Research Reveals Regional And Drive-Cycle Impact On PHEV Battery Degradation

1 month ago by Mark Kane 26

2017 Chevrolet Volt

2017 Chevrolet Volt

A research team at Carnegie Mellon University (CMU) has pubished an interesting article concerning battery degradation in PHEVs – “Plug-in hybrid electric vehicle LiFePO4 battery life implications of thermal management, driving conditions, and regional climate.”

2017 Ford Fusion Energi

2017 Ford Fusion Energi

According to the highlights, there are durability disproportions between various scenarios that are fairly large.

For example without battery cooling, aggressive driving can cut battery life (to 80% of initial capacity) by 2/3rds in hot regions.

In general, batteries should last 73–94% longer in mild-weather regions than hot regions, while air cooling can increase life by a factor of 1.5 to 6 – a huge range to be sure.

There is also important factor of battery size/total capacity, as smaller batteries often use more of their capacity to provide all-electric driving, thus reducing their usable lifespan more quickly (fractional discharges produce less strain on the cells).

“Battery degradation strongly depends on temperature, and many plug-in electric vehicle applications employ thermal management strategies to extend battery life. The effectiveness of thermal management depends on the design of the thermal management system as well as the battery chemistry, cell and pack design, vehicle system characteristics, and operating conditions.

We model a plug-in hybrid electric vehicle with an air-cooled battery pack composed of cylindrical LiFePO4/graphite cells and simulate the effect of thermal management, driving conditions, regional climate, and vehicle system design on battery life.

We estimate that in the absence of thermal management, aggressive driving can cut battery life by two thirds; a blended gas/electric-operation control strategy can quadruple battery life relative to an all-electric control strategy; larger battery packs can extend life by an order of magnitude relative to small packs used for all-electric operation; and batteries last 73–94% longer in mild-weather San Francisco than in hot Phoenix. Air cooling can increase battery life by a factor of 1.5–6, depending on regional climate and driving patterns. End of life criteria has a substantial effect on battery life estimates.”

Effect of air cooling on capacity fade and battery life when annual miles driven are 14,700 miles. The battery industry traditionally defines end-of-life (EOL) as the point where the battery’s energy storage capacity drops by 20% of its initial value—indicated by the solid horizontal black line on the first two charts. (a) Capacity Fade in Phoenix, comparison of air cooling vs no cooling for two drive cycles. (b) Capacity Fade in Phoenix and San Francisco, using GPS data. The comparison of air-cooling vs. no cooling is provided for two cities (c) Improvement in battery life by air-cooling for different cases simulated. Credit: ACS, Yuksel et al. source: Journal of Power Sources via Green Car Congress

Effect of air cooling on capacity fade and battery life when annual miles driven are 14,700 miles. The battery industry traditionally defines end-of-life (EOL) as the point where the battery’s energy storage capacity drops by 20% of its initial value—indicated by the solid horizontal black line on the first two charts. (a) Capacity Fade in Phoenix, comparison of air cooling vs no cooling for two drive cycles. (b) Capacity Fade in Phoenix and San Francisco, using GPS data. The comparison of air-cooling vs. no cooling is provided for two cities (c) Improvement in battery life by air-cooling for different cases simulated. Credit: ACS, Yuksel et al. source: Journal of Power Sources via Green Car Congress

source: Journal of Power Sources via Green Car Congress

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26 responses to "Research Reveals Regional And Drive-Cycle Impact On PHEV Battery Degradation"

  1. David Murray says:

    I thought most modern PHEVs used liquid cooling. I think the Ford products are air cooled. What PHEV out there has no cooling at all?

    1. Texas FFE says:

      The battery packs on both my Focus Electric and Fusion Energi are water cooled.

      1. Texas FFE says:

        With active temperature management.

