battery use

Don't do it. The more you use the battery, the worse your mileage. That may sound counter intuitive, but it's the truth. Check out the fuel economy forum, and look at the stickies in the nooby forum.

Tom

 

There are a lot of tricks you can use to try and put the vehicle in specific modes for specific driving conditions. You can read about many of them in the sticky thread at the top of the Fuel Economy forum that starts "New Owner, want MPG help?).

As mentioned, in general, the more you use the battery the worse your mileage will be. Thats an over generalization, but still pretty accurate. Typically to get into electric mode you get up to speed, then let all the way up on the accelerator. Now depress lightly until the green arrows go away all together and there are no arrows on the display. This is called glide mode. If you press a little harder, you will get electric drive arrows from the battery to the electric motor to the wheels (as long as your battery charge is high). If you press a little harder the ICE will generally restart.

While its cool to drive around on electric, it generally takes more gas to charge the battery up after doing so than if you had just driven around on gas. Glide mode is much more useful if you're trying to improve you mpg numbers. A simple way to start is just try to anticipate red lights as far in advance as possible and start gliding. Then hopefully the light will change before you get there, and if not you've at least used less fuel getting to it.

Rob

 

I typically drive my car pretty normal although I do pulse/glide from time to time. One thing I do do is use the battery for the last 1-1.5 miles through my neighborhood. The way I see it, the car's going to run the ICE anyway the next morning to get it up to temperature so I feel using up as much battery as possible before shutting it off makes sense. I have no hard data to back that up though.

Mike

 

You don't do anything to "use the battery". The Prius uses the battery as needed. All you do is drive and the control system does the rest.

Tom

 

I second Tom's statement!

ZC1

 

All the time

I guess I am different than most of you. I use the battery as much as possible. We have a lot of hilly roads around here. As long as the road is flat to slight downhill I will get up to speed and let off totally on the accelerator.Next I just give a little pedal and bring in the battery for power. I will keep it going as long as I can.On a flat surface I can sometimes ride on electric for upwards of two miles before the ice kicks in and charges the battery. The computer doesn't allow the battery to be drawn down too much and the ice can recharge it fairly quickly. A lot of time I will come upon a downgrade then allow the car to coast and charge the battery. I just keep up the same thing over and over.When you are using electric you are driving free..After a while it becomes second nature...Good luck and don't be afraid to use the battery. Thats what it is for.
John

 

Re: All the time

(emphasis mine)

Sorry to burst your bubble but that just isn't so. One way or another, you paid for that electricity with gasoline. When you are coasting down a hill at some point you used gas to get to the top. It may have been yesterday but you used gas to get there.

When you coast to a stop you are using energy to charge the battery that you burned gas to get up the speed up in the first place. Every electron came from gasoline.

And to top it all off you only get back about half of what you spent charging the battery. Some is lost as heat charging and discharging the battery. Some is lost as friction in the drive train and even some as heat in the motors.

Bob

 

Re: All the time

What you are doing is fun, but not efficient. I do the same thing going through a small village near where I live. I enjoy cruising through town at 25 mph on electric only, but I also realize that it's not an efficient way to drive. If you are after maximum fuel economy, use the engine to get up to speed, then release the go pedal enough to turn off the engine. Feather the go pedal just enough to keep the electric system neutral: no arrows into our out of the battery. You are trying to coast without any power. Unless you are going downhill or have a huge tailwind, your speed will drop. When your speed gets too low, repeat the process. This is the pulse and glide (P&G) technique so often discussed on this forum.

As the other poster stated, any power in the battery was put there by burning fuel, and it's a pretty wasteful way of getting power to the wheels: burn fuel to convert chemical energy to mechanical energy, spin the generator to turn mechanical energy into electrical energy, charge the battery to turn electrical energy into chemical energy, draw power from the battery to turn chemical energy into electrical energy, spin the motor to turn electrical energy back into mechanical energy. Using power from the battery adds four additional steps, each of which waste energy.

You might ask yourself why have the battery at all, which is a valid question. The answer is that while inefficient, storing energy in the battery is better than not having that energy at all. The battery allows the Prius to use a smaller, more efficient engine, and also recaptures some of the energy that would be wasted during braking. It also allows systems to run while the engine is off, and the Prius can be moved short distances without requiring a restart of the engine. That's why the battery is there. Energy from the battery is certainly not free.

Tom

 

Re: All the time

I will add that your downhill technique works very well on the interstate when you are going downhill, but otherwise on back roads you can get 65 MPG easy if you learn to not use the battery.

