Anybody tried this type of remote battery disconnect?

[David Womby]

Will it? The answer is complicated. A bad cable or (lets just call it a bad connection) will be a voltage drop. So lower voltage available at the starter. The bad connection dissipates the voltage drop as heat. A starter motor is a torque matching device - it will try to spin the motor at the speed it is commanded to regardless of how many amps it takes to do so. Speed in a DC motor is dependent on input voltage (minus IR losses) and field strength. Spinning a car engine is considered a constant torque application. Torque in a DC motor = Constant * Field Strength * Armature Amps (main cable amps). That means you have less volts (causes lower motor rpm) to basically provide the same torque but amps theoretically stay the same because its trying to maintain commanded speed (minus IR losses which can be significant and continue to increase as the starter is spinning and heating up). But a starter (even the newer designs as far as I know) have series fields. Series fields mean that as armature amps go up field strength increases slowing the motor down but also giving the motor more torque so amps go down until the circuit equalizes and vice versa. Eventually you run out of juice to do anything and my dissertation falls apart, but this is what is happening. That is why low battery voltage causes slow cranking.

I was bored. LOL!

Wow. I need to read that again a few times! Preferably after more coffee! Thanks

David

 

[Nxcoupe]

Will it? The answer is complicated. A bad cable or (lets just call it a bad connection) will be a voltage drop. So lower voltage available at the starter. The bad connection dissipates the voltage drop as heat. A starter motor is a torque matching device - it will try to spin the motor at the speed it is commanded to regardless of how many amps it takes to do so. Speed in a DC motor is dependent on input voltage (minus IR losses) and field strength. Spinning a car engine is considered a constant torque application. Torque in a DC motor = Constant * Field Strength * Armature Amps (main cable amps). That means you have less volts (causes lower motor rpm) to basically provide the same torque but amps theoretically stay the same because its trying to maintain commanded speed (minus IR losses which can be significant and continue to increase as the starter is spinning and heating up). But a starter (even the newer designs as far as I know) have series fields. Series fields mean that as armature amps go up field strength increases slowing the motor down but also giving the motor more torque so amps go down until the circuit equalizes and vice versa. Eventually you run out of juice to do anything and my dissertation falls apart, but this is what is happening. That is why low battery voltage causes slow cranking.

I was bored. LOL!

Ohm's law, resistance goes up, amperage goes down. Most people think reistance makes the circuit absorb more amperage. It is the most difficult thing to teach older techs is that not everything is a 'short'. Resistance lowers amperage flow. A slow cranking engine can have a huge amperage draw but it's due to either an internal short within the starter, bypassing the coil windings and causing high amperage draw(which will sometimes be accompanied by a high pitch whine or high resistance in the circuit where your amperage will not be high at all and not enough current and voltage will not allow a starter to turn. I see what you are saying but not my point.
And a bad connection indeed produces heat as it is resistance in the circuit. Resistance produces heat. It also resists current flow, or amperage. So yes.

 

[Nxcoupe]

But instantaneous inrush will be much higher than 200 amps. Much higher........ You are smacking a low impedance against as stiff 12 volt source and until you get get counter-emf the amps are very high. But it only lasts for milliseconds. But can't be ignored when using electronics.

A starter motor is not electronics. It's as complicated as a light bulb and with all the testing I've done using a peak hold meter, I've never seen 500 amps on a starter engagement, it would melt things as the cables can't carry that much current. They'd become toaster wires. In rush current will be higher to overcome the initial torque of turning the engine, but it will settle way down once turning. An amp clamp and a good fluke will give you hours of enjoyment. It did for me.

 

[twecomm]

Here's a good video that demonstrates typical starter current draw with a variable mechanical load.

The DC current measurement method here uses a "shunt" type device, which is basically a metal bar that has a specific very low resistance. The load (starter motor) is connected in series with the shunt and battery + lead. The meter is calibrated and designed to work with the shunt resistance. It measures the minute voltage drop across the resistance of the shunt. The current is indicated on the calibrated metering device. Quite basically, you can think of this as being a physical demonstration of Ohm's Law.
Current draw is a variable. It can be affected by many factors. Battery voltage and battery condition, any resistance in the wiring and integrity of any mechanical connections, ambient temperature, the physical engine load, electrical design of then starter, condition of contacts in starter solenoid. Even a heavy gauge of wire will have some rmeasureable resistance, although very small.

