Questions about normal Prius & PHEV Prius HV batteries

Normal Prius has 500 Wh of usable energy: 28*7.2V*6.5Ah*0.4. The 0.4 corresponds to the allowed 40%-80% SoC range. Of course, most people run out of gas at 60% SoC and can only go ~1 mile.

PHV Prius with 4.4 kWh, at 80% depth of discharge, is 3.32 kWh, and 14 mile range under the assumption of 240 Wh/mile.

 

There are three battery packs: the main pack, and two sub packs. The two sub packs will be drained first, then the main pack. It is not clear if the depth of discharge is allowed to be as much for the main pack as it is for the two sub packs. The sub packs will not be recharged by regen. If drained twice per work day, that is 5,000 cycles over a 10 year period, probably the worst case scenario for the batteries. Having two sub packs is probably a choice to allow for fault tolerance (car still runs if a sub pack goes bad) and better battery management (avoids having cells in parallel).

 

I think 80% range of discharge is too high. According to the European test cycle, the estimated electric range of the Chevy Volt is 51.6 miles (83km). The Volt uses 65% of it's 16 kWh pack. So, it goes 51.6 miles using 10.4 kWh which works out to 4.96 miles per kWh battery to wheel or 201.5 Wh per mile.

Toyota has stated that they are targeting the PiP to get 14.3 miles (23km) of estimated battery range on the same test cycle. Obviously, the Volt cannot be more EV efficient than the much lighter PiP...... so 14.3 miles at 201.5 Wh per mile is 2.88 kWh which is 65% of the 4.4 kWh pack.

I'm even willing to believe 185 Wh per mile for 60% of the pack since a smaller battery needs a smaller discharge range since it is likely to be recharged more often and will generally be stressed harder and the PiP's lighter weight should give it some efficiency advantage.

 

Mitsubishi and Nissan allow 80%, from what I've read. Anyway, range depends tremendously based on speed, accelerations, elevation delta. If somebody wants to bring me a PHV, I can put it through its paces and tell you exactly the Wh/mile average, depth of discharge, and configuration of the lithium pack. Mainly, this discussion was to address Grump's question, of how does 1.3 kWh get you 2 miles range, while 4.4 kWh gets you 15 miles (rather than 7).

 

Also note that you are comparing NiMH to li-ion. And if I'm correct US model of Prius V uses same 1.3 kWh NiMH, but in Europe/Japan Prius+ uses 1 kWh Li-ion (don't know if the EV range is the same but I'm guessing it is).

 

I'd bet that Toyota's lithium pack production capacity is more a limiting factor than their confidence in the tech.

 

Another factor is that the extra battery packs can (do?) have a BMS system that keeps the modules balanced. This would extend the pack life under deep discharge usage.

The non PIP battery does NOT have any mechanism for maintaining cell or module balance and this is the usual cause of module failure (normally well after the warranty expires).

JeffD

 

Thanks for the thread, Grumpy. Other than cost (which no, I never expect to make up in fuel savings), my big concern about buying a PIP is the longevity of the li-on battery, given all the contradictory opinions and uncertainty about long-term life in autos, compared to the known robustness of the NiMh batteries; especially since Toyota itself seems to have had doubts. Hopefully, after 3 or more years of testing, they are now fairly comfortable with what they are about to put on the market, but more expert opinions would certainly be welcome.

 

That was true for the prototype PiP. The production model has 3 sub packs, making the total of 4. It can regen to all the packs.

 

Allow me to introduce you to door #3; they're both correctly engineered given the way the computer uses their batteries. The less the depth of discharge, the more charges they get. I'll borrow a site of usb's to demonstrate: http://sites.google.com/site/usbseawolf2000/nimh-cycles-by-depth-of-discharge

The original Prius is designed to charge and discharge the battery in the course of normal use; you stop at a stoplight and that's a charge cycle, you start again, and it's a discharge cycle. You only use about 5% of the battery each time this happens, but it happens every mile or more. So the original Prius needs to handle hundreds of thousands of charge cycles. Worst case is about a 40% depth of discharge, which is limited to that point to make sure that even frequent stop-and-go traffic draining the battery, or frequent mountain descents charging the bettery, won't cut down the life of the battery to be less than the life of the car.