        1. SJC says:

          2013 Ford Fusion Energi

          Manufacturer: Panasonic Rated Pack Energy/Capacity: 7.6 kWh / 26.0 Ah
          Type: Lithium-ion Min/Max Cell Voltage: 3.00/4.20 V

          Number of Cells: 84 Pack weight: 272 lb
          Nominal Cell/System Voltage: 3.7/310.8 V Thermal Management: Active – Forced Air

          http://energy.gov/sites/prod/files/2015/02/f19/batteryFusion3776.pdf

          1. Texas FFE says:

            Okay I was wrong about the Fusion Energi but the owners manual of my 2013 Focus Electric states that the high voltage battery packs have an active liquid heating and cooling system.

            1. SJC says:

              Maybe they do, could be this article was wrong. This is the problem with the Net, not everything posted is true.

              1. Texas FFE says:

                The owners manual of my 2015 Fusion Energi also says the high voltage battery has forced air cooling.

    2. ¯\_(ツ)_/¯ sven says:

      FWIW, the Prius Prime uses air cooling.

  2. Alaa says:

    Who cares about the LiFePO4 batteries? As far as I know they have a very poor energy density! And as for the other types, well we have many on youtube that drove Teslas to gives us a good idea about that degradation. It hardly is worth mentioning.

    1. HVACman says:

      BEV’s require high energy density cells. PHEV’s require high-power-density cells, with energy density a secondary concern. It takes a different cell chemistry for a relatively-small 18 kWh battery to deliver up to 120 kW instantaneously (as in the Volt) as opposed to a 60-100 kWh battery.

    2. q says:

      I feel like, despite LiFePO4’s somewhat lower energy density, it’s relevant because of the metals supply chain. As reported before on this very site, cobalt/maganese supply may well be the long pole to getting lithium ion batteries built at scale. Lithium iron phosphate uses only common materials (iron and sulfur IIRC) as the cathode, sidestepping the issue.

      Maybe not important now, but 3-5 years down the line…

    3. Viking79 says:

      LiFePo4 have low energy density, but much higher power output. I think they also have longer cycle life. Great for commercial vehicles like busses and taxis, or if you give a higher priority to power output (like faster acceleration).

    4. R.S says:

      You can’t really use those high energy density cells on a PHEV for two reasons:

      1) Power: The Tesla 100kW pack can supply up to 560kW of power, build a 8.8kWh pack out of those cells, like in the A3 e-tron for example, and you are left with a very limited 50kW, the little Audi with its LiFePo has 75kW.

      2) Life cycle: If you get two thousand cycles out of a 100kWh Tesla with 315 miles of range, before you’D have to replace it, you end up with a very impressive 630k miles. But if you only have 38 miles of range like in the last generation Volt, you end up with a devastating 76k miles. Assuming that you would only drive 60% in electric mode, you would have to replace the battery after 125k miles, thats not acceptable.

      So if you have a long range BEV, you can go for high energy cells, but if you want a PHEV, you are poised to use a high power and high cycle life cell, which means sacrificing energy density.

      1. JeremyK says:

        As a data point, my Gen I Volt will have about 2600 cycles on the battery at the 5 year mark (in under 4 months). When new I would routinely get 38-46 miles of range in the summer. During the summer of year 4 I would get roughly 32-40 miles of range.
        This is a noticeable drop, but not unexpected as I have a 2011 (before battery chemistry tweaks and changes). The car still allows me to drive gasoline free 90%+ of the time, thanks to workplace and home charging. Great car.

  3. SparkEV says:

    I think charging causes more degradation than discharging. If it’s charged to 100% and plugged in and ambient is > 100F, even blowing air would be ambient temperature air. That’s the worst combo (high state of charge + high temp), so it would do little for saving the battery.

    This is why SparkEV recommends plugging in when the temperature is hot. It may charge to 100% (or not if you have it in delayed charging mode), but it will keep the battery cool and save it from heat related degradation. Same can’t be said about Leaf or eGolf; they’ll be cooking if plugged in at high ambient temperature.

  4. Kosh says:

    So, Elon was right about Nissan all along? 😉

    (I can def say our Leaf has lost capacity in jut 3 years).

    Best strategy, active cooled battery with large capacity, only charge to 80% most of the time.

  5. Fred Gibutr says:

    Almost worthless research. I expected better. Who does not cool their batteries? Air cooled and water cooled lithium ion batteries should have been compared.

