[blackpanther-st]

Tesseract,

I seem to remember and old trick of placing a small value capacitor across the contacts of a DC switch to reduce the arcing. I think the idea was for the capacitor to hold the voltage differential just long enough for the contacts to fully separate. Are you familiar with this? Does it work?

[Bowser330]

Cruising at 70mph takes 300A from a pack of 128V?? thats 38.4 kw...

Why so much? I didnt think that a light car would need so much power to cruise??

Do I have one of the figures wrong?

[dimitri]

Originally Posted by Bowser330

Cruising at 70mph takes 300A from a pack of 128V?? thats 38.4 kw...

Why so much? I didnt think that a light car would need so much power to cruise??

Do I have one of the figures wrong?

The air drag is the key, it takes 100A to cruise 40-45mph, but 300A to cruise 70mph. I think its somewhat normal from everything I read on the subject of air drag, which increases exponentially with speed.

I also had all 4 windows rolled down, which contributes to even more air drag.

[JRP3]

Originally Posted by Tesseract

Not for long if the load draws more than a few hundred mA and/or is at all reactive (i.e. - inductive or capacitive). I concurred with this suggestion only because it is an inexpensive way to get an adjustable thermostat that covers the range in question.

Why would you want to use the pack voltage to power a fan (or any other load besides the motor and dc-dc converter), anyway? Bad idea in my opinion.

Liquid cabin heater, unless you know of a cheap pack voltage to 120 volt AC converter.

[Tesseract]

Originally Posted by Bowser330

Cruising at 70mph takes 300A from a pack of 128V?? thats 38.4 kw...

Why so much? I didnt think that a light car would need so much power to cruise??

Do I have one of the figures wrong?

You are forgetting about the duty cycle.

Motor power = Pack Voltage * Duty Cycle * Motor Current

And this is the sort of data I was hoping we could keep quiet about for another week or so until after the next board and code revisions were done, Dimitri. I may have Qer change the code for your controller to randomly reduce your current limit and flash the idiot light like a disco strobe...

[Tesseract]

Originally Posted by JRP3

Liquid cabin heater, unless you know of a cheap pack voltage to 120 volt AC converter.

Ah... not something one worries about too much here in sunny Florida.

But they make these in various voltages, or does one have to stack up lower voltage elements in series to match a given pack voltage?

[dimitri]

Originally Posted by Tesseract

You are forgetting about the duty cycle.

Motor power = Pack Voltage * Duty Cycle * Motor Current

And this is the sort of data I was hoping we could keep quiet about for another week or so until after the next board and code revisions were done, Dimitri. I may have Qer change the code for your controller to randomly reduce your current limit and flash the idiot light like a disco strobe...

My references have always been to the battery current, so EV Power = Pack Voltage * Battery Current, who cares what the motor does if we can measure how much is being sucked out of the battery

[JRP3]

But they make these in various voltages, or does one have to stack up lower voltage elements in series to match a given pack voltage?

Well I'm using a 120 volt AC engine block heater that I'll hook to my 108 volt pack, but the AC thermostat won't survive, as Dimitri found out

[Tesseract]

Originally Posted by blackpanther-st

Tesseract,

I seem to remember and old trick of placing a small value capacitor across the contacts of a DC switch to reduce the arcing. I think the idea was for the capacitor to hold the voltage differential just long enough for the contacts to fully separate. Are you familiar with this? Does it work?

Oh sure, but one usually uses an RC - aka. a snubber. After all, it doesn't matter whether the switch is mechanical or a MOSFET, when you switch an inductive load you must deal with the kickback produced when the switch opens. You could also use a Transzorb (ruggedized zener diode) or a MOV.

BUT, you have know the total circuit inductance, resistance and whether these ever change (ie - whether the load is fixed and known, or variable/unknown) to be able to select the right values for both the resistor and capacitor, while the transzorb or mov has to handle a peak current at least as high as the load current, and a peak energy that depends, once again, on the inductance.

So, there is not a universal snubber, and remember that both MOVs and Transzorbs tend to fail shorted.

[major]

Originally Posted by dimitri

My references have always been to the battery current, so EV Power = Pack Voltage * Battery Current, who cares what the motor does if we can measure how much is being sucked out of the battery

The motor cares, so you might care too when the motor craps out