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Charging/Battery Capacity question

Discussion in 'Roadster' started by webzombie, Apr 2, 2014.

  1. webzombie

    webzombie Member

    Joined:
    Dec 9, 2013
    Messages:
    39
    Location:
    San Jose. Roadster 1.5
    I bought my 1.5 in Dec last year... At first, I charged only using the 120V mobile charger at home and consistently got 189/190 miles (ideal). Now, I mostly charge at work with ChargePoint 208V/30A charger and get 179-181 ideal miles. About six weeks ago, I installed the HPC at home, and get only 171-174 miles.

    I read many threads on this topic.... from what I understood the reason to be is that its like filling a bucket with a slow garden hose vs. a fire hose, so, you get less when you put energy too fast into the battery pack. To test this, yesterday at work, I manually set the amperage to 208V/16A, and I got 3 extra miles - 183IM.

    Is this correct understanding? I will never get 189IM if I used the HPC at home or ChargePoint at work?
     
  2. Doug_G

    Doug_G Lead Moderator

    Joined:
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    Messages:
    15,853
    Location:
    Ottawa, Canada
    It has been my experience that the Roadster will charge slightly higher when using lower power, especially if you use 110V.

    The technical reason is simple: the charging cutoff is ultimately based on battery voltage. The charger is aiming for is a specific open circuit voltage threshold, but you can't actually see an open circuit voltage unless the circuit is, well, open! When you draw power from the battery it will read slightly low. When you're pushing power into the battery it will read slightly high.

    So what happens if you push in a lot of power, say 15 kW, the battery will read slightly higher during charging than it would if you're pushing in only 1 kW. As a result the charger will trip out just a little bit earlier at high power then it will at low power. Then when you disconnect the charger you get the actual battery voltage, and your range number is slightly different.
     
  3. dhrivnak

    dhrivnak Active Member

    Joined:
    Jan 8, 2011
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    2,150
    Location:
    NE Tennessee
    Yes it is common for the VDS to show a few more miles if you charge at a lower rate. I typically get an extra 5 miles if I charge from a 120 outlet. Doug_G is right on the money as to why. If you charge at say 30-40 amps rather than the full 70 amps you will gain a few miles back AND use slightly less net energy as when you charge at the higher rate the AC to cool the battery tends to com on more often.
     
  4. Chris Lockfort

    Chris Lockfort 2008 Roadster 1.5 + 2015 Zero SR

    Joined:
    Dec 19, 2013
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    Location:
    Palo Alto, California, United States
    There's a balance to be had between a few factors, cooling (which is also a factor of the surrounding environment), overcoming some constant losses, and resistance heat loss in places that aren't even actively cooled (relay contacts, charger-related wires...).
    Tom Saxton did a few tests with his 1.5 and posted the efficiency in Wh/ideal-mile that he gained back from a few different charging levels; a lot of the data is close enough that is should be considered error/noise, but it's pretty clear at least that some of the lower current charging rates are not as efficient. Tesla Roadster Charging Rates and Efficiency - Tom Saxton's Blog
     
  5. wiztecy

    wiztecy Active Member

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    2,722
    Location:
    Santa Cruz, California, United States
    I'm wondering if that the 110v can over time hurt the pack due to heat. On 110v it won't kick the condenser/AC on to cool the pack unless the battery pack reaches 37-40 degress C. When the ambient temp outside in the spring is around 75 and I come home from work, my pack is at 31C, my condenser/ac kicks on only to drop the pack down below 26C within 15 mins. After that it doesn't run nor have a need to run if I chose to do a full charge. If you charge off of 110v, your heating the pack and in essence giving it a warm bath which they don't like in this example, and the pack would stay at 31C, possibly rise higher since its taking a charge.
     
  6. dhrivnak

    dhrivnak Active Member

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    if you live in a warm climate where the temperature is above 85 then I think it is likely the lack of cooling would be hard on the battery.

    As for the charge rate from Tom Saxton's finding 240v at 30-40 amps appeared to be the most efficient charge level. But I also seem t recall the differences were not large.

    I charge at 40 amps rather than 70 for several reasons:
    1) even at 40 amps I always have a full charge in the morning.
    2) 40 amps is slightly more efficient as the AC does not run much.
    3) it is easier on the power company
    4) as with most things and equipment slower normally equates to longer life.
     
  7. tomsax

    tomsax Member

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    Location:
    Sammamish, WA
    More recently I did a much larger study of about 8,000 charging sessions recorded anonymously via OVMS to develop a Roadster charge time prediction algorithm. Using the model I developed for that, I just created a graph showing how charging efficiency varies with charge rate. I've updated the charge time predictor blog entry with a new section on charge efficiency.

    Here's the graph:

    Wh-per-IM.png

    This shows that in moderate temperatures, charging efficiency increases with charge rate. There's a huge improvement between 120V/15A (1.44 kW) and 240V/24A (7.68 kW), but after that there's a much more gradual improvement with increasing charge rates.
     

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