Voltage drop while floored

[Helirich]

I have a PDS converted to 48V with XCT 500. Let's say my digital volt Guage reads 51 volts at rest. If I floor the pedal, it will read about 44 volts. This is with less than year old batteries. Does this seem normal?

I have no complaints on power, speed or endurance. (The cart hauls azz) Just wondering if that sounds right.

[teecro]

Assuming 6 8-volt batteries? 51 volts at rest is as good as one can hope for... And for the most part so is 44 volts at WOT with a 500 amp controller; mine drops that low on the flat and more on the hills where we live... What you should also be seeing is the voltage climb a little & level off once up to a steady speed while on the flat...

[kernal]

My experience is about the same with 2 month old T-875s and AC motor. (stock speed control)

[JohnnieB]

A typical 48V battery pack drops about 1.5V per 100A of current draw and the high current cable connecting the batteries in series, plus their connections will probably drop another 0.5V per 100A, for about 2.0V per 100A at the main battery pack terminals.

If your batteries, cables and connections are in near pristine condition, a 7.0V voltage drop means that 350A are being drawn from the battery pack.

[teecro]

Originally Posted by JohnnieB

A typical 48V battery pack drops about 1.5V per 100A of current draw and the high current cable connecting the batteries in series, plus their connections will probably drop another 0.5V per 100A, for about 2.0V per 100A at the main battery pack terminals.

If your batteries, cables and connections are in near pristine condition, a 7.0V voltage drop means that 350A are being drawn from the battery pack.

Dang then sometimes I must be drawing some serious amps then...

[JohnnieB]

Originally Posted by teecro

Dang then sometimes I must be drawing some serious amps then...

That is definitely a possibility.

[Helirich]

Originally Posted by teecro

Assuming 6 8-volt batteries? 51 volts at rest is as good as one can hope for... And for the most part so is 44 volts at WOT with a 500 amp controller; mine drops that low on the flat and more on the hills where we live... What you should also be seeing is the voltage climb a little & level off once up to a steady speed while on the flat...

The 51 volt was rounded down. Actually, if I charge it and then let it rest, it will read about 51.6V. But I wasn't asking about my battery health. I was just wondering about what a normal drop was. It does climb a little after I'm up to speed. (Maybe 45)

Originally Posted by JohnnieB

A typical 48V battery pack drops about 1.5V per 100A of current draw and the high current cable connecting the batteries in series, plus their connections will probably drop another 0.5V per 100A, for about 2.0V per 100A at the main battery pack terminals.

If your batteries, cables and connections are in near pristine condition, a 7.0V voltage drop means that 350A are being drawn from the battery pack.

My cables are new (2G), so I guess it is operating normally.

As always, thanks.

[Helirich]

Originally Posted by JohnnieB

A typical 48V battery pack drops about 1.5V per 100A of current draw and the high current cable connecting the batteries in series, plus their connections will probably drop another 0.5V per 100A, for about 2.0V per 100A at the main battery pack terminals.

If your batteries, cables and connections are in near pristine condition, a 7.0V voltage drop means that 350A are being drawn from the battery pack.

I was doing some thinking on this. (I know, dangerous) It would seem like if I had a 400 XCT, it would preform exactly the same as my 500 XCT. (Because I'm not using more than 400 amps.)

When you say the "connections are pristine" , it draws 350 amps. What if they are not? Would it draw more or less? I guess if they get hot, some amps are "burned" before they get to the controller. So for example, if 50 amps is resistance at the connections and 350 is at the motor, does that mean I need a minumum of 400 amp controller? Or are you saying that if I have 50 amps resistance at connections, that I'm only using 300 at the motor?

[JohnnieB]

Originally Posted by Helirich

1. I was doing some thinking on this. (I know, dangerous)

2. It would seem like if I had a 400 XCT, it would preform exactly the same as my 500 XCT. (Because I'm not using more than 400 amps.)

3. When you say the "connections are pristine" , it draws 350 amps. What if they are not? Would it draw more or less?

4. I guess if they get hot, some amps are "burned" before they get to the controller.

5. So for example, if 50 amps is resistance at the connections and 350 is at the motor, does that mean I need a minumum of 400 amp controller? Or are you saying that if I have 50 amps resistance at connections, that I'm only using 300 at the motor?

1. SWMBO has told me I have the same affliction.

2. If your motor will not draw more than 460A (Peak output is amp rating plus 15% for XCT controllers) during WOT acceleration from a standstill on a steep incline, than the performance of a XCT400 and a XCT500 would be nearly identical in your cart. (Assuming the same field map is used.)

Even if your motor was capable of drawing more than 460A, the only difference in performance would be at the low end of the RPM spectrum.

The max amps a DC cart motor can draw depends on the RPM it is spinning at and the voltage being applied. For example: If the max amps a sepex cart motor can draw at 2000RPM is 345A with a 48V battery pack, then there is no difference in cart performance with a XCT300, XCT400 or XCT500 when motor is spinning at 2000RPM and above. There will be a difference in performance below 2000RPM with the XCT400 and XCT500. Carrying it one step further, if that motor is capable of drawing 460A at 1500RPM, then there would only be a difference between the performance of a XCT400 and XCT500 when motor was spinning less than 1500RPM.

Of course, overkill is underrated, so I probably would have gone with a XCT500PDS if they had been available when I upgraded from a DCX500.
However, from the data I recorded from the DCX500, I saw only one instance my motor exceeded 460A and I had to work at creating a worst case scenario to do it, so a XCT400 meets my needs for my setup about 99.999% of the time and I didn't have to wait until Alltrax released the XCT500PDS to the marketplace.

3. The amp draw would be less due to greater resistance in the series loop.

4. Ideally, all the amps are converted to torque by the motor, but motors are mot 100% efficient, so some are converted to heat in the motor. Any amps converted to heat outside the motor are also wasted energy.

All the high current cables, connections and contacts have resistance and when amps pass through resistance they are converted to heat (Ohm's Law).

The MOSFETs in the controller have resistance, so they is a heat loss there also.

5. The batteries, cables, solenoid, controller and motor's armature are in a series loop, so armature amp flow is the same through all of them. (A SepEx motor system gets a little complicated because the Field current is in parallel with parts of the amp deliver system and in series with other parts, so let's just look at armature amps, which is in series with the high current loop.)

The amount of current a motor will draw depends on the RPM and Volts applied. The 48V battery pack is applying about 51V to the high current loop, but everywhere there is resistance in it, voltage is being dropped, so the the battery pack's full voltage is not being applied to the motor and the amps it draws is proportionally less.

Adding resistance in series with the motor is how a resistor drive cart works. It is the voltage dropped across the resistor coils not being applied to the motor, which reduces the amp draw of the motor, which in turn reduces torque and the cart's speed is lower.

The object is to minimize the resistance in the amp delivery system outside the motor.

[Clemsoncartguy]

Good gosh!!!

I feel smarter just having read that!