What is the usable power in KWh ?

You have to account for Peukert's for battery charging/discharging, and other inefficiencies in regen, like MG winding resistance, IGBT switching losses in the inverter, etc. Maybe 75% of the regen braking can be captured and re-delivered to the wheels later -- and that is a very optimistic estimate. The main point is that conversion of potential and kinetic energy is not very efficient, and adding weight will be a net loss. Otherwise, the process violates the First Law of Thermodynamics.

To the original poster, please tell your wife that she is getting lower MPG than is possible by trying to keep the battery fully charged, and in fact will cause it to wear out faster that way. These batteries do not like to be fully charged -- this is one reason why the car will start to burn off charge at full SoC.

 

The numbers that Plug-In Prius drivers report when they take long trips is proof that more battery DOES NOT result in higher MPG. Everything that goes into the battery, if not from a plug, comes from the ICE. For a non-plug-in, you only need the battery to start the ICE, provide assistance for acceleration, and have enough capacity to capture the regen when braking from 80mph to 0. The battery is about 2.5 times larger than that to allow for a long lifetime. Any additional capacity is unnecessary excess weight. That's one reason that the C has a 144V system, instead of a 202V system.

It is simple physics. I have a Physics Ph.D. but I've given up on expecting to convince you.

 

Since you are an engineer, calculate the optimal pack size given the (1) weight of cells, (2) Peukert's constant for those cells, (3) minimum and maximum acceptable charge levels for those cells (4) estimated efficiencies of MG1, MG2, ICE, inverter, (4) any other factors you think are relevant. Feel free to use either lithium or NiMH.

Here's a hint: if more is better, then your solution will converge to infinity.

 

Another way to look at the effect of battery capacity relating to efficiency is to look at Argon Labs testing of the conventional Prius. Argon found that mileage remained effectively the same even at half battery capacity. Put another way, the conventional Prius has twice the needed battery capacity. Presumably this was done to allow for good performance even as the battery degraded.

Extrapolating from that, one wouldn't expect better mileage from the PiP, other than the charge introduced from the wall. In fact one would expect some drop in mileage due to the extra weight. It's not a lot of extra weight, but even one extra passenger makes a measurable difference.

Tom

 

I had never even thought about that, but it's nice they made it as big as they did so we can enjoy great FE for years to come.

 

The only time a much larger pack is useful is if you're either charging it from the wall, or if you regularly go up and down mountains. Even on mountains, it may not be useful; I've gotten pretty good on some of my regular (small) mountain drives at draining the battery just before the peak, and then just barely hitting 80% SoC (full) by the time I'm at the bottom. In city driving, it's nearly impossible to reach 80% SoC, unless you're driving absolutely terribly (in terms of efficiency) - like by accelerating towards red lights at each block. In that case, it's more efficient (and cheaper) to modify your driving habits than to add a larger battery. Until you reach 80% SoC, you don't need the extra capacity.

 

The bottom line is, energy stored by the pack comes from the ICE. Yes, there may be peculiar circumstances that occasionally result in 80% SOC (the car tries very hard to put it to the wheels above 70%), but the vast majority of miles are spent in the steady state of 60% +/- on the highway, so larger battery will not help, unless you plug it in. This idea has been beaten to death so many times already over the years.

If you think your pack is too small, go ahead and experiment. Scientific and engineering advances only happen when someone challenges best practices.

Your Fuelly shows 51 MPG. Research driving techniques here to improve that. It takes courage to tell your wife that she is doing something wrong, but you will get better MPG if she doesn't try to keep the pack full. That implies a lot of hard braking, which is energy wasted. The pack will accept charge current to a certain level (about 25 kW) then the pads and rotors have to do the rest.

 

I cannot follow the reasoning because of the poor english. Rewrite please.

Talking of fuel economy,
The cost of a larger battery is its weight;
The benefit is additional regen capacity people who drive in hilly terrain might take advantage of, and (?) less conversion losses.

