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Variance in wall charger output

Discussion in 'The UK and Ireland' started by LEE3, Aug 16, 2019.

  1. LEE3

    LEE3 Member

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    Probably a silly question but charged the car with the wall charger on Wednesday with about 120 miles remaining in the car and was receiving around 26 mile per hour. Did the same yesterday and was receiving around 15 miles per hour with a similar remaining range on the car relative to the previous attempt.

    Does anyone know why this would vary so much when all known parameters appear equal?
     
  2. kevuwk

    kevuwk Member

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    Was it definitely 32amp both times? That does seem like 32a vs 16a
     
  3. LEE3

    LEE3 Member

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    Possibly different. Could something change the amp automatically?
     
  4. kevuwk

    kevuwk Member

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    I've not got a car yet so I'm not sure how it works but from what I remember the amps is in the bottom left corner of the screen and you can turn it up and down
     
  5. LEE3

    LEE3 Member

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    Yes I saw that but hadn’t changed anything from the day before and it was lower so I was wondering if either the car or the charger adjusts amp output based on other factors.
     
  6. Fullerene

    Fullerene Member

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    Battery charging isn't linear, depends on SoC and the BMS will charge based on temperature of battery, connectors etc. The charger advertises it's charge level to the car as described in SAE J1772 - there's an outside chance the control pilot signal PWM duty cycle has dropped due to a fault within the charger but I feel this is unlikely. Most likely car throttling the charge or some ancillary item consuming some of the charge power. Which charger do you have?
     
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  7. LEE3

    LEE3 Member

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    Tesla wall charger. I’ll keep an eye on it over the next few days.
     
  8. Fullerene

    Fullerene Member

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    TWC has a rotary dip switch that sets the advertised power level (not meant for user adjustment but by electrician during commissioning) - However the charge may vary if you are running more than one TWC, in which case they will load balance and the advertised charge will vary. Assuming you are running one then just keep an eye on it - I assume no fault messages popping up in app or on car and no fault lights on charger.

    Good luck, post back here to let us know how you get on :)
     
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  9. henleyregatta

    henleyregatta Member

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    You need to control for average-over-time vs just what the screen is saying. On my wall charger & car, the standard charging rate (for 240V, 32A charging) is 22 miles/hour.

    But at various times I've seen voltage sag as low as 230V... so that's 21 miles/hour.

    And if I'm still in the car, or (especially) if I've just got in it after leaving it for a while so the AirCon goes to max, at least 5A (as displayed) of the draw goes to service the auxilliaries.... dropping charge rate down to 15 miles/hour fairly quickly. It can be worse in the winter if the car's been cold-soaked and the battery heater needs to come on.

    But then if I leave the car, and check via the app an hour later, I'll see the average rate is back up to 21/22 miles/hour. This persists right up to about 95-97% SoC when "topping off" starts and the charge rate drops right back, shown on the Tesla as the amp rate dropping back down towards zero. That last 1% of charge takes a-g-e-s!

    In short: there can be a lot of short-term variations showing in the car as to the charge rate that are caused by lots of influences both internal to the car and to the charging infrastructure. By all means check the settings on the wall charger but also make sure you're measuring over a long period.
     
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  10. WannabeOwner

    WannabeOwner Active Member

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    have you got e.g. TeslaFi? The RAW DATA screen will tell you what the AMPs were for each [minute of! each] session

    IDK but is that likely to actually make a difference at 7kW? (Until the SOC gets into high 90%'s) [as compared to e.g. Supercharger power levels]

    Doh! I could have a look at my own TeslaFi data ... I'll report back :)

    That's definitely worth checking. TeslaFi et al would indicate if Climate was on. If the day was hot (have we had one of those recently ? :( ) then even if nothing scheduled the cabin protection could be "running". Or Sentry Mode?
     
  11. WannabeOwner

    WannabeOwner Active Member

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    TeslaFi04.jpg
     
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  12. Fullerene

    Fullerene Member

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    Sentry mode requires the normal processing on the car to be active which I believe is about 300W. Not horrendous but over a 24 hour would give 7.2kWh, approximately an hour of charging from your charge at full power. This would be about 10% of charge per day lost, which seems a little high compared to what other people are saying about the phantom power loss.

    This is most likely due to BMS cell balancing, a well known 'taking ages' thing for packs - that does actually consume a lot of power. I don't know the innards of the M3 BMS, but here's how a few of the BMSs I have worked with operate:

    Briefly as I can, a number of cells in series (they are also in parallel but let's skip that for simplicity) are charged with a voltage for the pack. The maximum voltage per cell is the voltage you really must have as a hard stop, otherwise cell damage will occur. Imagine 10 cells in series and we'll call the hard stop 4.1 volts. All cells get 41 volts charge voltage put across them and in an ideal world this would give us 4.1V per cell. In the real world some cells may have a little bit more voltage across as they don't always charge & discharge at the same rate. As they charge up we can end up with something like

    Cell 1 to 9 = 4.0V and Cell 10 = 4.1V (in reality the difference is usually low millivolts)

    with 41V applied across all cells this would start to put Cell 10 over 4.1V

    The Pack has a board in it that senses the Cell 10 is too high so it drains power away, balancing the pack and allowing the others to charge safely. As the cells start to reach the threshold of 4.1 V more of them are dumping power to avoid overcharging.

    The net effect is that it takes a long time to equalise the voltage across the cells and a lot of power is wasted in this phase of the charge as it is being dumped. A BMS that tries to recover this lost power is complicated and expensive, so in reality this balancing phase is seen as something that should be done on occasion and not all the time.

    I've no idea how Tesla managed this in their BMS, though things like this are usually take care of automatically so we're not left having something else to worry about.

    The last bit of charge gets progressively slower and power delivered by the charger vs power gained by the pack means it is not efficient compared to the lower SoC & faster bulk charge.

    Charging will get throttled back if the cell temperature gets too high (or is too low) but Tesla have water-cooled packs so you may find pre-heat or cooling kicking in which, at low-ish charge kWh rates, may end up being a high proportion of the overall power used. IF a bucket has a small hole in the bottom filling it up with a tap isn't a huge issue, filling it up with a teaspoon at a time may require much more than one buckets worth of water. (Dynamic Equilibrium)

    Final word is that the circuits in the packs that monitor the cell voltage can only handle dumping relatively small amounts of power, The balancing cables I have seen in most packs are quite thin, this really is a take it slow task and another factor in the charger power reducing as SoC increases.

    In conclusion - I think you're alright, just keep an eye on it ;)
     
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