Mach-Lee

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Hope you guys enjoy this, I did a complete teardown and brief engineering analysis on a HVBJB. I’ve been working on this for a while, but bad timing since a bunch of other people happened to all post about it at the same time. Warning, this post will have a lot of technical jargon and engineering data! Component data and measurements for engineers is included.

First, this is part # NK4Z-10C666-C on the box sticker, which is the version for vehicles with standard range packs. The front motor connections will be unused on a RWD vehicle. Dated 23-JUN-2022 (post recall). This will not work in a GT, the connectors won’t fit. This part is designed for a peak power of 266HP/198kW/560A. The GT version is very similar but will have some larger components in places.

Description and Operation:

The High Voltage Battery Junction Box (HVBJB) is located inside the sealed battery pack, and is used to “switch” the high voltage leaving the pack on and off. This internal switch is necessary for safety. When the car is fully shut down or in an accident, the high voltage is completely shut off to the external HV cables feeding the motors and accessories. The electromechanical switching devices inside are call contactors, because they contain a set of contacts that open and close. Closed is on, and open is off. The contactors are controlled and monitored by the Battery Energy Control Module (BECM) which also lives inside the sealed battery pack. There are six contactors inside the HVBJB:
  1. Main +
  2. DCFC +
  3. DCFC –
  4. Main –
  5. Auxiliary –
  6. Precharge +
1-4 are the large white cylinders in the photos, Aux is a black box, and Precharge is a small one inside the HVBJB under the cover. Main + and – need to both be closed in order to send power to the motors, cabin heater, and A/C compressor. The two DCFC contactors are only used for DC fast charging, and must both be closed before power can flow from the DC pins in the charging port. The DCFC contactors are in series after the main contactors, so 1-4 must all be closed to DCFC, and the charging current will pass through all four. The auxiliary contactor switches power to the DC/DC converter and the onboard charger (SOBDM) which is used for AC L1/L2 charging. The precharge contactor precharges the main + busbar to pack voltage through a 24Ω resistor before the main + contactor closes. Precharge is necessary to prevent arcing of the main contactors when they close, which significantly reduces their lifespan. When the vehicle is powered up, the contactors are closed in a specific sequence. First the negative contactors close, then the precharge, then the main + closes, and finally the precharge opens to end the sequence. The DCFC contactors are externally precharged by the DC charger to match pack voltage before they are closed and charging begins.

The HVBJB also contains four fuses for each of the high voltage circuits. The fuses are replaceable. There is a current sensor (round black circle in later photos) inside the HVBJB around the positive array terminal which measures all current flowing into and out of the pack. This is connected to the BECM and is used to calculate the % SoC in the battery (coulomb counting). Last, voltage sense wires are connected to the stages after each contactor and are monitored by the BECM. This allows the BECM to monitor if the contactors have opened or closed properly, and calculate the voltage drop across each contactor.

This is a photo of the HVBJB with all components and connections labeled:
Ford Mustang Mach-E Teardown & Analysis: Base AWD HVBJB (High Voltage Battery Junction Box) - NK4Z-10C666-C HVBJB Labeled


Reference: The front of the vehicle is towards the bottom of the photo.

Acronyms in photo:

Array = Battery pack array
CONT = Contactor
ISC = Inverter System Controller (rear motor inverter)
eFAD = Electric Front Axle Drive (front motor inverter)
DCFC = Direct Current Fast Charge
DCDC = DC to DC inverter (provides 12V power)
PTC = Positive Temperature Coefficient cabin heater

You will notice the layout is somewhat symmetrical, all the positive components and connections are on the passenger side, and the negative components and connections are on the driver’s side (LHD). FYI the high voltage negative side is completely isolated from the chassis ground for safety. Negative is not ground in a high voltage system.

Ford Mustang Mach-E Teardown & Analysis: Base AWD HVBJB (High Voltage Battery Junction Box) - NK4Z-10C666-C IMG_0937

Ford Mustang Mach-E Teardown & Analysis: Base AWD HVBJB (High Voltage Battery Junction Box) - NK4Z-10C666-C IMG_0940

Ford Mustang Mach-E Teardown & Analysis: Base AWD HVBJB (High Voltage Battery Junction Box) - NK4Z-10C666-C IMG_0945


The part number on the actual part is NK48-10C666-AB. It is normal for the part number on the box and the on the actual part to be different, although they share the same base (10C666 is the base part number for a HVBJB).

