2nd Battery for better milage ?

Leave well enough alone.

 

Just remember this:
100% of the energy used in moving the car comes from gasoline. Period. The engine drives the wheels, that's efficient and economical. If the engine must drive a generator which charges a battery which powers a motor which drives the wheels, that's much less efficient.

The ONLY reason there's an electric motor in the Prius is to compensate for the terrible low-end torque of the Atkinson-cycle engine. Even regen is no excuse: under the best of conditions the system recovers less than 25% of the accumulated kinetic energy of the car at speed.

Adding battery capacity might be interesting, but overall fuel economy would be degraded rather than improved (due to the fundamental inefficiency of the electric system).

If you want better mileage, KEEP YOUR FOOT OFF THE BRAKE. That's right - the brake. Remember, regen is inefficient. Friction braking is a total loss. Both cost energy. (btw - "B" is no help!) Plan ahead, never accelerate when you can see a stop coming, coast when you can, avoid the brake pedal, and mileage will improve.

 

Wolfi has a long hill. He could get up the hill with a lot less gas usage and recapture a lot more of that gravitational potential energy coming down.

The electrical side is not just to compensate for poor low-end torque on the efficient Atkinson/Miller engine. The electrical side is for load-balancing, and the brave souls who have added extra battery capacity have documented significant mileage improvements even on normal terrain.

However, there's a universe of difference between adding an EV switch and adding battery capacity. You can't just shove in another battery and clamp on the cables. The computer expects a certain-size battery with certain capacities, and I guess they have to do some pretty tricky stuff to make it work.

I would gladly pay $2,000 to double my battery capacity, but there's no way in *ell I'd ever attempt it myself.

 

I like the option of having two batteries and a plug in, so that you could run the Prius normally the way we do, but you would also have the option of plugging it in at night so you could do your commute to work, of say, 10-15 miles, on the electric motor. The ICE could charge up the battery for normal distance driving.
I know there is a group trying to get a few Prii so equipped. I expect that Toyota will introduce this as either an option or standard equipment within a few years.
B

 

A 15-mile electric-only range would require far more than a double battery. Under normal level-road conditions, with 6 bars of SOC, you'll only get about a mile in EV mode, and you cannot go fast or accelerate hard. There's no point in grid-charging even if you double or triple that.

Doubling the battery capacity will net you a small but significant increase in mpg, but a 15-mile gridable would need 10 or 15 times the battery capacity and a bigger electric motor and maybe higher voltage as well.

I'm hoping that the next major model revision will be gridable with a 25-mile EV range, though one person has speculated that 100 miles would make more sense given the nature of the required changes. But even at 15 miles this is a major revision, and not likely for an after-market modification.

 

In my geekmobile, a full charge goes 3.5 miles on a flat road at 25 miles per hour in EV mode.

Wayne's simulator shows that at 25mph, fuel consumption is better than 90 mpg. The battery is worth about four ounces, so considering acceleration that 3.5 miles is probably about right.

 

keep in mind that a good battery charger is only about 40-60% efficient

 

I never have a full charge. I usually have between 4 and 6 bars. I have seen 7 bars, briefly. And my 1/4-mile-per-bar is on residential streets with stop signs.

How many bars do you have when you say "full"? Do you mean 8 bars? And are you talking about a road with no stop signs? I need to check mine on a through street where I can get up to speed and then switch to EV mode and see how far I go per bar.

Oh, and do you have extra batteries?

 

3.5 miles?

a few have run out of gas and have reported going barely a mile before the battery was down to the red.

considering the "full range" of charge in the car is only 50% of its battery capacity, i wouldnt push that 3.5 mile limit too much.

