# Battery amps or motor amps???



## gmijackso (Sep 30, 2008)

I personally purchased two shunts, and 1 meter. A switch will allow me to switch between the two locations. For the extra cost of a shunt, it's a win win to me.

I can see both sides of the argument, motor amps won't tell you if something else is going south on you, and battery amps aren't going to accurately tell you what's happening with your motor. So, why not do both?


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## Guest (Oct 20, 2008)

This is the way to go. Two shunts, two fuses, one switch and one meter. Isolated from auto ground. Keep in mind that these are shunted off the main power pack and you must have them fused. It is nice to have a simple setup that allows for monitoring both with one gauge. 


Pete : )



gmijackso said:


> I personally purchased two shunts, and 1 meter. A switch will allow me to switch between the two locations. For the extra cost of a shunt, it's a win win to me.
> 
> I can see both sides of the argument, motor amps won't tell you if something else is going south on you, and battery amps aren't going to accurately tell you what's happening with your motor. So, why not do both?


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## Qer (May 7, 2008)

Motor mounted will give you some kind of coarse torque meter which might be fun but probably not entirely useful in every days life. It might be a good thing to have if you ever encounter problems (like a controller that misbehaves) though. Battery pack mounted will give you an estimate of your "eco driving", making it possible for you to see what gear that pulls the least energy out of the pack.

So without having an EV (but pretty experienced in the field of general short circuits ) I'd say the pack current is most useful as long as things work as intended.


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## Guest (Oct 20, 2008)

I find battery amps is most useful and meaningful. A lot of Motor amps is just hystereses maintaining current flow between pulses although it is the reason for the higher efficiency and higher torque output and smooth performance of modern PWM controllers, don't forget to keep wiring from motor to controller as SHORT as possible, so if you put the motor in the back, put the controller there too, or both in the front.. Of course the shunt goes in the battery wiring, after the contactor, then a one amp or less 250 volt rated fuse and holder go in each of the two wires from the shunt to the indicator, mount the fuses (2) as close to the shunt as possible. And connect to the indicator with 250 v. or better wires. I like to connect my pack charger to the junction of the shunt and the contactor, so the meter shows charging current too.
Engineering Manager


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## rctous (Jun 18, 2008)

What is the ball park differences you see on motor amps verses battery amps??? Is it 1/2 60% again just a ball park idea.

Brian


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## jaspersk (Jun 26, 2008)

rctous said:


> What is the ball park differences you see on motor amps verses battery amps??? Is it 1/2 60% again just a ball park idea.
> 
> Brian


I don't have mine up and running but theoretically the relationship between battery amps and motor amps should vary depending on what speed you are running the motor at, right? Power equals volts times amps. Theoretically, power at the battery and the motor should be the same (ignoring losses in the controller and accessories). However, if the car controllers are like most DC drives, the motor voltage will vary with speed. So if you have full voltage at the batteries but say only half the voltage at the motor (and you ignored controller heat loss) you'd see twice as many amps at the motor as you would at the battery. 

Someone who has real life experience should really chime but theoretically that should hold true.


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## ZenDaddy (Jul 22, 2008)

Engineering Manager said:


> I like to connect my pack charger to the junction of the shunt and the contactor, so the meter shows charging current too.
> Engineering Manager


E.M. 

Can you expound on this? My shunt is in series from pack positive to contactor then controller. My charger is connected at pack negative and at the shunt input (pack positive). How would I monitor charging current?
Would I simply connect the charger to the output of the shunt?

Thanks.

Z.D.


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## Tesseract (Sep 27, 2008)

Zendaddy - See attached picture.

In general: there should be a microswitch in the charger inlet port that interrupts power to the battery pack contactor coil whenever a plug is inserted into it (or some other means of preventing said contactor from being energized while the charger is plugged in).

Battery Amps: the average amps drawn from the pack times the pack voltage gives you the kw consumed - i.e., the EV equivalent of the gas gauge. Battery amps = motor amps * PWM duty cycle, more or less.

Motor Amps: practically irrelevant to the driver; critically important to the controller.

Hope this helps.