 

[Nxcoupe]

Here's a good video that demonstrates typical starter current draw with a variable mechanical load.

The DC current measurement method here uses a "shunt" type device, which is basically a metal bar that has a specific very low resistance. The load (starter motor) is connected in series with the shunt and battery + lead. The meter is calibrated and designed to work with the shunt resistance. It measures the minute voltage drop across the resistance of the shunt. The current is indicated on the calibrated metering device. Quite basically, you can think of this as being a physical demonstration of Ohm's Law.
Current draw is a variable. It can be affected by many factors. Battery voltage and battery condition, any resistance in the wiring and integrity of any mechanical connections, ambient temperature, the physical engine load, electrical design of then starter, condition of contacts in starter solenoid. Even a heavy gauge of wire will have some rmeasureable resistance, although very small.

Very interesting video. An almost completely locked up flywheel still only generated 300 amps. I would have assumed more. The cables are going to limit the current ultimately, as will the starter internals. Pretty cool stuff.

 

[David Womby]

Here's a good video that demonstrates typical starter current draw with a variable mechanical load.

The DC current measurement method here uses a "shunt" type device, which is basically a metal bar that has a specific very low resistance. The load (starter motor) is connected in series with the shunt and battery + lead. The meter is calibrated and designed to work with the shunt resistance. It measures the minute voltage drop across the resistance of the shunt. The current is indicated on the calibrated metering device. Quite basically, you can think of this as being a physical demonstration of Ohm's Law.
Current draw is a variable. It can be affected by many factors. Battery voltage and battery condition, any resistance in the wiring and integrity of any mechanical connections, ambient temperature, the physical engine load, electrical design of then starter, condition of contacts in starter solenoid. Even a heavy gauge of wire will have some rmeasureable resistance, although very small.

Excellent video - thanks.

 

[65CopCar]

It's as complicated as a light bulb and with all the testing I've done using a peak hold meter, I've never seen 500 amps on a starter engagement, it would melt things as the cables can't carry that much current.

You didn't understand what I was saying. It is complicated. You will never measure inrush current with a peak hold meter. You have a chance if you have a good current transducer with a top of the line Fluke meter that has 1 ms capture. The good ones toggle between 1 and 100 ms. Since inrush current is a peak function with high dv/dt, I would measure it with a Rogowski coil and an oscilloscope.

I never argued that inrush was a problem for cabling. It could be a problem for an electronic disconnect switch.

Do you understated CEMF and what happens before you build CEMF?

 

[65CopCar]

Ohm's law, resistance goes up, amperage goes down. Most people think reistance makes the circuit absorb more amperage. It is the most difficult thing to teach older techs is that not everything is a 'short'. Resistance lowers amperage flow. A slow cranking engine can have a huge amperage draw but it's due to either an internal short within the starter, bypassing the coil windings and causing high amperage draw(which will sometimes be accompanied by a high pitch whine or high resistance in the circuit where your amperage will not be high at all and not enough current and voltage will not allow a starter to turn. I see what you are saying but not my point.
And a bad connection indeed produces heat as it is resistance in the circuit. Resistance produces heat. It also resists current flow, or amperage. So yes.

One thing you need to understand is that an electric motor does not follow Ohm's law per se. Ohm's law does apply when the windings heat up and the winding resistance goes up, but what most people that quote ohm's law on electric motors don't understand is that a motor is a torque matching device - it will draw whatever current it takes to drive the load at commanded speed. Put the same voltage across a resistor and Ohm's law applies.

You also need to understand how a series field DC motor works (see my write-up above). A starter is a 100% series field motor.

Your argument infers a pure voltage source but because because its a torque matching device one could argue that the battery becomes a current source. How much battery energy is available affects these arguments, including battery internal resistance. And now that I think about it, the battery internal resistance would limit inrush. My Odyssey battery has an internal resistance of 2.9 milli-ohms so using ohm's law the max inrush current would be 4138 amps, but the battery voltage would be collapsing so the battery is actually rated at 2400 amps short circuit. So during inrush you have 2400 amps available and the only thing to limit the inrush amps at that point is the wiring and brush resistance (really impedance during inrush) - if you know that impedance you can loosely determine what the inrush current would be. Actual inrush current would be very complicated to calculate.