 

I am an engineer who has been involved with hybrid / electric vehicles for the past 6 years. The midterm answer for a truly efficient vehicle is the plug-in or rechargeable hybrid electric vehicle. I own a Toyota 2004 Prius and love the car. The car is capable of a full EV mode (available with this option only in Europe and Japan). Some American Prius owners have on their own installed the “EV buttonâ€, but even with this setting the car can travels little more than 2-3 miles. The reason for this is not that the Prius battery pack is small, but the Prius engineers have gone out of their way to prevent the packs State-of-charge (SOC) form having large excursion. In fact it is limited to about 15% to 20% of the total SOC of the battery. The reason they do this is be able to provide a car that has a battery pack that lasts the life of the car. The deeper you discharge the pack the shorter the life span of the battery. The best NiMH battery can be fully discharged about 700 times before it needs replacement.

If Toyota were to make a Prius with a full EV mode of 25 – 30 miles with a battery pack that last the life of the car ( using only max 20% SOC) the pack would weigh 1500 lbs and cost as much as the car. Sure they could make a Prius with a pack that is just slightly bigger than the one they use now, that utilizes the full SOC of the battery, but then the pack would need replacing every 20,000 miles or so for about $4000. I believe Toyota made the decision to not go plug-in because they could not build such a car economically for the public.

Again-

Usually we think of using the full charge of a battery to supply power. The problem with using the whole charge of a battery is that you only get a small (500 – 700) discharge cycles. Now the less deeply you discharge a battery before recharging it the life cycle goes up. The Toyota engineers took advantage of this by placing a 117 lbs battery pack that only uses a narrow band of 15 to 20% of the batteries charge before recharge. This shallow discharge allows for thousands of cycles. In actuality the Prius SOC hovers between 60 to 80% never reaching 100%. This works well for a hybrid since energy for the battery is only need in spurts of accelerating and assist. Now in EV mode all the energy would come from the batteries. As I mentioned, the Prius computers will not allow the battery SOC to fall bellow 60%. At that point the ICE (internal combustion engine) must turn on to give propulsion to drive the car and charge the batteries. That narrow band can supply only 2 to 3 miles of full EV mode.

Since the Prius is design to be a Hybrid with a life of the car battery pack, only a small portion of the battery energy can be used. The GM EV1 on the other hand used almost 100% of the battery charge. The EV1 was designed to go 120 or more miles on a charge with 1,300 lbs of batteries. With a life cycle of 500 - 700 cycles on the battery that equals 60,000 to 100,000 miles per battery pack. As you can see the two cars are different animals.

What to do?

Some breakthroughs and commercialization in technology need to be made before a commercially viable plug-in HEV can be produced. The key is to make an energy storage system that can cycle at least 5,000 times before requiring replacement. I personally believe that the present idea of the fuel cell car will not be the answer. The answer will come from a system that uses electricity to recycle spent hydrogen in a closed-loop fuel cell like a NiH (not NiMH) battery. This can be accomplished using carbon nanotube hydrogen storage supplying hydrogen to the NiH battery and an onboard fast electrolyzer unit powered from a wayside power source. I have been working on such systems and they show great promise.

Until we address the issue of where the source of energy that powers our cars comes from, we will be shooting in the dark about what platform to develop for our future transportation. Is Hydrogen the answer? It takes energy to make hydrogen. If it takes energy then where is the energy coming from. Oil? Coal? Solar? Fusion ( I hope). The engineers I have been working with and the understanding that I have developed is that energy production, transmission and vehicle transportation can no longer be separated. Their linking is the answer to our future.

Distributed energy generation, combined heating and power (CHP), on-board fuel recycling and non-contact electromagnetic energy transfer are some of the technologies I have been working on, and they show great promise in providing a solution to our energy and transportation needs. The combustion engine must eventual go. Even with the best possible scenario the combustion engine is on 35 to 40% efficient. An EV powered from a CHP facility has an overal fuel efficiency of 85%. Waist heat in this system is not dumped into the atmosphere as in the combustion engine, but collected at the CHP facility and used for heating and cooling. We must reassess where energy should be produced, how it is transmitted (hydrocarbon fuel, hydrogen gas, electricity, etc.), how it is stored and how it is used. Each stage must be efficient. Our studies have shown, once an above system is realized, an overall savings in energy of 40% is achieved and a cost savings of at least 35% is achieved for the same result.

