Measuring DC AMPs W/Clamp-on meter

[rlw]

Radioman,

Sorry for hijacking your thread! I'll shut up now (or is it "shunt" up? ;^)

RLW

[cgtech]

I would have loved to have the opportunity to have met him. I have tons of respect for the people who know so much that it would blow my mind. No calculator required...here's the answer... I do the easy stuff in my head (amps & watts & volts). I thought the ammeter in a golf cart charger simply used the magnetic field ("flux" which is directly related to amp flow through a wire) from in the "buss bar" going across the back of the meter to fight against the magnet wire that kept the ammeter needle at zero normally to indicate the amps delivered. I didnt think the ammeter was a "resistor" or shunt.

[kab69440]

Originally Posted by rlw

CGTech,
For very high currents like in our carts, you'd need more than just a copper strap, you'd be looking at a copper bar a couple of inches long, maybe 3/4" wide, and 3/16 or a quarter inch thick.

RLW

I have a length of copper bar stock out in the garage that just about fits that description. I used it to make a couple of heat sinking "spoons" to back the seam when I stitched a 70 Road Runner power bulge into my Charger's hood. Long stitch runs in thin sheet require a delicate touch. That much flat sheet will warp beyond any hope of salvation in an instant. The copper bar stock fit the bill perfectly.

[sunking]

It can be done quite easily, accurately and inexpensively. First thing to understand Motor Current does NOT EQUAL Battery Current due to back EMF of the motor and the buck nature of the controller. a 2.2 Kw motor at 48 VDC will only draw a maximum of 50 amps from the battery.

All decent battery fuel gauges use two Hall Effect Transducers. One on the charge lead to count Amp Hours Into the battery, and another on the Battery Supply lead to count Amp Hours out of the battery called Coulomb Counting. About $10 to $20 in parts for a 0 to 200 amp sensor. With such a device you can easily monitor current, and with some controllers and logic can determine Amp Hours used over a period of time.

With some of the advanced motor controllers out there it is already built-in the controllers. All you have to do is put the sensors on and add a display/selector panel.

[rlw]

Originally Posted by cgtech

I would have loved to have the opportunity to have met him. I have tons of respect for the people who know so much that it would blow my mind. No calculator required...here's the answer... I do the easy stuff in my head (amps & watts & volts). I thought the ammeter in a golf cart charger simply used the magnetic field ("flux" which is directly related to amp flow through a wire) from in the "buss bar" going across the back of the meter to fight against the magnet wire that kept the ammeter needle at zero normally to indicate the amps delivered. I didnt think the ammeter was a "resistor" or shunt.

CGTech,

Your thinkin' is right - the ammeter is really a millivoltmeter reading a voltage across a resistor (the shunt) that has a very low resistance.

Think back to high school (or GRADE school) science when they showed you a galvanometer - it was a needle on a fulcrum with a little magnet on one end, surrounded by a coil. A current through the coil pulled the magnet, deflecting the needle.

That's the basic concept of ALL analog meters. Ammeters, Voltmeters, Ohmmeters are all just variations on that theme.

You want to measure HIGH current? Run MOST of the current through a shunt, and measure the tiny voltage drop across the tiny resistance that's carrying most of the current.

You want Ohms? Measure the voltage drop from a known reference voltage across an unknown resistance.

You want Volts (in various ranges)? Measure voltages across known resistances.

I've got my dad's old Heathkit multimeter. It's basically a single millivoltmeter tied into a bunch of switches and precision resistors that change the way the meter "views" the item it's measuring. The reason that old school Ohmmeters start at infinity on the left side of the meter (0.00000 volts, in reality) is that when you're measuring infinity ohms, there's no voltage to measure!

That meter has 8 different scales on the card behind the needle, depending on what it is you're measuring (and which rotary switch position is selected). Very ingenious technology, halfway between steampunk and virtual-digital, which pretty much describes the "remote sensing" stuff my dad worked on in the 50s and 60s.

Here's a really good description of how to build your own multimeter:

http://www.brats-qth.org/training/advanced/measure0.htm

Again, sorry for hijacking this thread, but it got real interesting, so I guess I'm not as sorry as I think! ;^)

We should probably start a new thread: "Halfway between SteamPunk and Virtual-Digital" to discuss this kind of tech.


RLW

[radioman]

I am from the old school in electronics too. Brings back memories.

[JohnnieB]

All electric analog meters are ammeters, as is the sensing element in digital meters.
What unit of measurement they display ( Amps, Volts, Ohms, Etc.) and the range of measurement, is determined by circuitry external to the basic meter movement or sensor element.

That attached shows the same meter movement being used as an ammeter and as a voltmeter.

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Yes, the high current cables can be used as ammeter shunts, but as CG mentioned, the tricky part is finding the Ohmic value of the cable since it is only a fraction of the values most Ohmmeters will quantify.
Specifically:
6Ga = 0.0003951Ω per foot
4Ga = 0.0002485Ω per foot
2Ga = 0.0001563Ω per foot

I have access to a NIST traceable meter that will resolve 0.001Ω and toggles to 0.001 if the fourth digit to the right of the decimal is equal to or greater than 5, but the longest of the 2Ga cables on my cart didn't make the Milli-Ohm digit flicker, so I have no idea how much is added by the terminal lugs.

By adding a low Ohmic value resistor the the charge circuit (0.1Ω or so), you can measure the voltage drop across it to get the current flow and then measure the voltage drop across the interconnecting cables in the battery pack to calculate their resistance. Then you can measure the voltage drop across one of them to calculate the total current flow while the cart is being driven

If you want to measure current flow though motor alone, substitute a motor cable for one of the shorter battery cables to determine its resistance.

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I also have a clamp-on DC current probe (Hall effect type). The problem with it is that it is position sensitive (Angle the cable going through it is at), so it doesn't work very well when cart is moving.

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If you have an Alltrax controller, the data log file you can get from it is reasonably accurate, but tricky to read.
The armature current column is the peak current flow though the MOSFETs and does not take the duty cycle of the PWM output into consideration.
The throttle position column is the duty cycle.
The battery current column is the peak current times the duty cycle, or average current flow through the armature.

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I'm old school also. Emphasis on old.

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As for the batteries, they are basically water jugs at this point. All the sulfate ions are in the plates.
Chances for redemption is somewhere between none and nonexistent