 

My Opinion. Best mpg comes from a light foot on the gas pedal and a lighter foot on the brakes on normal roads. I totally agree with letting the car take care of itself with EV and ICE. Drive SMOOTHLY like you have a filled fish bowl on the seat next to you and you will be rewarded. Hills ..... You pay on the way up and are rewarded on the way down. It is better to think of the Prius as a small 4 cylinder car with electric assist than an electric car with an engine to charge it!

 

One of the best high mileage techniques (pulse and glide) is, in fact, an electric only mode. The traction battery will be discharging from 3 to as much as 10 amps during p&g, while the engine is not running. This pretty much defines an electric mode, and is one of the the reasons that p&g in the Prius works so well.

Toyota has broken down the various systems with regards to their contribution to overall fuel efficiency, and the regenerative charging represents about 1/3 of the total. Even on Interstate 90 in the bad lands of Montana, slowing and stopping are part of getting from point A to point B. If you are going to stop anyway, the regen energy is, in a real sense, free, since the alternative is to heat the environment. If you are accelerating only to slow down and charge the battery, then you are obviously on the wrong side of the efficiency curves. However, using the regen system to its fullest is a major reason that the Prius works as efficiently as it does.

Pat

 

Efficient "gliding" is not in "electric only" mode. In order to efficiently glide, you should not be using any of the car's power sources (i.e. neither the electric motor or the ICE); you should see no arrows on the MFD. Moreover, efficient pulsing is achieved by using only the ICE as much as possible in it's most efficient RPM range (I've heard ~80% of maximum RPMs is about right).

Remember: charging and discharging a battery is inherently an inefficient process. As Tom says, you should try to use the battery as little as possible. A lot of the time, it is not practical to completely cut out battery-driven driving, but if you're trying to drive the most efficiently, you should avoid using the battery whenever possible.

 

If you are using the brakes you're doing it wrong most times and there is room to improve. The ultimate if you have to stop is to use a no energy glide right up to the point where even with your foot off the accelerator the electric motor drives, 8mph I believe and only then should you need to brake.
And in preference to that try not to have to stop at all.
I wouldn't deliberately try to use the battery. I also drive a lot on the cruise control.

 

What you are saying is generally true compared to a conventional car, but not necessarily the optimal way to drive the Prius. If you drive up to a red light and then regen brake to a stop you will recoup some of the energy normally lost stopping the car. However, if you had anticipated the light turning red and started gliding well before the light you would have saved more gas than what will be offset by the energy stored in the battery during regen. This is just the nature of the efficiency penalty associated with the electric system. Further more, by slowing sooner there is a decent chance that the light will change before you get there which saves a lot of energy. Accelerating from a stop is where you burn a lot of energy. This basic idea applies to all cars, not just the Prius. The Prius is just well set up to take advantage of it.

Regen does help efficiency, but not as much as you would think. In the EV world, regenerative braking is commonly accepted to add ~5-10% to your range. The Prius' efficiency gains are estimate more in the 30-50% range, so something else is going on. That something is basically a smaller than usual engine, running on a more efficient but less powerful cycle, and maintained in an efficient operating state. If the electric motor wasn't there, when you stepped on the gas to accelerate the ICE would have to rev out of its peak efficiency range killing your mpgs, and it would be really slow so you'd spend a lot of time in this mode of operation. The electric motor adds power during acceleration to try and keep the engine from having to rev, while providing a level of acceleration that is deemed acceptable by most people. Even though gas later has to be burned to recharge the battery, this efficiency hit is smaller than the hit you would have taken if the engine had needed to rev up. Regen comes in to help offset some of that gas needed to recharge, particularly in stop and go driving or hilly terrain. However, if you are willing to accelerate in such a way that the extra push of the electric system is not required to maintain the ICE in an efficient range, then you don't need to burn the extra gas later to put the energy back in. In this case there is no real benefit to regen, and you might as well glide instead. Thats the basic idea behind the "use as little battery as possible" method of driving the Prius.

This is basically the busting of one of the most common Prius myths. "Prius' are only efficient in stop and go driving, on the highway they're no better than any other car." If the key to the Prius' efficiency gains were primarily from regen as many people believe, this would be true. On the highway, the Prius is efficient because it has a very small efficient engine, good weight/volume ratio, and good aerodynamics. The electric system is mostly the key to allowing you to run that engine in a car this size with performance that the market will accept.

