (usually always full after the gentle downhill freeway drive home for about 3 miles.) But honestly, I'd rather it not be in either place for too long, because when that fan next to the rear seats come on, it is an annoying rush of air to have to listen to....sounds like the windows are leaking.

 

I guess this makes me the heretic. My battery spends most of its time at 7 or 8 bars. I've been concerned that I'm not always getting the full benefit of regenerative braking. I was gonna ask if this was normal. Now I'm afraid to Terrain in S. CA is mostly flat with the occasional canyon hill, steep but less than a mile.

The only time I've seen two bars is when I put it in EV mode and drive around side streets. So at least I know that part is working.

Anyone else have this "mostly full battery" thing?

 

I get the mostly-full battery thing in the winter, when the ICE runs a lot to make heat. That shouldn't be the case for you in S. California.

How many miles and how old is your Prius? Do you see fast swings in the battery SOC?

Tom

 

Well, I was wrong. Thanks to this discussion and the one that was linked to, I took a photo and counted the bars. Seems it runs between 6 and 7, not 7 and 8, and that just makes me like everyone else: I've never seen it full. I didn't realize there was space for one more bar. I just always thought 7 was max.

I'm posting a shot because in the other thread there was some discussion about colors on the display. The red dot reveals how the pixels are put together.

 

It's easier to see with the Gen II Prius, since the bars are color coded. The bottom bars are purple, the middle bars blue, and the top bars green.

Tom

 

Not sure exactly because I didn't reset the trip odometer. But the average mpg climbed from 37 up to 48. So I got 48mpg over 180 miles. That means overall the car used 3.75 gallons for the round trip. And I got 37 mpg over the first 90, which means I used 2.432 gallons over the first 90 miles. Subtracting 2.432 from 3.75 leaves 1.318 gallons used on the return trip. Based on all those assumptions I got something like 68 mpg on the way down. I was hoping the average would get back closer to my normal rating of about 55 after the return trip, and was surprised that it didn't do better. But perhaps driving 180 miles over flat terrain is easier on the engine than climbing 6600 feet during the 180 mile trip. Another factor: it wasn't all down hill going home - there were a number of ups and downs as we descend into the foothills.

 

Still have it and use it occasionally. Started calculating mileage 3 years ago, and since then I've covered just over 30,000 miles at 38 mpg. I am a slow driver (usually slow lane 60mph) and did a lot of coasting to a stop with the Corolla whenever it was safe. That probably helped the mileage alot.

 

My experience is that the ups and downs don't matter. The round trip up and down should average out to about the same as flat terrain -- or slightly better, with certain hypermiling methods and reduced air drag at the higher elevation. But weather (particularly common headwinds when heading west) and steep downhills requiring braking (friction or engine) will mess this up.