Proper jump starting

[Ducati1098]

How is NOT possible to "see" current when the BMS shares the same common negative ground with the rest of the Bronco?
Please let me know.

He already explained it.
The path of least resistance is key to understanding how it works.

Since I have no time to explain how electricity works, here is AI to help out:

In Normal Vehicle Operation:
• The only electrical path from the battery negative post to the rest of the vehicle (chassis/engine grounds) is through the internal shunt (that precise, low-resistance copper strip inside the BMS housing).
• There’s no lower-resistance “shortcut” around it because the vehicle-side negative cable is attached exclusively to the output side of the shunt.
• So, all current (starting, accessories, alternator charging the battery) has to flow through the shunt—no choice. The BMS measures the tiny voltage drop across it to calculate exact amperage in/out.


When Charging/Jumping with Negative Connected Directly to the Battery Post:
• You’re creating a new, direct path straight into the battery negative post (battery side of the BMS).
• This direct clamp-to-post connection has near-zero resistance compared to going through the shunt + vehicle wiring.
• Charging current takes the path of least resistance: right into the battery, bypassing the shunt entirely.
• The battery charges fine (often even better/faster), but the BMS sees zero incoming current → it doesn’t update state-of-charge properly.


When You Connect Charger/Jumper Negative “After” the BMS (to chassis ground or vehicle-side cable):
• Now there’s no lower-resistance bypass.
• Current must flow: charger positive → battery → out through the shunt → to chassis ground → back to charger negative.
• Full current goes through the shunt → BMS sees and logs it accurately → smart charging and SOC stay happy.

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[Sparkie]

Gosh, physics...Maxwell's equations, classical electromagnetism, etc. might be a place for you to start.

Think of it like water (at least water in a gravity field). If electricity is a river, amps is the measure of the width/depth of the river, its volume. Volts is the measure of its speed, pressure. Resistance is the path it takes and the things that direct it, block it, and route it.

Like all other engineers, we had to study those principals.
This is a simple DC circuit, so it does not involve electromagnetism.

 

[Sparkie]

He already explained it.
The path of least resistance is key to understanding how it works.

Since I have no time to explain how electricity works, here is AI to help out:

In Normal Vehicle Operation:
• The only electrical path from the battery negative post to the rest of the vehicle (chassis/engine grounds) is through the internal shunt (that precise, low-resistance copper strip inside the BMS housing).
• There’s no lower-resistance “shortcut” around it because the vehicle-side negative cable is attached exclusively to the output side of the shunt.
• So, all current (starting, accessories, alternator charging the battery) has to flow through the shunt—no choice. The BMS measures the tiny voltage drop across it to calculate exact amperage in/out.


When Charging/Jumping with Negative Connected Directly to the Battery Post:
• You’re creating a new, direct path straight into the battery negative post (battery side of the BMS).
• This direct clamp-to-post connection has near-zero resistance compared to going through the shunt + vehicle wiring.
• Charging current takes the path of least resistance: right into the battery, bypassing the shunt entirely.
• The battery charges fine (often even better/faster), but the BMS sees zero incoming current → it doesn’t update state-of-charge properly.


When You Connect Charger/Jumper Negative “After” the BMS (to chassis ground or vehicle-side cable):
• Now there’s no lower-resistance bypass.
• Current must flow: charger positive → battery → out through the shunt → to chassis ground → back to charger negative.
• Full current goes through the shunt → BMS sees and logs it accurately → smart charging and SOC stay happy.

This AI text of the first paragraph contradicts itself with second and third paragraphs.

For the record, I just retired from a company that is developing AI for the medical/pharmaceutical industries. AI is NOT yet ready to be trusted.

Please see this following industry article from last summer regarding AI models.
https://spectrum.ieee.org/ai-misinf...dmbT1nY4Q9UL04otoCI5UxrhN5-fZMrkG828oqUyEVDnu

 

[CalvinT]

o, since the BMS is essentially a voltimeter and ammeter that automatically sends info to the Bronco's computers, it must be connected in series between the load and the power source (in this case between the negative power terminal and the load) in order to tell the computers how much load there is on the charging system.

Almost all amp meters are very sensitive volt meters. The scale is just calibrated in amps instead of volts. The voltage drop across the sensing resistor is called the burden voltage. The exceptions are ampmeters that measure the magnetic field in the wire. They're all clamp on meters. The DC clamp on meters use a hall effect device. A hall effect device changes it's resistance in the presence of a magnetic field. The clamp for an AC amp meter acts as the secondary winding of a transformer. The wire you're measuring is the primary winding.

