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IssacZachary
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Re: Extra Battery, How to Integrate with 24kWh Traction Battery?

Wed Mar 28, 2018 1:16 pm

davewill wrote:Isn't controlling the voltage the same as controlling the current? If the DCQC outputs a voltage above the battery pack's, a charging current flows. To control the charge rate, the BMS would request a particular voltage. With the add-on pack there, the DCQC has to supply more current maintain the voltage. The part I'm curious about is what happens if the BMS requests a voltage that exceeds the DCQCs current capacity because it doesn't know about the second pack. Hopefully, the DCQC would just modulate the voltage to output it's maximum current. However, it could decide it was an error for the BMS to request a voltage that exceeds the charger's capacity.

The question is where the current and voltage are being measured and where they are being regulated.

Current coming into the main bus splits into everything connected to the main bus. This not only includes the traction battery, but also accessories if the car is turned on. For an example, I've ran the A/C while charging from CHAdeMO once. The same must be true for any auxiliary range extending battery.

From what i understand the Leaf asks for current. I would guess it ramps up current until it reaches a voltage threshold, at which point it maintains that current until the voltage changes. At the same time the CHAdeMO station is communicating back to the car the amount of current it is sending. Basically the car asks and the station gives that much or less. If the car asks for 30 amps but the station can only put out 20 for whatever reason the station replies back "I'm sending you 20" and sends just that.
2013 SL SOLD :cry:
2013 Toyota Avalon Hybrid CURRENT

lorenfb
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Re: Extra Battery, How to Integrate with 24kWh Traction Battery?

Wed Mar 28, 2018 2:01 pm

wwhitney wrote:
lorenfb wrote:So you're implying that a DCQC device is a current source versus a voltage source?

I guess that is the implication. I certainly believe that the only parts in the Leaf that participate in a Chademo charge session are the BMS computer and the battery itself, I don't believe there are any power electronics on the Leaf involved.

lorenfb wrote:If that's true, how would the current be divided between
Mux's add-on battery and the Leaf's actual battery which is controlled by the Leaf's BMS? And how would the add-on battery ever become
fully charged, e.g. the required current is unknown?

Well, I haven't been following the details of this thread, but I presume the new pack is installed in parallel with the existing pack. In which case they would necessarily be at the same voltage at all times. The current from the Chademo station would divide between the packs according to their resistances. That would presumably be related to pack size: e.g. if during charging the add-on pack climbs the voltage curve twice as fast (for a given charging current) as the original pack (if the add-on pack had half the capacity), then the requirement that the two packs maintain the same voltage would imply that 1/3 of the current would go to the add-on pack, and 2/3 of the current would go to the original pack.

Cheers, Wayne


1. Yes, his pack is in parallel and at the same voltage.

2. If the input is really a current source and current divides, how would one ever be assured that the add-on battery was fully charged
when the Leaf's BMS reduced the charging current to a very low value. Since the Leaf's battery is larger, its series resistance would
be lower (an assumption) resulting in a lower charging voltage for the second battery, thereby potentially resulting in an incomplete
charge for the add-on once the Leaf's BMS stopped the charging or reduced it to a very low level. Furthermore, if the Leaf's BMS
found that the charging current was less or greater than desired at any point in time, it would change the current affecting the add-on.

3. We are both attempting to determine how the DCQC might function with two batteries connected in parallel and where one battery is controlling the total charging current. Unless you can document that the DCQC is a current source, the charging method whether using
L1/L2 or DCQC is the same - using a voltage source input and pulse width modulating the input voltage to achieve a desired charging current.
Obviously the DCQC device becomes much more costly for the electric company providing the charging station and they're no longer providing a simple service as usual which is just supplying a simple voltage to its users.
Last edited by lorenfb on Wed Mar 28, 2018 2:17 pm, edited 2 times in total.
Leaf SL MY 9/13: 70K miles, 49 Ahrs, 5.1 miles/kWh (average), Hx=70, SOH=78, L2 charges to 100% > 1000, max battery temp < 95F (35C), min discharge point > 20 Ahrs

lorenfb
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Re: Extra Battery, How to Integrate with 24kWh Traction Battery?