Rob

 

Actually you're both partly right. In a perfect world a glide would draw no power from the battery. But in reality it does. Depending on vehicle speed and go-pedal pressure it can be anywhere from 1-2 amps up to 5-6 amps, even with no arrows.

Sounds like another worthwhile chart to build from my CAN-View data, to post here.

More of a semantics issue, I think, but I agree that "electric only" suggests electric propulsion rather than gliding.

 

My understanding is that basically you are getting just enough power from MG2 to overcome the drag of MG1. Ideally MG2 would use the same amount of current that MG1 is generating. This should be a pretty high efficiency operation, as in this case the current should flow directly from MG1 to MG2 without having to charge/discharge the battery. But of course, efficiency will still be less than 1, so I suppose it makes sense that some battery current will be used. At an average of 3 Amps, I would think you should be able to glide for about 1 hour before the battery is run down

Rob

 

If you watch the ScanGauge during glide, you will see that the battery is discharging at up to 10 amps/hr. This does not show up on the display. Gliding is, in fact, helped by the battery doing the work that the engine would normally be doing (such as turning an alternator to power the displays and various electronic devices). The battery will have to be recharged at some point, so glide is not a free ride.

The engine only provides about 10% of the efficiency gains in the car, while the regen system provides about 35%. Unfortunately, the common thinking here is to not use the regen system any more than possible. Reclaimed energy is free in the purest sense of the word. It is energy that would be discharged as waste heat, but has been recaptured for later use. The two ways to charge the battery are by using the engine (very inefficient), or by using the regen system (free). The regen system is 50-50% efficient, while the engine, running through the MG, is more on the order of 15-20%. The battery wants to be at 60%, so you have a choice as to how to keep it there.

The point here is that you have to stop once in awhile, and using the brakes is an efficient way to do it. Use glide up to the point where you have to start stopping and then apply the brakes at a charging rate of around 75 amps. Gliding to a stop (which has been recommended here more than once), wastes the opportunity to reclaim some of the energy used to get you going in the first place, and converts that wasted energy to heat. Just because you can't get it all back is no reason not to get as much as possible.

Pat

 

You get amps/hr from your ScanGauge? How do you set that up? All I can find is amps (coulombs/second). When I glide I typically see less than 6 amps.

There is not "alternator" in the Prius. All electricity comes from the generators. The 12V system is always powered from the HV system through a DC/DC converter.

True, since the car is "On" and using energy, the energy will need to come from somewhere. However, using the smallest amount of energy possible while covering the largest distance possible is what efficiency is all about. The less energy you use now, the less you'll need to generate later.

Where do these numbers come from? This doesn't sound right to me.

Or rather any more than necessary.

When possible is is more efficient to avoid creating the kinetic energy in the first place. Reclaiming it means conversion losses both in the reclaiming and the reusing. Certainly if your choice is use friction brakes or regen brakes, then the reclaiming is more efficient. However, a better choice whenever possible is to avoid needing to reclaim in the first place.

Again I'm not sure these numbers are accurate, but for the sake of discussion, I'll accept them for now. The problem is that in order to "reclaim" the energy must first run through the engine to create the kinetic energy that you are trying to reclaim. Therefore, using your numbers, the energy is generated at 20% and then 50% of that is reclaimed. This means that the end result is only reclaiming 10% (half of 20%) of the starting energy in the fuel. Far better to avoid this as much as possible.

This is commonly accepted. If or when you need to slow down, the Prius certainly does it more efficiently than other vehicle. It shuts the Internal Combustion Engine (ICE) off if it can, and it tries to recapture some of the excess energy. The discussion, on the other hand, is about gliding when not needing to stop.

Where do you get this number. I've hear that you can more efficiently recapture the energy at substantially less than 75 amps.

The only way to glide to a stop is to be going slow enough or to begin your stop early enough. Either way you've avoided generating the energy in the first place that you are now trying to recapture. Seems like trying to recapture energy at 50% that could have been kept in the tank as fuel at 100% is less efficient. On the other hand, it is rare to have a situation in normal driving conditions where you can actually glide all the way to a stop. Therefore by trying to minimize the use of the brakes, you will still be forced to reclaim some energy.

Agreed, if you needed to generate excess kinetic energy to maintain normal driving, and you now need to eliminate the excess kinetic energy then by all means try to recapture as much of it as possible. However, whenever you can drive in such a way as to avoid using the brakes, you will most certainly use less energy (and since all the energy in the vehicle comes from gasoline, you'll use less gasoline).