The components on the top of the HVBJB use spade terminals, and can be removed in order to open the top cover. The bottom cover can also be removed to reveal the the internal wiring and busbars.

Ford Mustang Mach-E Teardown & Analysis: Base AWD HVBJB (High Voltage Battery Junction Box) - NK4Z-10C666-C IMG_0951


Cover removed. The current sensor (black donut) for battery current measurement has been taken off the positive array terminal and can be seen lower left. It has a 4-pin connector (C4815E). The white ceramic pre-charge resistor can be seen above. It is pretty hefty at 30W since a lot of power will flow through it for a brief second during precharge. The black auxiliary contactor bridges the gap in the small busbars below it when closed, and sends power to the DC/DC and SOBDM modules. The 50A fuse to the right of it is next in series. The very large 630A cube fuse bolted in is for the rear motor. The 150A front motor fuse is the largest orange one. And the last small 50A fuse is for the PTC heater and A/C compressor.

The metal between the white contactors are the busbars. It is 3mm thick copper sheet that is tin-coated. Very beefy. The large spade terminals for the array connections are 20x3mm.

Ford Mustang Mach-E Teardown & Analysis: Base AWD HVBJB (High Voltage Battery Junction Box) - NK4Z-10C666-C IMG_0953


The center circuit board is used to condense the contactor coil connections into a single 8-pin connector (C4815D) to the BECM. The contactors are ground-side switched. The yellow component is the precharge contactor, which connects the precharge resistor to the + main bus. R1 next to it is a 249Ω resistor in parallel with the precharge coil to suppress inductive kickback and increase operation speed.

Ford Mustang Mach-E Teardown & Analysis: Base AWD HVBJB (High Voltage Battery Junction Box) - NK4Z-10C666-C IMG_0992

Ford Mustang Mach-E Teardown & Analysis: Base AWD HVBJB (High Voltage Battery Junction Box) - NK4Z-10C666-C IMG_0950


Underside. You can see the bottom of the contactors and how they are bolted to the busbars. There are two small wires coming out of the white contactors, those are the wires for the control coil. When 12V is put on these wires, the contactor will close. They are not temperature sensors. There are no temp sensors anywhere in the HVBJB. The small control wires lead back to the center and connect to the bottom of the green circuit board with flag terminals.

The flag terminals scattered around that are not under the center board are the voltage sensing taps. These wires lead back to the orange connector (C4815C). The sense taps are connected to the busbar after each contactor stage so the BECM can detect the voltage after each one. If a contactor sticks open or closed, the BECM will see the voltage on the other side of the contactor where there shouldn’t be and set a stuck contactor code. When this happens all the contactors are disabled and you will see a Stop Safely Now message in the cluster. Since the DC/DC converter is powered through the Aux contactor, when that shuts down the 12V system will not be charged and the 12V battery will die after a while.

The last components we see are the four rectangles with leads, those are 0.1 µF suppression capacitors that are placed on either side of the DCFC contactors and connected to chassis ground. These should suppress EMI coming in on the DC charger that might interfere with vehicle systems.

Ford Mustang Mach-E Teardown & Analysis: Base AWD HVBJB (High Voltage Battery Junction Box) - NK4Z-10C666-C IMG_0973


Ford Mustang Mach-E Teardown & Analysis: Base AWD HVBJB (High Voltage Battery Junction Box) - NK4Z-10C666-C IMG_0967

Ford Mustang Mach-E Teardown & Analysis: Base AWD HVBJB (High Voltage Battery Junction Box) - NK4Z-10C666-C IMG_0972


I removed a DCFC contactor to examine the busbar connection. It appears the busbar shape near the contactor terminals has been maximized to greatest extent possible to fit the angled dividers on the contactor. It is the same shape on all four contactors. This should maximize heat dissipation. You can see the two coil control wires which are used to close the contactor. The contactors are hermetically sealed so the primary heat dissipation is conduction through the bolt terminals.

