Useable battery kWh

Discussion in 'Clarity' started by Jordan, May 31, 2018.

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  1. Jordan

    Jordan Member

    Hey all, new to EV's! I bought a Honda Clarity plug in hybrid a couple of weeks ago and am loving it. I had a question that I couldn't find an answer to so sorry if this has been answered in previous posts. The battery size is 17kWh. But how much of that do I actually charge when I plug it in from a totally empty battery? It's not all 17 right? Before I got the Clarity I was looking at the prius prime and they publish the % useable battery but I couldn't find it for the Clarity. I've been getting 50-55 miles on my battery and would like to know how much power I'm actually using for costing purposes. Thanks so much!
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  3. jdonalds

    jdonalds Well-Known Member

    We have a JuiceBox charge station which keeps records of time to charge, total energy used. The most its ever recorded was 14.75kWh. The longest charge time was 2 hours 6 minutes and 47 seconds.

    Our car has over 8,000 miles, only about 500-1000 or so were gas engine (my guess).
  4. Pushmi-Pullyu

    Pushmi-Pullyu Well-Known Member

    @ Jordan:

    Unfortunately, you are not likely to get an authoritative answer to your question. At best, you can find estimates.
    And that 14.75 kWh will be the amount pulled from the wall, including charging losses. The amount actually added to the battery pack will likely be 10-15% less due to charging losses.

    More discussion of this issue in another IEVs Forum thread:

    "Clarity PHEV Usable Battery kWh"

    Last edited: May 31, 2018
  5. Viking79

    Viking79 Well-Known Member

    For cost purposes, MPGe gives the necessary information, indirectly, but requires math. The Clarity gets 110 MPGe combined, there are 33.7 kWh per gallon equivalent (constant found on Wiki), so that is 110/33.7 = 3.26 miles/kWh. The combined range is 48 miles, so 48/3.26 = 14.72 kWh per charge (from the outlet). So what jdonalds said ;)

    It can vary some depending on conditions, etc, I think I usually see from 14 to 14.75 as well.
  6. Viking79

    Viking79 Well-Known Member

    Ps, often if you see people give mi/kWh figures they see a value that the car gives them, not what it actually takes to charge, so these values will be a lot higher than numbers like the 3.26 mi/kWh from the EPA rating.
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  8. Pushmi-Pullyu

    Pushmi-Pullyu Well-Known Member

    Yes, the miles/kWh rating is usually figured by use of energy stored in the battery pack, just like MPG is figured by gasoline stored in a gasmobile's gas tank. Miles/kWh does not include charging losses, just as MPG does not include refinery losses.

    Both ways of figuring energy use -- MPGe, which (confusingly) includes charging losses, and miles/kWh, which does not -- are informative, but one must be careful not to mix them up! Figuring the energy usage from the wall will tell you how much you have to pay for on your electric bill, but won't tell you how efficient the car is in terms of miles per kWh using energy stored in the battery pack.

    The term "usable capacity" does not include charging losses. It refers to the amount of energy actually stored in the battery pack.

  9. Viking79

    Viking79 Well-Known Member

    Mi/kWh in the car is somewhat meaningless other than while driving as the car might not know efficiency of a particular charger, however, the only figure that really matters in the end is the total energy used by the car, including charging losses.

    You can calculate mi/kWh with or without charging losses. As I show in calculations MPGe and mi/kWh are only different units. MPGe shows that EVs use less thermal energy than gas cars in most cases.
  10. Viking79

    Viking79 Well-Known Member

    PS, I agree completely, but read what the OP wrote, they were concerned with charging costs.

    In the end "usable capacity" is not at all necessary for end user to know. In fact is a bit like HP ratings in the US on an engine taken at the crankshaft. It doesn't tell you the entire picture (what is HP at wheels? ).

    If people knew how much HP was lost in an automatic they probably wouldn't have taken off like they did. This analogy is a bit weak, but any necessary information for an EV can be taken from the wall figures (for Clarity 110 MPGe/3.26 mi/kWh leading me to 14.7 kWh from the wall).

    The wall figures tell me how long it will take to charge, how much it is costing to drive, how large of PV solar setup to charge it, how it compares to other vehicles for energy usage etc.
    KentuckyKen likes this.
  11. Pushmi-Pullyu

    Pushmi-Pullyu Well-Known Member

    Well that's true, the OP does end with " much power I'm actually using for costing purposes." So his question addresses both: Usable kWh and kWh drawn from the wall, which will actually be two different figures.

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  13. M.M.

    M.M. Active Member

    TL;DR version: Around 70% until the ICE comes on, 77% if you count the two bars/10% that is reserved but usable to assist the ICE.

    This isn't accurate; the car carries its own charger, so it more or less does know what its efficiency is, and it isn't going to change measurably over the life of the car. All modern EVs carry an AC/DC converter onboard, so the power that comes out of any Level 1 or Level 2 charger is just AC directly from the grid. (DC fast chargers like in Teslas are a different case.)

    I had my Siemens Versacharge open, for example, and you can see quite clearly that there's just a contactor and some safety electronics inside, nothing in the current path. Regardless, though, AC/DC converters are extremely efficient these days--I'd expect 98% or higher. The charging efficiency of the battery, however, is much worse--probably in the 85-90% range.