In the PHV, I don't know the exact mechanism; it sounded like in the trial vehicles, it had the same philosophy for one of the three sub-packs (which got used in CS mode), while the other two were only used for CD mode, and were discharged more deeply. This means those other two packs won't last for as many cycles, but you're inherently limited to how many times you can charge them anyway because of the time it takes both to charge them, and to drive and use up that charge. There are probably assumptions that the owner won't run more than 2 or 3 cycles on them per day - so a cab driver like you might be in trouble if you quick-charge the battery between every fare or two. But that's highly unlikely, given that it'd eat into your time, and charging stations aren't that ubiquitous yet.

For the final vehicle, I don't know if they keep the HV and PHV battery packs separate still, but the idea is the same; they can get more miles out of the pack because they expect the pack to only be completely charged and discharged once or twice per day, and it's just not feasible to do it more often. With the smaller pack, it would be possible to get maybe 10 times more charge cycles on the battery, so 10 or 20 per day, and the battery has to be able to handle that many more cycles during its lifetime.

Basically, the more battery you have, the greater depth of discharge you can use; as an extreme example, imagine a battery that could propel you for 200,000 miles on a single charge. Most people would probably be OK if they could never recharge it, and it would be "over-engineered" if they got a second or third charge into it. So that battery could use 100% depth of discharge - or 120%, for that matter. But if I give you a battery that can only propel you for 1 mile, you need to charge it 200,000 times to get the same EV range, so it needs to have a very shallow depth of discharge - like 5% to charge that many times. So the battery's true capacity is more like 20 miles, even though only 1 mile is available due to the expected pattern of usage.

 

Grumpy,
You are not thick, your questions are valid and worries in place. The exact mechanism of PiP batteries strategies is still unknown. We have to wait to more details from Toyota.

 

These concerns are the main reason the cars won't be charging themselves up while running around. You'll have a deep discharge from full through the charge depleting mode, then it will sustain charge in a narrow band for the "normal" hybrid operation.

The packs could not take dozens of full charge discharge cycles a day. In the narrow charge sustaining band however they can take hundreds.

 

That mean in EPA combined cycle, PIP will get about 10 miles (using 73 miles @ 21kwh for Leaf).

 

This paper used the automotive grade Lithium battery manufactured by Panasonic. I believe it will be in PiP as well as in Tesla Model S.

It is a true automotive grade battery. The standard definition of the battery's end of life is 80% of the original capacity. Lithium NCA cells will have 82% capacity left after 5,250 cycles. At the end of it's life, it should get 5,500 to 6,000 cycles. If you use one full cycle (~80% DOD) every day, it'll last 15 to 16 years.

As for reference, Volt's battery is expected to last 8 years and the end of life is 70% of the capacity.

This work encompasses two primary objectives. The first is to understand the chief mechanisms of degradation in lithium-ion batteries using a LiNiCoAlO2 -based positive electrode LNCAO and a graphite negative electrode that are cycled under typical HEV conditions high rate and deep depth of discharge, especially for plug-in HEVs. The second goal is to document a quantitative database of capacity fade and impedance rise with cycle number through accelerated testing for extensive cycles representative of HEV application. These data will enable high fidelity, model-based cycle-life prediction under dynamic HEV conditions...

...cycled nonintrusively at high power 5C rate and elevated temperature 40°C. The aged cells were characterized at prescribed cycle numbers up to 5250 cycles by a three-electrode cell, capacity measurement, and electrochemical impedance spectroscopy EIS. Excellent cyclability of these cells under typical hybrid-electric vehicle conditions is demonstrated by 18% capacity fade after 5250 cycles...​

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