Why is the Prius traction battery so incredibly inefficient?

Hummmmm:
So recently, this was posted in "Prius, Hybrid, EV and Alt-Fuel News":

• Joined: Yesterday
• Messages: 2
• Location: xcvcxv
• Your Vehicle Year: Other Hybrid
• Model: N/A

There are a number of 'red flags':

1. First-time poster - we should give newbies the benefit of the doubt. Their ignorance and lack of skill does not mean they are a deliberate troll, seeking to pick a fight. It could be they are just unskilled, inartfully trying to understand something. It would have been better had this posting gone in the "Newbie Forum" where patient folks are willing to help them get a clue. Alternatively, post in the Prius specific forum of their questions.
2. "Why is the Prius traction battery so incredibly inefficient?" - this is a good newbie question or something to address in a Prius specific forum. Since he claims these are the Prius hatchback battery specification, that would be a much better forum.
3. "Prius hatchback battery has 1310 w/hr capacity, it weighs over 200 lbs, and it takes up as much room as about five group 31 diesel truck batteries" - so 5 group 31 diesel truck batteries weigh each 67 lbs or 335 lbs. The original poster does not understand volume is not the same as weight. Lead acid batteries require a stronger body and suspension further increasing the vehicle gross weight.
4. "So why is the traction battery so oversized for such an undersized capacity?" - the newbie does not understand the role of the Prius traction battery in the transmission. The Prius hatchback traction battery is part of the transmission and has specific roles that are not tied to the absolute, "1310 w/hr capacity." In fact the state of charge ranges between 40% to 80%, or in effect, only a 40% range. In operation, the Prius traction battery runs at ~60% +/- 5% with rare excursions to the 40% and 80% limit. It is a common, newbie mistake because they do not understand the transmission.
5. "Why dident Toyota just use a bank of AGM marine batteries and a voltage step-up converter?" - weight.
6. "There are plenty of batterys on the market that are many times smaller, but have the same capacity." - not in 1997 when the first Prius was sold in Japan. However, the 2012-current, Plug-In Prius uses a LiON battery.
7. "what is the capacity of the traction battery in the Prius V?" - ask the folks in the Prius v forum but I believe it is similar to the Prius hatchback.

Ignorance can be forgiven and treated by responding factually and accurately. Perhaps the OP will 'get a clue' and in the future be a little more skillful. They might show good faith by hitting the "Report" link and ask the moderators to move this thread to a better forum. They might also be a little more forthcoming in their location and what their other ride is.

Bob Wilson

 

The DieHard's capacity rating is for its 20 hour discharge rate, i.e. when it delivers just 60 watts. If you double the power to 120 watts, capacity drops significantly. Boost the power delivery even more, and the capacity drops off even more sharply.

The Prius battery is rated to deliver at 27,000 watts.

 

I was hoping we were dealing just with ignorance:

Well that is a weird pair of photos equating a 202V Prius traction battery to one "Diehard Platinum" battery:

• 9.5"x13"x6.8"
• 75 lbs
• 92 Ahr @10 hours (source:
  DieHard Marine Battery : Buy High-Performance Battery at Sears
  )

For the Prius, volume is not as important as weight and energy. It is also important to have the 202V battery voltage level more closely matched to the MG1/MG2 voltages whereas a 12V battery requires 202V/12V = 12 times more current for the same amount of power. This would require much larger and heavier cables using the 12V battery compared to the higher voltage, 202V battery.

We have a 2003 Prius and it has a maximum current charge rate of 50A and discharge at 70A. This is limited by the inverter electronics but provides more than enough capacity for regenerative braking that after 160,000 miles, the brake pads are not even half way worn.

Well I'm not ashamed and hiding. I also enjoy sharing nine years of 2003 Prius and five years of 2010 Prius experience. Do you have any?

Once again:

• "I am aware the battery is only cycled through 60% of it's capacity." - no the state of charge range is 40-80% which in arithmetic is a 40% range.
• "I am aware the battery contains a small portion of the transmission." - actually it is the primary function to handle transition between the two modes, normal and energy recirculate. It also absorbs energy during braking and returns it when the car needs it for acceleration or to offload the engine. The Prius transmission energy recirculate mode is like an 'overdrive' gear so the engine can turn slower. In normal operation, the traction battery hovers around 60%. In my Prius experiements I have run from 80% to 40% but it requires a forced charge, floored accelerator up a 500 ft hill. But normal operation including regenerative braking seldom exceeds +/- 5% around 60%.
• "I am aware AGM batteries are heavy. None of that has anything to do with my question." - it has everything to do with it. Weight makes the car accelerate slower, climb slower, and increases the rolling drag. Slow acceleration means the engine has to run longer, burning more fuel. Climbing slower means the engine has to run longer, again burning more fuel. Increased rolling drag means the engine has to run at a higher power setting, again burning more fuel. All of these adversely impact mileage, payload, and utility.
• "So the battery is not 200 lbs, and it's 120—whatever, again, irrelevant to my question." - then your understanding suggests profound ignorance. The 75 lb lead-acid battery is smaller but does less than the Prius traction battery, a lot less. For example, does your 12V battery help climb a hill or accelerate from a standing start or provide an overdrive gear?
• "I am asking about the size-to-capacity efficiency ratio of the battery, and in that aspect it is VERY inefficient as I just showed above." - not really, you've simply revealed an abysmal lack of knowledge about vehicle efficiency and what the Prius traction battery does.