Overall Physical Specs:

Part No.NK4Z-10C666-C
Overall weight11.730 lb
External dimensions (LxWxH)23.3 x 5.9 x 4.0 in


Components:

PartQtyManufacturerPart No.
Main contactor4TE2406901
Auxiliary contactor1TE2203997-1
Precharge contactor1TEV23700-C0001-A408
24Ω 30W Precharge resistor1TESQH30 24R?
Current sensor1LEMDHAB S/161
630A Rear motor fuse1EatonEV1XSQ-630-TE
150A Front motor fuse1EatonEV20-150-PX
50A Auxiliary Fuse (3/8” spade)1Eaton10EV50F4TEJP9
50A PTC/ACCM Fuse (1/4” spade)1EatonEV10-50-P
0.1 µF Safety Capacitors4KEMET413N310040M1K

Contactor electrical data:

ContactorCoil ΩCoil mH @ 1020 HzCoil Hold Current @ 12V APull-in VDrop-out VClosed contact mΩ
Main +23.22620.5187.642.320.74
DCFC +23.02220.5217.552.380.70
DCFC –22.92700.5257.782.570.50
Main –22.82250.5267.652.540.70
Aux –21.622.70.5556.552.390.43
Precharge +51.51650.2236.423.307.60

Main contactor physical specs:

Part No.2406901
Weight631.2 g
Diameter51 mm
Can height78 mm
Overall height95 mm
ThreadsM8-1.25
Bolt flange diameter15.8 mm
Nut13 mm
Torque10 Nm

Component resistance:

Precharge resistor23.5 Ω
Front 150A fuse0.67 mΩ
Aux 50A fuse1.81 mΩ
PTC 50A fuse1.70 mΩ

Busbar specs:

Main bar thickness3 mm
MaterialCopper bar, tin coated
Front motor connection3/8 x 3/64" spade
Main Array Spade Terminal20 x 3 mm

Findings:

Overall the part seems very well constructed. The components are all automative grade, as expected. The contactors are all made by TE. All measured data suggests the four white main contactors are internally identical, the different suffix of the part numbers (-1, -2, etc.) is likely for minor differences in the external coil wire length and colors. They are a custom part, with no specifications available. They appear very similar to the TE EVC 500A main contactor (whose datasheet is available), but are physically larger and heavier. Another difference is the coil, based on inductance and resistance measurements the internal coil is much larger than the EVC 500 (22Ω/250mH vs. 3Ω/12mH). It seems probable the extra height and weight of these contactors is due to the larger coil alone. The larger coil is probably to reduce the amount of holding current required while the contactors are closed, so smaller wiring and connectors can be used. The size of the terminals is the same as the EVC 500, which would seem to suggest this may still be a 500A rated contactor.

All the contactor measurements seem nominal.

My milliohms meter isn’t great, the readings are slightly over the 0.5 mΩ spec but it's a difficult measurement to make and I wouldn't worry about it too much. The readings still seem low enough to support 500A with a minimal 0.35V voltage drop. However the heat dissipation would still be several hundred watts, so the contactors will get quite warm handling 500A. Heatsinking through the busbar is very important to control internal temps. There are no temperature sensors in the HVBJB.

The busbar design seems to have been made to maximize the contact area with the bolted contactor terminals for better heat transfer. There isn’t much further improvement that can be made to busbar coverage without making the overall HVBJB larger. Terminal nuts were correctly torqued to 10 Nm. The high-current busbars are 3mm thick. The main bus terminals for the battery array are 3 x 20 mm, which is very close to 1/8 x 3/4”. From a busbar sizing reference, this size busbar would support 325A continuous at 105ºC rating. It’s possible a higher temp rating (such as 125ºC) was chosen for the design, which would increase the current rating slightly.

I will try to post a wiring digram eventually, but another important finding is the DCFC contactors are in series with the main contactors. This means all four contactors must be closed in order to DCFC, and the current from DCFC will pass through all the main contactors and heat them.

Conclusions:

A reminder this is the HVBJB for a standard range pack with lower specs (approx 560A peak). And that HVBJB failures are caused by overheating.

Many of you are wondering if the HVBJB is properly engineered, and the short answer is I’m not qualified to answer that definitively. I can only give you my thoughts and opinions. Based on the size of the busbars connecting the pack, I would estimate the continuous rating of this particular HVBJB to be around 350A, which is about 60% of peak power. This should be adequate for everyday driving with occasional 560A maximum acceleration.