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## dimitri (May 16, 2008)

When I think about monitoring charging and discharging amps using same meter 2 things come to mind, charging current is opposite direction than discharging and generally 10 times less amount. So the needle on the meter will be moving to the left of 0 mark and will only move a small fraction on a typical 0-400 Amp meter if charging current is only 15-20 Amps. Of course I am assuming small analog 0-400 ammeter that I have, results may vary with digital meters, etc.
In my scenario this doesn't look very practical.


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## gmijackso (Sep 30, 2008)

You can often find dual direction amp meters. Most automotive are dual direction. If you're using a digital meter you'd have less problems obviously. You can also use a bridge rectifier to ensure your amp meter always runs forward. You could even get tricky and make it multiply the reading when the "backward" charging current is turned around to make the meter move forward by running it through an opamp.


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## Qer (May 7, 2008)

gmijackso said:


> You can also use a bridge rectifier to ensure your amp meter always runs forward.


Uhm. Bad idea. If you try to rectify the signal from the shunt the Ampere meter will most likely always read zero Ampere since the shunt typically gives some mV out (I'd say between 50 and 200 at max current depending on the shunt) and the rectifier demands a little more than 1 Volt before it even opens.

If you rectify the motor current "around" the shunt, you will have a voltage drop of a few Volt, 2 or more, which will mean that the rectifier will generate a kW or two in heat dissipation when the motor starts to pull heavy current.

I'd say it's better to buy two Ampere meters and connect them to the same shunt but opposite directions, unless the Ampere meter can show negative current that is.


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## gmijackso (Sep 30, 2008)

Good point, I thought past the fact that you're only dealing with a few milivolts. With that in mind, I guess if I really NEEDED to do one meter for both charging and discharging, I'd use an op-amp to buffer the signal and I think you can even use it to invert only when necessary. So, when you're charging the signal is inverted and buffered, and when you're discharging the signal is just buffered.


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## Qer (May 7, 2008)

gmijackso said:


> Good point, I thought past the fact that you're only dealing with a few milivolts. With that in mind, I guess if I really NEEDED to do one meter for both charging and discharging, I'd use an op-amp to buffer the signal and I think you can even use it to invert only when necessary. So, when you're charging the signal is inverted and buffered, and when you're discharging the signal is just buffered.


you mean something like this:










It's a bit over complicated, but it has the advantage that it's pretty straight forward at least + that it's what Google managed to find fast.


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## gmijackso (Sep 30, 2008)

Qer said:


> you mean something like this:
> 
> 
> 
> ...


Yes, I mean something like that. You can then "tune" the op-amps to be unity so that .05V in is .05V out and you're good to go. I agree, it's a bit complicated, but if it's what somebody needs/wants, to each their own. 

Good find btw. I was going to look in my old "Linear Electronics" book for the circuit, but I forgot. I think google did a fine job. The only question for that circuit is if it requires both the positive and negative 15V supplies normally associated with op-amps. Some people won't have problems with that, but others would. The DC-DC converters I have provide + and - 12V so I'm set


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## Guest (Oct 30, 2008)

Yes, Zendaddy, by connecting the charger so that charging current flows thru the shunt when charging an indication of charging is displayed, HOWEVER, polarity is reversed! so be sure the display can Handle reversed polarity. See also, Dimitri's comments, he is quite correct!


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## kugmo (Oct 31, 2008)

battery amps, why use motor amps when it's for a car...


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## gmijackso (Sep 30, 2008)

kugmo said:


> battery amps, why use motor amps when it's for a car...


To let you know if you're about to turn your motor into a puddle, of course.


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## ZenDaddy (Jul 22, 2008)

Engineering Manager said:


> Yes, Zendaddy, by connecting the charger so that charging current flows thru the shunt when charging an indication of charging is displayed, HOWEVER, polarity is reversed! so be sure the display can Handle reversed polarity. See also, Dimitri's comments, he is quite correct!


Understood, thank you all.

Unfortunately my display only reads positive current. Perhaps it would be worth upgrading.

ZD


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## patzke (Nov 1, 2008)

It makes sense just to have both readings available to your viewing. 