 

[CalvinT]

This is a simple DC circuit, so it does not involve electromagnetism.

Not true. A magnetic field is created by current flow. Maxwell confirmed this in the 19th century.

 

[Ducati1098]

This AI text of the first paragraph contradicts itself with second and third paragraphs.

I'd love to hear how

 

[crenca]

This is a simple DC circuit, so it does not involve electromagnetism.

Yes it does, the magnetic lines are static - Maxwell.

Look, your a typical engineer. When engineers don't understand something they argue. Most people don't like this but I get it and even like it.

But you ain't going to "put this thread to bed" unless you break away and do some basic research.

 

[Sparkie]

Not true. A magnetic field is created by current flow. Maxwell confirmed this in the 19th century.

Yes, I know that, but this a steady state 12 VDC circuit, so the electromagnetic field is also at steady state and has no bearing on the BMS shunt resister.

 

[CalvinT]

Yes, I know that, but this a steady state 12 VDC circuit, so the electromagnetic field is also at steady state and has no bearing on the BMS shunt resister.

True. But since resistors aren't considered reactive, ie, not capacitive or inductive, the same statement would be true if it were an AC circuit. Magnetic fields from either AC or DC current flow have no effect on shunt resistors.

 

[Sparkie]

I'd love to hear how

First paragraph, second bullet.

• There’s no lower-resistance “shortcut” around it because the vehicle-side negative cable is attached exclusively to the output side of the shunt.

Then the next paragraphs mention that the IS a short cut. That makes no sense in a common DC circuit, right? Hence, this AI was BS and why I sent the link describing how BS exists in AI responses.

 

[Sparkie]

True. But since resistors aren't considered reactive, ie, not capacitive or inductive, the same statement would be true if it were an AC circuit. Magnetic fields from either AC or DC current flow have no effect on shunt resistors.

But the BMS is not measuring magnetic fields.

(By the way, I used your PDF for recalculating my Fuel MPG bias on a summer road trip. Thank you. It worked great.)

 

[crenca]

Then the next paragraphs mention that the IS a short cut. That makes no sense

Think of it as water, a river (who said that? ). At that point (i.e. when the clamp is connected directly to the battery post) the shunt is then "uphill" (instead of downhill when the clamp is on the other side of the shunt) of the path of least resistance (i.e. the post).

Water does not flow uphill does it? Of course not. Thus the BMS does not "see" any water flowing past it.

 

[Ducati1098]

First paragraph, second bullet.

• There’s no lower-resistance “shortcut” around it because the vehicle-side negative cable is attached exclusively to the output side of the shunt.

Then the next paragraphs mention that the IS a short cut. That makes no sense in a common DC circuit, right? Hence, this AI was BS and why I sent the link describing how BS exists in AI responses.

One is talking about NORMAL operation, AKA nothing connected, nobody charging or jumping it.

The other 2 are stickily talking about jumping/charging situations.

-There is no short cut when the system is on it's own. It HAS to go though the BMS.
-When charging it or jumping it, someone can create a short cut (bypassing the BMS) depending on where they place the cables.

There is no contradiction, you just don't understand. You can try and blame AI, but it isn't wrong in this case.

 

[Sparkie]

Yes it does, the magnetic lines are static - Maxwell.

Look, your a typical engineer. When engineers don't understand something they argue. Most people don't like this but I get it and even like it.

But you ain't going to "put this thread to bed" unless you break away and do some basic research.

Yikes! "typical engineer".
You have not yet explained how the Bronco's BMS measures magnetic fields!

In the absence of any knowledgeable electricians, I will "do some basic research".
I will use my own multimeter and measure voltage in the various locations described in this thread.
However, I am fighting off a flu virus right now and it is only 20 degrees outside.
So, hopefully I can get back to you in a few days.

 

[Sparkie]

One is talking about NORMAL operation, AKA nothing connected, nobody charging or jumping it.

The other 2 are stickily talking about jumping/charging situations.

-There is no short cut when the system is on it's own. It HAS to go though the BMS.
-When charging it or jumping it, someone can create a short cut depending on where they place the cables.

There is no contradiction, you just don't understand. You can try and blame AI, but it isn't wrong in this case.

In theory, the "distance" of the does cause a very slight voltage drop, but that would have to be hundreds of feet away to cause even a small detectable drop.

Yes, in a few days, "I will try this at home".

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