Prolonged or repeated acceleration will cause the contactors and busbars to heat up to temps over 100ºC for short periods, this is expected. In extreme cases, temps may reach 200ºC, which is too hot and will melt the contactors. As previously mentioned, heat from the contacts is mostly dissipated through the busbars. Since the busbars must remain isolated, convection and radiation would be the only means of dissipating the heat from the busbar. In a sealed pack, heat saturation could easily occur in the HVBJB with prolonged high-current usage. Thermal saturation remains a question of when, not if. To be successful, the design needs to absorb enough heat under anticipated normal acceleration conditions to not overheat, and slowly dissipate that heat while driving. Since active cooling of the HVBJB really isn’t an option, the only solution is to make the entire HVBJB bigger if the design is deemed inadequate. Unfortunately there is limited space in the front of the Mach-E pack, so only so much can be done to improve the design retroactively. This revised HVBJB seems to maximize what can be done within the confines of the existing design.

The choice to put the DCFC in series with the main contactors also surprised me because DCFC will only continue to heat up the main contactors during a charging stop. I’m not an expert in HV design, but it seems like the DCFC contactors could have been given a completely separate path directly to the pack. This would give the main contactors time to cool down during a DCFC session. And it should still be safe since it would require a double contactor fault before voltage would be seen at the exposed DC charge port pins. I feel this design decision has contributed to HVBJB failures since a number of people had a stuck contactor happen shorty after DCFC. The extra heating from DCFC puts the failing contactor over the edge.

In terms of the recall software interacting with the part, it can measure the resistance of the contacts by monitoring the voltage drop under load. Normally this voltage drop should be well under 1 volt, but if it rises much above that, it means the contacts are going bad and likely overheating. So a power cut can be made. Excessive voltage drop across the contacts is what triggers the “Service vehicle soon” message and 1/3 power cut. This should prevent further damage to the contactors while allowing the vehicle to continue to drive until replacement.

Preventing contactor damage with software is more tricky, and requires a thermal model of the contacts since there are no temperature sensors in the part. A thermal model such as this would require instrumented testing to develop. The software could integrate the pack amperage over time and correlate that to a heating model to “guess” the contactor temperature, and provide intelligent reductions in power when it thinks the contactors are getting too hot. I haven’t seen evidence of this behavior yet, perhaps in a future software release.

It is hoped the combination of improvements in the HVBJB hardware and better software will prevent the part from failing again in the future. Time will tell.

It would be interesting to see an original design HVBJB to understand what changes were made, but unfortunately those all go back to Ford.

Special thanks to @breeves002 for logistics. If you want higher quality pics, PM me.
 
Last edited:

Garbone

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Excellent. I wonder if the design is different in the F150, with the charge and discharge paths not being in series. It would be interesting to compare the two and see..
 

HuntingPudel

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Nice write-up Lee. 😊🐩

Small red line: “The main bus terminals for the battery array are 3x10 mm, which is very close to 1/8x3/4”.” 10mm is 0.3937”, which is closer to 3/8” than to 3/4”. 🤪🐩
 


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Mach-Lee

Mach-Lee

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Nice write-up Lee. 😊🐩

Small red line: “The main bus terminals for the battery array are 3x10 mm, which is very close to 1/8x3/4”.” 10mm is 0.3937”, which is closer to 3/8” than to 3/4”. 🤪🐩
Thanks for catching that typo, they're 3 x 20. Fixed.
 

JohnFoxeSheets

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Wow, thank you Lee. This is very helpful and informative. Do we know if Kelly at Weber disassembled an original (GT) HVBJB? If so, then if @breeves002 can somehow get you a new GT HVBJB then we would have a basis for comparison between the two...?
 

Nklem

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Thanks. Great work.
 

Logal727

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Is there a way to see inside a welded contactor?
 

breeves002

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Wow, thank you Lee. This is very helpful and informative. Do we know if Kelly at Weber disassembled an original (GT) HVBJB? If so, then if @breeves002 can somehow get you a new GT HVBJB then we would have a basis for comparison between the two...?
In the works.......
 

Mathington

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I'm very curious to see the HVBJB and contactor designs from other vehicles like the Lightning, Model Y and Rivian to see how they compare.
 

voxel

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They are hermetically sealed so you'd have to cut one open to see inside.
So without temp sensors and no active-cooling possible in the HVBJB... should we be concerned about DC charging and WOT in hot climates like Florida?

Are there any EV drivetrain contactors with active cooling? Just curious.
 
 




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