I believe that most automakers have the right intensions, but the old slave master of ours “money†is holding the reigns. Until we understand that we must invest in a new paradigm of energy / transportation for its huge social, ecological and financial benefit, the change will come to slow.

NiMHPrius

 

I was under the impression that the Prius battery is limited at 25% or 30% at the low end, and around 75% or 80% at the high end, so that it uses about half its total capacity, though it is reluctant to go to either extreme, so that it commonly (but not absolutely) stays within about a 25% range.

Lithium batteries have a greater energy density, but are much more expensive. However, costs come down with tenchological improvements and mass production. So there is hope in that direction.

However, going with your general theme that we must look at overall systems, it's clear that private cars are a very bad idea. The independence they provide the individual is nice to have. But it's probably not sustainable. Mass transit has the potential to be much more energy-efficient.

 

great explanation on the whys of the SOC.

 

great explanation on the whys of the SOC.

Danny: This post should be put in the Q and A for Prius or something...

 

I have what I think is a related question. For a very short part of my commute I have a 1/4 mile stretch of road posted at 35. It has some gentle rolling rises and falls in the road. Calling them hills is an exaggeration. Just up down up down. I usually do them at 35 but last time I was distracted and in a hurry and I hit resume on the cruse control and went to 45 mph and got better mph. Does this make sense? I would guess that the momentum is letting me go over the humps with out engaging the ICE? I have done a limited experiment (twice) and this finding seems to hold. I am gaining .3 mpg on this stretch vrs speed limit. I would like to see others ideas on this.

 

i think that your car may have had a higher SOC or the car was warmer when you noticed the higher mpgs.

another thing to realize is that the cruise control does have somewhat of a leadfoot and the flattened rolling rises and dips do make the cruise control race at times. lower speeds will be affected by more by small elevation changes than higher speeds simply because there is more kinetic energy at a higher speed than a lower one. so a greater energy need on a percentage basis is needed to kick in the cruise control...

ummm that make sense?....thats what i thought.

ok lets say you are driving down the road at 30 mph. you hit a hill and without increasing your gas pedal, you slow down to 27 mph. you have just lost 10% of your velocity.

now you are driving down the same road and hit the same hill doing 40 mph and with increase of the gas you slow to 38 mph. you have just lost 5% of your velocity. the reason you lose less at higher speed is because momentum is greater.

in both cases gravity, change of elevation are the same. but other factors change with higher speeds resulting in the difference.

in your case, the speed difference might be a factor, but i think you would have to reproduce the results several more times over a much longer distance to make any real conclusions.

with all things equal, the slower you go the better your mpg. in a car with a CVT, this is even more true.

 

I take a different view than Dave. Both very slow and very fast speeds are inefficient. Fast is inefficient because of wind resistance. Slow is inefficient becasue of the engine and PSD characteristics. There is some speed, or range of speeds at which any given car is most efficient, and I suspect that with the Prius this is closer to 45 than to 35.

On the other hand, 1/4 mile is too short to draw firm conclusions in the Prius, because you may have covered part of that in golf-cart mode. And a battery icon with only 8 discreet levels is not precise enough to tell you really if your SOC is the same going in and coming out.

But I would discourage driving 10 mph over the limit just to save a teaspoon of gas a month. I'd say drive at or near the limit, or with the flow of traffic.

Hills will take a bite out of mileage, but this bite is less in the Prius than in other cars because it's able to recapture on the downhill some of the energy invested on the uphill.

 

yep daniel is right... besides henry, i know where you live and we both know that any set speed around here aint possible for a little inconsequential bit.

you are better to drive the way that requires the least amount of braking.

however, if you have the time you could experiment for us, but you may have to move to do it. its pretty hard to get that much clear road around here, although it can be done, i do it nearly every day but i guess driving at 4:30 AM does have SOME advantages (i guess...)