EV tech explained: Why do EVs restrict the amount of battery capacity that can be used for driving?

Tesla-Battery-Charged-EVs-AdobeStock

All EVs that use lithium-ion batteries are designed so that their full capacity can’t be used in driving – a few kWh are always left over as a buffer. Sometimes you’ll find this figure in spec sheets, which might (for example) quote capacity as “60 kWh (55 usable)” or some such. What is the reason for this?

There are three main reasons why you shouldn’t use all of the energy stored in the typical Li-ion EV battery. The first is that cycle life increases dramatically as depth of discharge (DoD) decreases. Nailing down the precise ratio of DoD to cycle life is difficult, as it depends on many factors, some beyond the control of the manufacturer. One illustrative example is from the Winston Battery 100 Ah LFP cell spec sheet, which claims a lifespan of 5,000 cycles at an 80% DoD or 7,000 cycles at a 70% DoD. That’s 90,000 more Ah delivered over the life of the cell for a mere 10 percentage point decrease in DoD!

The second reason is that a higher capacity pack artificially limited to a lower delivered capacity will be stressed at a lower C rate (a measure of the discharged rate relative to a battery’s maximum capacity) when delivering any given amount of current. For example, a 125 Ah battery limited to 80% DoD is equivalent to a 100 Ah battery drained to 100% DoD, but at, say, 250 A, the 125 Ah pack is operating at 2 C compared to 2.5 C for the 100 Ah pack. Since the internal resistance of any type of battery tends to go down as its capacity goes up, a larger-capacity pack will waste less energy as heat at the same discharge current.

Tesla Batteries 18650 Li-ion Cells

The third reason is that restricting the DoD window to, say, 0-80% (or 10-90%), runs the pack in the flattest part of its discharge curve, and that can make for a better overall driving experience (and allows for an emergency “limp home” mode, as there is something left in the tank, so to speak).

This practice of restricting the DoD window should not be confused with EVs that allow for battery pack upgrades. For example, some of the 60 and 70 kWh versions of the Tesla Model S can be “upgraded” to the 75 kWh version using software to unlock the extra capacity. After a Model S is upgraded, it continues to safely employ the required Li-ion battery capacity buffers. However, it does raise an interesting question: Will the battery pack in a 60 kWh Model S that is never upgraded have a useful life that greatly exceeds the normal life expectancy?

SEE ALSO: A closer look at how batteries fail

 

  • Bruno Bevilacqua

    So you’re saying that people who actually buy the 75kWh Tesla to begin with are left unprotected? Is that, seriously, your argument?

    • http://ChargedEVs.com/ Christian Ruoff

      Hi Bruno, Thanks for the note.

      No, of course that’s not what Jeff was trying to say. I’ve updated the last paragraph to (hopefully) be more clear.

      Regards.

  • Eco Logical

    Good article! One more thing, a larger (75 ~ 100 kWh) battery isn’t deep cycled nearly as often (maybe weekly or monthly instead of daily) thus dramatically increasing it’s calendar life.

  • jstack6

    Cooling and limiting the top and low end of the power can make lithium battery packs last for 20 to 30 years or many more. But cooling is the biggest key.
    I had a LEAF and only charge to 80% and at a slow 5 kW rate for over a year and still lost over 10% capacity. I never even Fast Charged. It’s the cooling that is the biggest key to long life. along with not over charging or discharging them too low.

  • Vincent Wolf

    This will all be a moot point when they finally finalize Lithium or Aluminum-air batteries or plastic supercapacitors. Range will be so high that even if they lose 50% capacity they will still have thousands of miles of range. So you just charge up at home and not worry about range anxiety at all. Supercharging networks then become just an emergency or ‘bootsting’ network nothing more.

    Lithium-air has 15 times the capacity of LiON (proven in the labs), Aluminum-air at least 12, and supercapacitors up to 50 times the capacity of LiON.

    Thus a Tesla P100D with 315 miles range, factored up 15 times would conceivably have a range of 4725 miles. A SEV (a supercapacitor powered EV) could potentially have a range of over 15,000 miles.

    Get the picture? Super charging is all fine and dandy for the short term but in the end it wont’ be necessary for most folks–they will just charge at home (or specially equipped apartment charging locations) using solar and when full you just forget about it for a couple of months!

    For some retired people like myself you just charge at home and forget about it until NEXT year!!

  • http://www.energylabusa.com simon

    70% of people in metro areas cannot charge from home – I think a EV filling station is required #EVHUB