    This, on the other hand, is absolutely true, and that's why the MPGe measurement takes that into account.

    Incorrect. The MPGe figure on cars does include charging losses. To quote from Wikipedia:
    So the MPGe figures are indeed already factoring in the onboard charger inefficiency and the charge inefficiency in the battery. Your refinery example would be the efficiency of the power plant and transmission lines, not the charger. So the MPGe number is the most useful number when it comes to figuring out how much money you'll spend, or how many miles you can drive for each kWh that comes out of the grid and goes into your car. That doesn't, however, actually get you to the usable % of the battery, which was the original question.

    It so happens that I recently discharged my Clarity to 2% (probably 2.0%; the A/C ran for several minutes after the app first showed 2% SOC, and I turned it off immediately after the ICE cycled on), then charged it from there to 100% with a high-precision Fluke 435-II power monitor hooked to the output of the charger (that is, I dismantled the charger and hooked the voltage and current probes up downstream of the charger's contactor, so literally the only losses other than what's inside the car will be from resistance in the cable going to the car). The meter calculates power at the waveform level, so the energy readings should be extremely accurate.

    Per that, the car absorbed 15.343kWh of energy to get from 2% to 100%. That would be 15.656kWh to get from 0% to 100%. Keep in mind that the car will start the ICE at 9-10% and tries to maintain about that state of charge when it's forced itself into hybrid mode; you will generally only get it below there if you're either driving up a long hill or leave it in park for half an hour with the climate control running, meaning that under normal circumstances an "empty" battery (two bars on the dash, ICE had just started running) would need around 14.1kWh from the wall to charge fully. Those numbers are before charger and battery inefficiencies, so the MPGe numbers line up.

    As for the original question, we don't know exactly what the AC/DC converter efficiency is or what the battery's charge efficiency is, but we can make an educated guess. I'm sure people have good numbers for other automotive batteries, but I do have access to publicly-available analysis (not published yet, but soon will be) of a 2-year-old industrial Tesla system that uses lithium-ion battery packs similar to automotive batteries, a high-efficiency industrial charger, and should have similar efficiencies. That system was measured at 83.9% charge efficiency for high-throughput events (discharge efficiency is much higher), and I would expect the Clarity to be relatively close to that.

    That would put the total available capacity of the battery at 13.15kWh, or 77% of rated 17kWh total
    . When you factor in that the ICE kicks in at around 10% SOC, that would put usable EV-range capacity at 11.84kWh, almost exactly 70% of rated capacity which sounds just about right for a plug-in hybrid.

    Battery charge efficiency might move those number around a bit, but it's going to be pretty close. Also keep in mind that lithium-ion batteries are much more efficient going the other way, so you probably get 97% or more of that energy back out at the wheels.
    insightman, KentuckyKen and Sandroad like this.
  14. Jordan

    Jordan Member

    Hi all, thanks so much for all this information. Some of it went over my head but I think I understand the concept. Let's take 14.75 as the amount I'm going to pay to "fill up" my battery. And I've been getting about 55 miles on a full charge. So that means I get 0.27kWh/mile. At my current summer rate of 12 cents, that means I'm paying 3.24 cents every mile I drive. With gas, at my local 2.80/gallon and the EPA rating of 40 (I've only used 2.5 gallons of gas so I don't really know my true MPG yet) that means I pay 7 cents every mile I drive on gas. Those numbers make sense right? One poster used the EPA's MPGe number but that doesn't reflect what I actually have been getting right? Understanding that in the winter it sounds like I won't get as many miles and that number will change. Am I understanding it correctly?

    Editing to add that yes, I guess I don't really care what my useable battery amount is. What I really care about is how many kWh it's going to take to fill the battery since it sounds like it's a little more than what the battery is going to hold due to inefficiencies.
    Last edited: Jun 1, 2018
  15. M.M.

    M.M. Active Member

    Yes, your math and logic are all sound, with one correction:

    If my calculations are correct, it should actually take closer to 14.1kWh to charge from when the ICE comes on (2 bars) to full, so if you're really getting 55 miles at the point it switches to hybrid mode, you should calculate your cost based on that.

    Measurements of greater than 14.1kWh should, I think, be due to people who due to where they live (for example, up a hill) drained the battery below 10% before plugging in, so the additional 0.6kWh would be "spent" increasing fuel efficiency while in hybrid mode, not as part of the EV mileage calculation.
  16. Jordan

    Jordan Member

    Awesome, thanks for the help!
  17. Proteus

    Proteus New Member

    Beware the fact that Honda rates it's Clarity plug-in at 47 mi per full charge. If you are getting, say, a maximum of 55 mi/chg in Summer, in order to average 47 you may get a minimum of "x" in Winter, where: (x + 55)/2 = 47.
  18. Pushmi-Pullyu

    Pushmi-Pullyu Well-Known Member

    That's exactly what I said. I said the miles/kWh figure doesn't include charging losses, but the MPGe figure does include those losses.

    Please read what I wrote more carefully next time, before venturing to "correct" my statements.

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