Proper jump starting

[Dan-O]

@Sparkie wants to know why when you connect a jumper/charger negative clamp to the battery post directly, the BMS does not "see" current...why does the current flow only from the two posts to {the inner chemistry of the battery} - and not at the same time "flow" past the BMS as well so that it can measure it? It's all connected by conductive metal is it not? It appears to be all one big interconnected circuit...

Yes….thats what I was not understanding as well.

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[crenca]

Yes….thats what I was not understanding as well.

Think of electricity as water, like a river (who keeps saying this? ) - directed (actually, "attracted" ) in its path by gravity, the shape of the land, boulders, etc.. If the river winds back and forth down a hill, and with your Bulldozer you cut a straight path between two points in a meander, what is the river going to do? If you stand on the bank of the meander after the cut you just made (but before the cut enters back into the river) how much water will you see flow by? Yet, the meander still exists and is "connected" to the river is it not - it still has "potential" right...or does it?

 

[Ducati1098]

Those of us that aren't engineers don't overthink right or wrong and just have to believe "because that's how it's supposed to work".

Tom

I mean I'm certainly no engineer, yet it's pretty crystal clear not only how it's supposed to work, but also how it does work.

 

[Rydfree]

@Sparkie wants to know why when you connect a jumper/charger negative clamp to the battery post directly, the BMS does not "see" current...why does the current flow only from the two posts to {the inner chemistry of the battery} - and not at the same time "flow" past the BMS as well so that it can measure it? It's all connected by conductive metal is it not? It appears to be all one big interconnected circuit...

Because inside this molded plastic that goes around the grounding stud is the same as a clamp on amp meter. It measures the amperage passing through that stud. if you attach directly to the battery post you bypass that amp meter .

 

[crenca]

Because inside this molded plastic that goes around the grounding stud is the same as a clamp on amp meter. It measures the amperage passing through that stud. if you attach directly to the battery post you bypass that amp meter .

Yes, but that just begs the question as to how the amp meter is bypassed. Why does the current (i.e. "the electricity") bypass this pathway, which is connected to conductive metal which is connected to the battery post which is "receiving" the electricity from the jumper/charger? In other words, why does the electricity "flow" to the left into the battery, but not to the right and past the BMS amp meter at the same time?

 

[BillTheCat]

If anyone is still reading this far into this thread, THIS is a decent video from a B6G'r from a while back...

 

[Dan-O]

Think of electricity as water, like a river (who keeps saying this? ) - directed (actually, "attracted" ) in its path by gravity, the shape of the land, boulders, etc.. If the river winds back and forth down a hill, and with your Bulldozer you cut a straight path between two points in a meander, what is the river going to do? If you stand on the bank of the meander after the cut you just made (but before the cut enters back into the river) how much water will you see flow by? Yet, the meander still exists and is "connected" to the river is it not - it still has "potential" right...or does it?

Yes I understand all that, I just thought that bolt and tab on top of BMS mount was straight flow thru current and that the sensor itself was only measuring voltage. I was never arguing about it, it just didn’t make sense why the post and tab would be any different.

 

[Rydfree]

Yes, but that just begs the question as to how the amp meter is bypassed. Why does the current (i.e. "the electricity") bypass this pathway, which is connected to conductive metal which is connected to the battery post which is "receiving" the electricity from the jumper/charger? In other words, why does the electricity "flow" to the left into the battery, but not to the right and past the BMS amp meter at the same time?

because the positive is on the other side .

 

[crenca]

I just thought that bolt and tab on top of BMS mount was straight flow thru current and that the sensor itself was only measuring voltage.

Ah, but it is straight thru voltage and the sensor is measuring current, right?!

because the positive is on the other side .

Ah, so gravity flows, or rather attracts, in a certain direction?! What could get in its way - what can resist gravity (or at least what gravity is acting upon) and its pull in any certain direction while at the same time still being connected in some manner??

 

[MilesTeg]

@Sparkie wants to know why when you connect a jumper/charger negative clamp to the battery post directly, the BMS does not "see" current...why does the current flow only from the two posts to {the inner chemistry of the battery} - and not at the same time "flow" past the BMS as well so that it can measure it? It's all connected by conductive metal is it not? It appears to be all one big interconnected circuit...

What you learn in electric circuits 101 is that current (electrons) always flows through the path of least resistance. The path created by a battery charger connected directly to the battery posts creates what is the lowest path of resistance, thus all current flows through it and bypasses the path that includes the BMS.

However, that is a simplification, because current actually flows through all connected paths, but proportionally to the inverse of resistance (i.e. the conductance). Because the path that includes the BMS has typically far more resistance very little current will flow through that path. If that whole path is fully open ("infinite" resistance), it will get zero flow.

So the real answer is that even with a battery charger connected to the terminals directly, the BMS will still get current flow, but only a fraction of the flow through the path created by the battery charger which has much less resistance. And in some cases the cars/BMSs normal path will have "infinite" resistance and thus zero flow. Importantly, it will NOT be seeing all the current flow through the battery and thus cannot do its job correctly.