Wed Mar 28, 2018 2:09 pm

IssacZachary wrote:
davewill wrote:Isn't controlling the voltage the same as controlling the current? If the DCQC outputs a voltage above the battery pack's, a charging current flows. To control the charge rate, the BMS would request a particular voltage.


Or use the DCQC's standard output voltage (~400V) and pulse it (switch it on/off in the BMS or possibly in the DCQC itself) to achieve the
desired average charging current just as it does when a L1/L2 charging occurs, since the BMS is constantly measuring the charging
current. That's the most efficient method from an energy transfer process.
Last edited by lorenfb on Wed Mar 28, 2018 2:23 pm, edited 1 time in total.
Leaf SL MY 9/13: 70K miles, 49 Ahrs, 5.1 miles/kWh (average), Hx=70, SOH=78, L2 charges to 100% > 1000, max battery temp < 95F (35C), min discharge point > 20 Ahrs

wwhitney
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Re: Extra Battery, How to Integrate with 24kWh Traction Battery?

Wed Mar 28, 2018 2:19 pm

lorenfb wrote:2. If the input is really a current source and current divides, how would one ever be assured that the add-on battery was fully charged
when the Leaf's BMS reduced the charging current to a very low value.

If the chemistry is the same as the original pack (which I assume), then the way the voltage reflects state of charge should be the same for both packs. So if the BMS throttles charging current using a voltage-based estimate of the state of charge of the packs, the add-on pack should be at the same state of charge as the main pack.

Look at it this way: the Leaf BMS is going to run the packs in a operating range from some V_max down to some V_min. Whatever capacity the add-on pack has between V_max and V_min is the extra capacity you are going to get from the add-on pack. If due to different thermal environments or for some other reason the add-on pack would optimally be charged to V_max + 10 mV, then that small portion of the potential capacity of the add-on pack will not be realized.

Cheers, Wayne

jkenny23
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Re: Extra Battery, How to Integrate with 24kWh Traction Battery?

Wed Mar 28, 2018 2:26 pm

There's several threads about it in detail, here's a video with some more detail, in the beginning he mentions the car asking for a certain current which was more than the charger can output so it delivered the max it could instead (25kW): https://youtu.be/ddKkffUKZWE

lorenfb
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Re: Extra Battery, How to Integrate with 24kWh Traction Battery?

Wed Mar 28, 2018 2:31 pm

wwhitney wrote:
lorenfb wrote:2. If the input is really a current source and current divides, how would one ever be assured that the add-on battery was fully charged
when the Leaf's BMS reduced the charging current to a very low value.


wwhitney wrote:If the chemistry is the same as the original pack (which I assume), then the way the voltage reflects state of charge should be the same for both packs. So if the BMS throttles charging current using a voltage-based estimate of the state of charge of the packs, the add-on pack should be at the same state of charge as the main pack.


Assuming that the chemistry is the same is not realistic. Read how Mux sourced the add-on battery.

wwhitney wrote:Look at it this way: the Leaf BMS is going to run the packs in a operating range from some V_max down to some V_min. Whatever capacity the add-on pack has between V_max and V_min is the extra capacity you are going to get from the add-on pack. If due to different thermal environments or for some other reason the add-on pack would optimally be charged to V_max + 10 mV, then that small portion of the potential capacity of the add-on pack will not be realized.

Cheers, Wayne


I totally agree! Let's get back to determining how the DCQC device functions when connected to the Leaf. Then we can discuss the details
of how the add-on battery gets fully charged during a standard Leaf QC. I may be wrong, as I always assumed a "dumb" DCQC that only
supplied a fixed and constant voltage with fold-back current limiting as the power output of the DCQC exceeded a certain level.
Leaf SL MY 9/13: 70K miles, 49 Ahrs, 5.1 miles/kWh (average), Hx=70, SOH=78, L2 charges to 100% > 1000, max battery temp < 95F (35C), min discharge point > 20 Ahrs

jkenny23
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Re: Extra Battery, How to Integrate with 24kWh Traction Battery?

Wed Mar 28, 2018 2:51 pm

I believe I posted this earlier but since the discussion has come up again, I did a small scale experiment a while back charging/discharging 2 parallel different capacity/chemistry cells and found the current sharing depends mostly internal resistance and capacity, but shifts depending on the discharge curve as mux has found: https://secondlifestorage.com/t-Differe ... 2#pid25652

wwhitney
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Re: Extra Battery, How to Integrate with 24kWh Traction Battery?