 

[65CopCar]

Here's a good video that demonstrates typical starter current draw with a variable mechanical load.

The DC current measurement method here uses a "shunt" type device, which is basically a metal bar that has a specific very low resistance. The load (starter motor) is connected in series with the shunt and battery + lead. The meter is calibrated and designed to work with the shunt resistance. It measures the minute voltage drop across the resistance of the shunt. The current is indicated on the calibrated metering device. Quite basically, you can think of this as being a physical demonstration of Ohm's Law.
Current draw is a variable. It can be affected by many factors. Battery voltage and battery condition, any resistance in the wiring and integrity of any mechanical connections, ambient temperature, the physical engine load, electrical design of then starter, condition of contacts in starter solenoid. Even a heavy gauge of wire will have some rmeasureable resistance, although very small.

Oh my that is a misleading video! I have done plenty of engineering studies (one current project I'm working on is building a 1000 hp 600 volt DC motor dyno) in my career and my professional opinion is that this testing sucks!

Problems:
- He treated the test as a variable torque load - engines are considered a constant torque load. He did not measure torque or hp (kw). The starters are rated in torque and hp (kw).
- High resistances in the circuit including that shunt (lamest shunt I ever saw and what is the resistance of the shunt?). Never saw him tighten the battery terminals. The high resistances cause voltage drops.
- Most importantly, he never shows battery voltage. More than likely the battery (it is not a high end battery like the ones that could be in our cars) is severely collapsing the voltage so the current is artificially being limited. This test is null and void by not showing actual voltage at the starter.
- His explanations were totally bogus - it is a series DC motor................

A much more beneficial test would have been putting a starter on an engine on a test stand and then varying the voltage to the starter. Then you would be able to start drawing some actual conclusions.

Electric motors have been very good to me. I made a real good living around motors. I know my motors..........

 

[twecomm]

I do not believe that the maker of that video had intentions of it being an exhaustive and detailed engineering study of DC motors or motor theory. It's just a simple demonstration of what the current draw is in a basic starter setup under a simulated mechanical load. Bar type shunts of the type shown in the video for measuring DC current levels is negligible, typically not more than .01 ohms, sufficient to produce a measurable voltage drop.
Just my opinion. All are welcome to disagree.

 

[65CopCar]

I do not believe that the maker of that video had intentions of it being an exhaustive and detailed engineering study of DC motors or motor theory.

We are going to disagree. Didn't say the video needed to be a detailed engineering study. My point that it really sucked because it is very misleading and missing critical data.

Attached is a real dyno test sheet of my starter. This is what I received with my J&N mini starter last year. You will notice that it shows kilowatts (kw), torque, amps, speed and voltage. These are all the parameters needed to determine the performance of a starter. The video missed most of these components.

This is a 1.4kw mini starter for a 1990's Dakota. Its the one with the higher gear reduction as compared to the 80's Diplomat starter. You will see in the data that the starter pulled 408 amps around rated kw and on their battery it was at 1001 rpm and 7.93 volts. This dyno sheet does not paint a 100% picture, but is MONUMENTALLY better than that video. Not sure the torque rating of the starter but I think its around 14 Nm. I have that data somewhere. Max power was achieved at 408 amps and 7.93 volts.

 

[RemCharger]

Attached is a real dyno test sheet of my starter. This is what I received with my J&N mini starter last year.

That thing sure spent some time on a shelf.

 

[65CopCar]

That thing sure spent some time on a shelf.View attachment 1795314

That is the spec date. It was made/tested on 8/28/2023. It was less than a year old when I bought it. Look at the bottom - last row of data.

 

[Runner 68]

It’s hard to make out … what’s the breaker for and what is the positive connected to up on the hump?

The breaker is a 175 amp unit that protects the starter system. I ran the 1/0 starter cable through the interior of the car. It's wired so that cable is only hot while cranking. MAD method. Seamed smart to me. The positive you refer to is connected to a solenoid attached to the trunk hinge mount to get the whole system to work. Link to MAD diagram below. Pretty much what I did with a couple of modifications.

Catalog