This may explain the confusion by some, or just muddy the waters even more, hah.

 

[crenca]

So the real answer is that even with a battery charger connected to the terminals directly, the BMS will still get current flow, but only a fraction of the flow through the path created by the battery charger which has much less resistance. And in some cases the cars/BMSs normal path will have "infinite" resistance and thus zero flow. Importantly, it will NOT be seeing all the current flow through the battery and thus cannot do its job correctly.

This may explain the confusion by some, or just muddy the waters even more, hah.

Ah, so when @Sparkie connects his meter to the tab (or some other grounded part) with the charger directly clamped to the battery posts (step one), he will read 12 volts. When he then connects his meter (in series - unless he has a seperate clamp meter) to read the current it won't = 0, but that is not just due to other electrical activity of the Bronco beyond the BMS but because some amount, though trivial in the scheme of things, of current is making it from the charger to the other side?

Almost as if the Bronco's electrical system was like the Mississippi river basin, and even though the lower Mississippi is now flooding from an unusually strong storm system in Louisiana (i.e. the charger), the water in it still flows in one direction and the Ohio river is not flooded at the same time, so little if any of the flood can be measured up there...

I feel sorry for the poor Ford techs who have to figure all this out

 

[MilesTeg]

Ah, so when @Sparkie connects his meter to the tab (or some other grounded part) with the charger directly clamped to the battery posts (step one), he will read 12 volts. When he then connects his meter (in series - unless he has a seperate clamp meter) to read the current it won't = 0, but that is not just due to other electrical activity of the Bronco beyond the BMS but because some amount, though trivial in the scheme of things, of current is making it from the charger to the other side?

Almost as if the Bronco's electrical system was like the Mississippi river basin, and even though the lower Mississippi is now flooding from an unusual strong storm system in Louisiana (i.e. the charger), the water in it still flows in one direction and the Ohio river is not flooded at the same time, so little if any of the flood can be measured up there...

I feel sorry for the poor Ford techs who have to figure all this out

Yup. In a modern car something is always pulling significant amps from the battery (computers, etc.) even when the car is off. By design, that path is low resistance so the fraction of current that path will get is still significant. It all depends on the relative resistance of available paths for electrons to flow.

 

[crenca]

...It all depends on the relative resistance of available paths for electrons to flow.

So it seems to me "electrical potential" is like gravity in a river basin, and the lay of the land - the ups and down, the boulders, even the meandering river bed itself is like the "resistance" found within the properties of an electrical circuit (such as the Bronco's) - it's various metals, lengths, etc.

Gravity, like electricity, seems like a "pushing" force, but really they both are "attractive" forces...kinda of...and only they seem like a pushing forces when your in the middle of them, resisting, and in the case of electricity your body resisting its flow (current kills, not volts - which is why people can do those 10000 volt hair stand up exercises) until is heats your biology up and your cells are damaged beyond all repair and you die...

 

[Roger123]

Because inside this molded plastic that goes around the grounding stud is the same as a clamp on amp meter. It measures the amperage passing through that stud. if you attach directly to the battery post you bypass that amp meter .

Best explanation of this yet, well done Sir.

1. If you ground a test light on the tailpipe (or pretty much any other piece of metal) and touch the tip to the battery positive post your light will light.
2. If you ground the light to the point circled in red and touch the positive post the light will light.
3. If you ground your light directly to the negative post the light will light.

In 1 and 2 the current lighting the light will flow via the BMS, in 3 it will not.

 

[MilesTeg]

So it seems to me "electrical potential" is like gravity in a river basin, and the lay of the land - the ups and down, the boulders, even the meandering river bed itself is like the "resistance" found within the properties of an electrical circuit (such as the Bronco's) - it's various metals, lengths, etc.

Gravity, like electricity, seems like a "pushing" force, but really they both are "attractive" forces...kinda of...and only they seem like a pushing forces when your in the middle of them, resisting, and in the case of electricity your body resisting its flow (current kills, not volts - which is why people can do those 10000 volt hair stand up exercises) until is heats your biology up and your cells are damaged beyond all repair and you die...

You have the right idea.

Voltage is like water pressure in a pipe created by a pump (battery). The volume of water going through the pipe is like current, and that volume depends on how much pressure (voltage) and how big the pipe is (resistance). Smaller pipes have more resistance, larger pipes have less resistance. If water pressure in both pipes is the same, the larger pipes will have a higher flow of water than the smaller pipes. But, if you crank up the pressure with a bigger pump (more voltage), you can get more water through the smaller pipes (more than the larger pipe at lower pressure even).

If you have a single pump connected to two pipes of different sizes, you'll get water into both, but the bigger pipe is still going to carry more water. The only way to know the total volume of water is to measure the amount of water going through both pipes.

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