Net net, I suspect a little benefit for some, a little penalty for others, and not much difference for most. As others have pointed out, the PiP is good empiric proof against the idea in general. Wayne Brown put a lot of work into this idea 5 years ago by adding capacity to his G2 Prius, and IIRC reported some benefit. Perhaps he could be contacted and asked for details.

 

kwh is not a unit of power, it is a unit of energy.

 

You are incorrect on many of your points:

1) There is not a LOT more energy that could be reclaimed - there is some. As you stated you only reach the 8 bars occasionally. You would only improve overall energy consumption when you are at 8 bars and you have to waste energy that would otherwise be wasted.

2) The weight of any additional battery does cause extra energy to be consumed just to carry the battery around:

For example if the battery weighs 100 lb (same as existing Prius battery) and the rolling resistance coefficient is 1% (some LRR tires may be a bit better than this). Then it requires an additional 1 lb of traction thrust to move the battery @ 60MPH. This requires 88 ft-lb/sec of power. This is equivalent to 0.16 BHP (550ft-lb/sec is 1HP), ~120 Watts

The optimum efficiency of the engine is ~230g/KWh.

The extra weight of the battery would therefore require the consumption of 27.6g/hr of fuel.

If we assume that in steady state conditions on level ground the Prius has a fuel consumption of 60MPG @ 60mph then it consumes 2760g/hr of fuel. This will increase by ~1% with the added weight of the battery. (i.e. a reduction from 60MPG to 59.4MPG).

The battery would have to improve fuel consumption by more than 1% for there to be any overall increase in MPG.

3) The engine very rarely produces excess power - if it did the ECU would reduce the throttle setting (and fuel consumption) to lower the output until it matched the demand.

Why would the engine be set to produce more power than needed?

There are a couple of situations where it may be done too advantage:

If the power output would be so low as to be less efficient than running on electric power and the battery has a suitable state of charge (SOC) then the engine is stopped and the car runs on electric power. If the battery does not have a high enough SOC then yes the engine is run to provide power to propel the car as well as restore SOC in the battery.

4) The best efficiency from mechanical - electrical - chemical - electrical -mechanical is probably in the 60% region. The motor/generators each have a best case efficiency of about 90% so there is only ~80% efficiency mechanical - electrical and back. The battery itself has an efficiency of 70-80%. 90% * 90% * 80% = 64.8%

5) The hybrid system does give an improvement in fuel consumption even o freeway driving. The engine size required in a car has to allow for reasonable drivability. Without the the hybrid system the engine has to be size to be larger than necessary to accommodate the extra power required for hills, passing, accelerating etc. With the Prius those transient loads can be provided by the battery so the engine can be sized for the average load - it is no coincidence that the Prius has the best freeway fuel consumption of any car sold in America.

Toyota will have modeled the Prius under multiple scenarios to obtain the optimum battery size and it will be a rare situation where a larger battery will have an advantage (unless you charge it from the wall as in the PIP).

kevin

 

Perhaps you need to tell us what's so different about your way of driving, then. I have never even come close to filling the battery in city driving, and highway driving will always leave me almost exactly at 60% SoC. Only going down long hills is enough to fill the battery.

Another question; does your car ever start using engine braking on its own? Even when the battery reaches 8 bars, the battery isn't full yet. Without a gauge of some sort, the easiest way to tell is that the engine will typically remain on until you are stopped (whether accelerating, coasting, or braking), and will spin up to high RPMs while you are braking.

If you're not regularly hitting this condition, you're not regularly filling your battery, and so there's no need for more capacity.

Also, keep in mind that when the Prius shows 6 bars out of 8 filled, you might think that the battery is 75% full; however, the battery is roughly 43% full at that point ([60-45% SoC] / [80-45% SoC]). Like most gas gauges, the bars are not all sized equally, and they will show full or nearly-full when they are not even close.