Before, I was willing to assume a lack of skill and ignorance. But now, I see you're also selectively deaf to the facts and data . . . suggesting poor ability to learn. But I'm patient and we enjoy dissecting such posts.

Bob Wilson

ps. You might hit the "Report" link in your first post and ask that the thread go to a more appropriate forum. There you'll find historical posts detailing Prius traction battery and other functions.

 

The only person tackle the real reason why the Prius battery so much bigger than one crappy marine battery is fuzzy1.

I'll keep it as short as I can.
1 horse power = just over 745 watts. Let's keep it 745 watts for the sake of argument.
Prius electric motor is rated at 80 horse power. However in order to fully use that 80 horse power electric motor, the battery also has to supply 80 horse powers. The Prius can't. It can only supply 36 horse powers. This is why the Prius is rated at 134BHP. (The gas engine is rated at 98 horsepower in case you want to know)
What's 36horse power in electricity? 26,820watts.

We need a battery that can

1. supply nearly 27kilowatts of electricity
2. recharge at 27kilowatts during regen
3. as compact as possible
4. as light weight as possible
5. run as cool as possible without liquid cooling
6. lasts at least 150,000 miles
7. lasts at least 10 years.
8. cost as cheap as possible to manufacture
9. as fully recyclable as possible at its end of life.

Once you know the criteria for a hybrid battery, you can start eliminating all the battery technologies. First one to go is lead acid battery for reasons stated in the previous 20+ posts.
in case you've missed it, 1 lead acid battery to supply 27kw would need to discharge at over 2,000 amps, not possible. 2 batteries - over 1000 amps, still not possible. Realistically, in order for lead acid battery to last 150k miles or 10 years, it can't charge or discharge at more than 50 amps at the most. This means it would take 45 lead acid batteries wired in series to yield over 550 volts. Do you want to work with 550+ volts? I don't. I get chills just thinking about working around 201v Prius battery. You can lower the voltage down to 190v by parallel 3x15 batteries in series. I have never seen a lead acid battery with enough energy storage to be used on a hybrid car that weighs less than 10 lbs. 45 batteries at 10 lbs is 450 lbs. Current Prius battery module weighs less than 100 lbs. I'm sure 45 lead acid batteries will be at least 5 times the size of current Prius battery.
Toyota spent millions of dollars in research. They did their homework. They ruled out lead acid battery.

GM, however, used lead acid battery in First Gen EV1. The performance and reliability were lacking. They fixed it by replacing the lead acid batteries with Nickel Metal Hydride batteries, the same kind of battery as the Prius.

If you still don't understand why Prius didn't use lead acid batteries then you're beyond help.

 

The OP is a troll, simply because he cannot be bothered to Google search his simple questions for answers even if he is too ignorant to understand them. For everybody else, a pretty picture:



P.S. Troll, the energy unit you are trying to write is watt*hr, NOT watt/hr. Go back to Physics for 14 year olds and read the chapter this time.

 

OP's location is very easy to type, that's a plus.

 

I think it's hilarious the number of posts before someone pointed out the prius battery is 200V while marine battery is 12V.

 

as mentioned in my post above, they don't have equivallent power levels, or energy. The prius needs to recharge its battery fast from regen braking. IIRC this is over 20kw rate, while the agm battery can only be charged at a 0.6kw rate. If you take a agm battery that can handle the power levels that the prius requires it will be heavier than the prius battery. The marine battery also is not safe to sit inside the car. Adding that protection adds even more weight.

1) power requirements
2) ballance they wanted the battery more centered which adds the requirment that it can sit in the body
3) waranty requirements

After doing all this an agm battery would be much heavier than the prius battery. I do not know if agm would take up more or less volume if it hit all the requirements in the prius, but my guess is it would be larger. Why look at agm when lithium polymer is lower volume and lighter? When size and weight are considered lithium polymer should be compared to nimh, and cost is likely the reason the next gen prius will stick with nimh as its base battery. lithium polymer is about 1/5 the volume of agm for a given required power. AGM lead acid really doesn't compete when you look at all the requirements.

Again refer to my post above if you have questions.

 

not really a problem. you could build a 48v battery the same size with lower current, then use 48v inverters.

The problem is the power levels of agm technology, not the voltage. Nimh provides much more power, and can be charged at higher powr, for a given energy

 

Just to restate your post a little, the power density (w/kg) of lead-acid is I think, in the range of 100w/kg. So to reach the performance spec of a Prius (27 kW) we would need 270 kg of lead battery.

An EV might carry this weight and volume, but a hybrid would never be built this way.

 

To extend the life of a lead acid to near the extent of the Prius battery you can only discharge it 20%, which is half of the 40% range in the Prius.

Looks like you have found another market for old hybrid and EV batteries, which still have plenty of life in them after becoming 'dead' for electric driven cars.