I'm having a tough time finding a digital or analog gauge that goes past 60A.... any suggestions?

What instruments are used for monitoring (for example) a 200A current to the motor?


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## gmijackso (Sep 30, 2008)

patzke said:


> It makes sense just to have both readings available to your viewing.
> 
> I'm having a tough time finding a digital or analog gauge that goes past 60A.... any suggestions?
> 
> What instruments are used for monitoring (for example) a 200A current to the motor?


There's a bit of a "trick" when you're looking at measuring high current. The trick is that you're not REALLY measuring current. To measure high current you use a shunt.

A shunt is basically a very very tiny resistor (tiny as in low resistance not physical size) but is very very accurate in resistance. These shunts are rated in amps and generally milivolts. Most common seem to be 50mV and 75mV. 

What happens, is when you run current (amps) through the shunt (which is just a resistor) a voltage drop is created across the shunt due to ohms law. The rating of the shunt tells you how much voltage drop will be measured at max current rating. For instance, if you have a 400A 50mV shunt, you will have a 50mV voltage drop across the shunt when drawing 400A. This logically means that when drawing 200A you'd have 25mV (it's ohms law V=IR). 

All of that means that your high current meter isn't reading current at all. It's reading voltage. Think of an analog sweep style meter. Consider if we mark this meter off in segments ranging from 0-75mV. When you apply 75mV your meter swings full scale to read such. When you supply 38mV the meter sits in the middle to read 38mV. 

Now consider that you've hooked this meter to a shunt that creates 75mV when 400A is passing through it. So, if the meter now read 75mV we know your motor is drawing 400A. If the meter is reading 38mV, your motor is drawing 200A. What if it read 19mV? (100A) Ok, so now, since we don't want to do all this math while whipping down the road, how about we just change the way the meter reads, sweeping from 0A to 400A. Same meter reading milivolts, but they now represent current.

I should also add that if you have a meter that measure 0-100A using 75mV, and you hook it to the 400A 75mV shunt, you're still fine. Nothing will blow up. Your meter is just marked wrong now, cuz it's really metering 0-400A instead of 0-100A. That 0-100A meter was really just a 75mV meter marked to represent 0-100A.

Digital high current meters do the same thing, the only difference is that it's done electronically. This is often accomplished with a set of switches or jumpers, or even may be hard set by the factory.

Hope that helps some.


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## patzke (Nov 1, 2008)

gmijackso said:


> There's a bit of a "trick" when you're looking at measuring high current. The trick is that you're not REALLY measuring current. To measure high current you use a shunt.
> 
> A shunt is basically a very very tiny resistor (tiny as in low resistance not physical size) but is very very accurate in resistance. These shunts are rated in amps and generally milivolts. Most common seem to be 50mV and 75mV.
> 
> ...


Hey, that helps a bunch. I didn't think about how you could use a 400A shunt with a 100A meter and it just being marked wrong - but it makes sense. That would work.

I understood the use of a shunt for the most part, and this polished my knowledge on them.

I bought two digital gauges from a seller in Hong Kong for 65$ (with shunt). So that should be a decent deal as long as they work... I plan on ordering another shunt and using it to have the option to switch between battery side and motor side. I should be able to achieve this with a SPDT switch. Yes? I want to do the same thing with my voltmeter to monitor traction battery volts and AUX voltage. Would be convenient. 


Thanks for the help and clarification,

Nick


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## gmijackso (Sep 30, 2008)

patzke said:


> I should be able to achieve this with a SPDT switch. Yes?


Yes. You can even consider a SPDT switch with a center off if you want to be able to read nothing. Dunno why you'd want to, but it's probably what I'll do.


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## Guest (Nov 13, 2008)

Mine is up for motor amps. Down for battery amps and middle for off. It will always be either up or down. : )


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## patzke (Nov 1, 2008)

gmijackso said:


> Yes. You can even consider a SPDT switch with a center off if you want to be able to read nothing. Dunno why you'd want to, but it's probably what I'll do.


That got me-a-chucklin:

"Dunno why you'd need it, but i'mma do it." 

Love the attitude. I have that as well. I'll take some pictures, they're pretty clean switches.


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