    1. bro1999 says:

      Ford does not properly cool its PHEVs. Large numbers of both Fusion and C-Max Energi owners have reported significant (like 30-40%) degradation of their HV batteries in vehicles not even 3 years/36k miles old!

      The kicker is Ford REFUSES to acknowledge there is anything wrong with their batteries….going so far as to state the the terms on the capacity warranty are PROPRIETARY!

      So Ford can deny degradation claims all day long, and hide behind their proprietary info wall! Now that is something worth reporting on in an article, I think.

      There is currently research being done by a law firm to see if there is basis for a class action lawsuit….that is the only way Ford will divulge the capacity warranty conditions….via legal strong arm.

      1. ModernMarvelFan says:

        That is a first to hear… I know there has been problems reported with LEAF, Prius Plugin, but this is the first that I have heard about Ford Energi.

        Is there any study to back that up? I am curious.

        Thanks.

      2. Texas FFE says:

        Where did all this Ford bashing come from? My FFE was sold over four years ago and I only have about 18% battery degradation. I doubt most people with the C-Max or the Fusion even know to measure their battery degradation.

        If you are going to post based on negative data please provide a link to the data so it can be evaluated.

        1. Texas NRG says:

          Just visit the Fusionenergi and Cmaxenergi forums for battery degradation examples with 2013s and 2014s, especially in warmer climates. The dash display shows the kWh used for each trip after you power off so the driver is reminded regularly. When new, my Fusion NRG would use 5.6-5.8 kWh before switching to ICE hybrid mode. By 3yr/36k miles it was down to 3.9-4.1 kWh in similar warm conditions and climate off. They should have used a liquid cooled battery thermal management system.

  6. jmhays says:

    Does anyone else think that this study was produced on purpose so it could be “generalized”? I’ll be within the week (hour?) that we will start seeing headlines screaming “Electric cars will fail! Battery life falls by 2/3 in less than one year”. In the current “facts are no longer needed” environment, I can see this happening.

  7. Jeff Songster says:

    Did they actually specify whether passive air cooling ala Nissan LEAF 1.0 versus active forced air ala eNV200 van from same company and same battery prismatic cells? That would be apples to apples in terms of chemistry and cell format.

    1. Pushmi-Pullyu says:

      Since the article specifies “air cooling can increase life”, apparently it means forced air cooling, as opposed to the passive cooling which Nissan uses for the Leaf.

      I’m surprised they focused on forced air cooling. So far as I know, only one mass produced PEV (Plug-in EV) uses that. Perhaps liquid cooling has too many variables to computer model easily?

  8. Pushmi-Pullyu says:

    While theoretical modeling of the sort described here — I presume they used a computer model — can be useful, we should take care not to rely overmuch on the results. Multiple factors can interact with each other, and with environmental factors not included in the computer model, in complex ways which may be difficult or impossible to predict.

    “In theory, there is no difference between theory and practice. But, in practice, there is.” — Johannes Lambertus Adriana van de Snepscheut, computer scientist and educator

  9. Bill Howland says:

    This is also the first I’ve heard about Volt Degredation. I’m curious since I just purchased a used 2012 with 50,000 miles on it, and it does seem to have a slightly less range than my 2011 had. But I’m still familiarizing myself with both of my current cars. The ELR on the other hand, with the more modern bigger battery, is somewhat more unnerving in that you are never quite sure what the car is doing. The battery must be heated – but the only way to get the heater to ‘cooperate’ is to turn the recirculate on, and the blower off, and then drive around cold and windows fogged up – so you can only do that in spurts.

    Otherwise the engine runs much to much in moderate weather when I dont want it to, as it regulates the coolant temperature between 120 and 145 deg F. Many short trips will uselessly quickly run through a tankfull of gasoline, whereas I would get along with just the battery if the car didn’t always try to outsmart me.

    This is the part about GM arrogance that I don’t like. Putting the heater controls on “MIN” in this car is useless since it will turn on the electric heater if it wants to even on this setting.

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