 

Thank you! The source: Battery and Cell Chemistries. Battery primer.

I had a question about why the "Cylindrical" is listed as having such a higher energy volume than the Prismatic. For a single cell, I agree. The cylindrical cross-section minimizes the case overhead but it leads to unusable space when multiple cells are assembled into a higher voltage pack. So let's compare equivalent Ahr capacity, NiMH batteries in cylindrical and prismatic configurations:

• D-size NiMH rated 50A, 10 Ahr (Source: http://www.batteryspace.com/prod-specs/325.pdf)
  • six cells ~7.2 V
  • (6*37) * 37 * 60.5 = 496947 mm{3} = 496.9 cm{3}
  • 10 / 496.9 = .0201 Ahr/cm{3}
• Prius Gen I module, handles +50A, 6.5 Ahr (Source: in my spare battery pile and measurements)
  • six cells ~7.2V
  • 10" * 4.5" * .75" = 254 * 114.3 * 19.1 = 554515 mm{3} = 554.5 cm{3}
  • 6.5 / 554.5 = .0177 Ahr/cm{3}

The volume densities show ~88% difference in energy volume versus the chart suggested change of 165 / 250 ~= 66%. There are other aspects such as the 'head space' inside the Prius module is a larger portion of the volume. Also the cylindrical cell uses a thin steel outer body versus the less dense, thermoplastic case of the Prius module.

Bob Wilson

 

The OP stated that the prius battery has W/hr capacity comparable to large 12V acid lead battery.
Now I realized I don't know what W/hr is. Is he talking about Whr?

 

OP really meant wh (energy) instead of w/h which is a meaningless measure. The problem with the marine batteries is they can only sustain high power out for short bursts of time (watts out steady state is much lower per unit energy), and can not be charged fast (watts in about 2% charge rate for regen braking as the prius battery). Here nimh is a much better applictions, especially where lower weight is important.

 

I plan to restore/upgrade my 2000 Honda Insight one day, and believe me it's traction battery pack and supporting hardware is clearly inferior to the Prius.

Originally, the SoC parameters was from 85%-15%, then Honda changed it to 85%-40% like Toyota. Still, the IMA does not charge/discharge the pack in a way to promote sustainability, nor is the quality of batteries as good as Toyota's. Using heavy-duty NiMH D-cells do not cool as well.

I hear Honda is eventually going to come up with a different full hybrid system (IMA can't do EV in a practical way.) Using a manual tranmission with a hybrid has been problematical, as lugging can hurt battery life (yes, Honda has built manual transmission hybrids like mine.) CVT hybrid Hondas have fewer battery problems.

The redeeming quality of my car is it's 3-cylinder 995cc engine is super-efficient and acts like a diesel in lean burn mode...both Ford and GM are starting to build 1 liter 3-cylinder engines. So while the IMA system sucks, this 3-cylinder engine is sweet.

 

I'll try to put it in a language you can understand. You obviously understand watts. Google "watts per horsepower" returns 745.7 watts per horse power. I'll give you an example of other EV
The super bad nice person Tesla Model S P85D - Front motor 221 HP, Rear motor 470 HP, combined 691 HP 691*745.7=515,278.7watts. The Tesla engineered a battery pack that can discharge over 515kw. To put that into a 12v's prospective, it needs to discharge just under 43,000 amps. 2 marine battery would need to discharge 21,500 amps. You can keep multiplying the battery count by2 and divide the discharge amp by 2 until you come to an amount of batteries that discharges what a marine battery can easily handle. 2,048 marine batteries at a discharge rate of 42 amps each. Imagine your 2,048 group 31 marine battery. Tesla uses 7,104 18650cells in their battery pack. This means Tesla manged to put the size of group 31 marine battery into 3 and a half of these

Also remember that the marine battery example I use is at 100% depth of discharge. This would quickly kill the battery. Tesla's battery technology is 10 years newer than the Prius.

Now back to the Prius battery
As I've mentioned before Prius uses 27,000 watts of power and would require 45 marine batteries. Toyota uses 168 NiMH cells. 6 cells per module. Each module measures 11.2 x 0.8 x 4.6 inch. 28 modules total measuring 11.7 x 23.2 x 0.42 in https://techinfo.toyota.com/techInfoPortal/staticcontent/en/techinfo/html/prelogin/docs/3rdprius.pdf page 12. Toyota manged to cram almost 2 marine batteries into a single module.

I kept going on about the battery supplying power that I forgot the regen. The Prius regens at a rate of 27,000watts. This equates to 1,875 amps at 14.4V. A high output alternator puts out anywhere between 200 and 400 amps. You would need 5 to 9 high output alternators. What does 5 to 9 alternators look like?
Here's 6 alternators

Here's 12 alternators

So somewhere in between.

 

Isn't that the system in the Accord now?

 

I am pretty sure it is not. The last technical article I read indicated the Accord system is EV series-mode below 45 or 50 mph. Above that speed, it is parallel. If so, a reasonable approach.

Bob Wilson

 

In some cases, troll posts can lead to stimulating conversations among the sane. I like that.