Wed Mar 28, 2018 2:58 pm

lorenfb wrote:Assuming that the chemistry is the same is not realistic. Read how Mux sourced the add-on battery.

OK, as I said I haven't been following. If the V_min / V_max of the add_on pack doesn't match the original pack, that's problematic. If the new pack has a range that is strictly bigger on both sides, then the only effect is that the add on pack is used at less than its full capacity. If the new pack has a higher V_min or a lower V_max, then the Leaf BMS may end up damaging the add on pack if you exceed those limits during your use of the combined pack.

lorenfb wrote:I may be wrong, as I always assumed a "dumb" DCQC that only
supplied a fixed and constant voltage with fold-back current limiting as the power output of the DCQC exceeded a certain level.

As I and others have said, that is not how DCQC works. All of the power electronics are in the DCQC station; fundamentally all the onboard BMS does is ask for a particular current level. There may also be communication about maximum and minimum allowable voltages, I don't know.

Cheers, Wayne

lorenfb
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Re: Extra Battery, How to Integrate with 24kWh Traction Battery?

Wed Mar 28, 2018 3:34 pm

lorenfb wrote:
I totally agree! Let's get back to determining how the DCQC device functions when connected to the Leaf. Then we can discuss the details
of how the add-on battery gets fully charged during a standard Leaf QC. I may be wrong, as I always assumed a "dumb" DCQC that only
supplied a fixed and constant voltage with fold-back current limiting as the power output of the DCQC exceeded a certain level.


Additionally since the Leaf's BMS already has the current monitoring function, as do all BEVs' BMSs, it's superfluous to also have
the DCQC do that function, i.e. more costly for the DCQC device than just having the Leaf's BMS switch the DC voltage (~ 400V) on/off
from the DCQC via communications versus actually encoding & sending an actual charging current value. The battery will integrate
that on/off voltage from the DCQC, resulting in the desired charging current. That would result in a simpler and more reliable system.
From the Leaf's charging function, it would be basically the same as when charging via L1/L2.

It would be valuable if there were a spec sheet on a DCFC device that would explicitly define its functions. Maybe someone can link to one.
Leaf SL MY 9/13: 70K miles, 49 Ahrs, 5.1 miles/kWh (average), Hx=70, SOH=78, L2 charges to 100% > 1000, max battery temp < 95F (35C), min discharge point > 20 Ahrs

jkenny23
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Re: Extra Battery, How to Integrate with 24kWh Traction Battery?

Wed Mar 28, 2018 3:46 pm

lorenfb wrote:Additionally since the Leaf's BMS already has the current monitoring function, as do all BEVs' BMSs, it's superfluous to also have
the DCQC do that function, i.e. more costly for the DCQC device than just having the Leaf's BMS switch the DC voltage (~ 400V) on/off
from the DCQC via communications versus actually encoding & sending an actual charging current value. The battery will integrate
that on/off voltage from the DCQC, resulting in the desired charging current. That would result in a simpler and more reliable system.
From the Leaf's charging function, it would be basically the same as when charging via L1/L2.

It would be valuable if there were a spec sheet on a DCFC device that would explicitly define its functions. Maybe someone can link to one.


That is an overly simplistic description of a switching power supply, and would not be practical to have "feedback" via the Leaf and the relatively low speed CAN bus that CHAdeMO uses. The DC fast charger is a very large switching power supply, which takes in (typically I believe) 3 phase AC, rectifies it to high voltage DC, then steps it down to DC near the battery voltage, and feeds it at constant current to the car's battery which is directly connected through another set of contactors. From what I gathered in the video I linked earlier, the car (BMS) requests a charging current, and the DCQC feeds that current (or a lower amount if it cannot deliver as much as was requested), and also tells the car what voltage it is outputting. The car then modifies this current request as needed to make sure the battery is in the safe operating area (limited by temp, current, voltage).

There used to be a pdf from the CHAdeMO association that described this all, but it has since been removed/restricted, and I can't find any mirrors.

Edit: Found it at the web archive: https://web.archive.org/web/20151223161 ... erface.pdf Good read and overview of how the CHAdeMO system works.

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