# Kickdown switch for full pack voltage



## Matej (Dec 4, 2015)

This is another one of my dumb ideas, and I am curious about the feasibility of it.
Most automatic cars have a kickdown switch under the throttle pedal. Even in a manual car, it should not be difficult to adapt the switch from an automatic model.
The switch is used to tell the ECU/transmission when the throttle pedal is depressed all the way, so the transmission can be 'kicked down' into the lowest gear possible for quick acceleration.

My idea is, what if I use the switch to close the controller contactor, and open a contactor connecting the battery pack directly to the motor?

This would allow me to use nearly any controller for commuting and driving around town, and if I press the pedal all the way down I would have a robust high amp capability comparable to high performance controllers such as the Zilla 2K, for a fraction of the cost.
Obviously it would not be as smooth and refined, but it should work great for what I would want it for, which is breaking the rear tires loose.

The main thing I wonder about is whether this would somehow adversely affect the controller and quickly damage it, otherwise I do not see what anyone has not employed a similar setup already. 

Of course, there would be a fused safety disconnect and a cutoff switch. I definitely would not want the battery pack to weld itself directly to the motor.

Thank you for any input.


----------



## bigmotherwhale (Apr 15, 2011)

no, just no this time! a controller is a switch of sorts you gain next to nothing and there is a possibility as you correctly stated of a welded contactor which would be a very large disadvantage indeed.


----------



## Matej (Dec 4, 2015)

bigmotherwhale said:


> no, just no this time! a controller is a switch of sorts you gain next to nothing and there is a possibility as you correctly stated of a welded contactor which would be a very large disadvantage indeed.


Is a welded contactor not a possibility at all times? Even electric drag cars run contactors. I hoped a 500A Kilovac should be more than enough for my single motor application.

If there is nothing to gain, what makes a Soliton or Zilla with a 144V battery pack so much livelier compared a Curtis 1231 with the same pack? I assumed the difference is how instantaneously they can 'switch' that voltage on, and people are paying a lot of money for those fractions of a second.


----------



## dcb (Dec 5, 2009)

Matej said:


> If there is nothing to gain, what makes a Soliton or Zilla with a 144V battery pack so much livelier compared a Curtis 1231 with the same pack?


the 1231 is a 500 amp controller. It will limit the current (via duty cycle) to 500 amps, to protect itself and everything else you might have purchased with 500 amps in mind.

the zilla starts at 1000 amp
at 144v, it can deliver twice the power. Though it is often coupled with more expensive motors and etc.

price is a market function. 



Matej said:


> I assumed the difference is how instantaneously they can 'switch' that voltage on.


No, it is the difference between a garden hose at 144psi and a fire hose at 144psi.


----------



## Matej (Dec 4, 2015)

dcb said:


> the 1231 is a 500 amp controller. It will limit the current (via duty cycle) to 500 amps, to protect itself and everything else you might have purchased with 500 amps in mind.
> 
> the zilla starts at 1000 amp
> at 144v, it can deliver twice the power. Though it is often coupled with more expensive motors and etc.
> ...


Exactly.
There is something to gain by bypassing the controller completely at full throttle, especially if it is a lower amp controller.

The question is whether that can be done safely or not, and without causing damage to the controller or any other components.

From what I read about the White Zombie car they ran an on/off contactor for a while. It eventually welded up on them, but that was long before they switched to the high amp Kilovac. Obviously on/off is not sustainable on a street car and a controller would be employed for regular driving. The controller would only be bypassed when the pedal is depressed completely.


I imagine it would function like Knight Rider's turbo boost button.


----------



## dcb (Dec 5, 2009)

Matej said:


> The question is whether that can be done safely or not.


It isn't really a question, contactors can't really break tons of current repeatedly. If you launch with it, you might have to wait till you hit 100mph before you can open it again, or until the motor comes apart.


read the datasheet on the component (and do some de-rating for inductive loads)
http://www.te.com/commerce/Document...313_._5-1773450-5_Sec7_EV200A.pdf2-1618396-7

since you aren't actually controlling, and are thinking of 1000 amps, if you have to break contact at that current, you are into single digit cycle life (you might get 10 on-off cycles out of it, or you might not).

So after a few panic stops your contactor fails... Safe?!?


----------



## Matej (Dec 4, 2015)

Thank you for explaining. That was exactly what I needed to know.

My assumption was that contactors work the same as relays, except they can handle higher voltage and current. Well, I suppose they do function that way, but even they cannot handle the current levels required. Obviously there is still quite a bit I have to learn about EV components, so I apologize in advance for any more stupid things I say.

I do not suppose there is another electronic component that can perform a similar function at such high currents, is there? Short of a controller, of course.
I am guessing not, or it is really expensive and/or big and heavy.


----------



## dcb (Dec 5, 2009)

Matej said:


> I do not suppose there is another electronic component that can perform a similar function at such high currents, is there? Short of a controller, of course.
> I am guessing not, or it is really expensive and/or big and heavy.


There is a $99 1200v 1000a surplus igbt on ebay, but igbts require drivers and heat sinks and some logic and sensing to stay within operating parameters and respond to user demands, aka a controller. The other common approach at lower voltages is to use lots of parallel mosfets, since they have a lower voltage drop.

Note these devices are not foolproof either. You still have quite a bit of homework to do to begin to understand all the nuances.
i.e. https://en.wikipedia.org/wiki/Latch-up

nobody said high power electronics was easy


----------



## Duncan (Dec 8, 2008)

Hi Matej

dcb is right - it's not a good idea BUT it may not be as bad as all that

It's not the closing of the relay that does the damage but the opening

Assuming you are using a std DC series motor approach 

As the motor rpm's rise you will get more Back EMF - 
If you have a relatively low battery voltage the current will self limit as the rev rise

Example - my "Device" on 130v 
I would take off with 1000amps - as the motor revs rose the controller would go to 100%
Once the controller was at 100% and the revs continued to rise the motor current had to drop
At the same time the wind drag was increasing 
In my very unaerodynamic device at 100kph I was down to 200amps - and the drag was about the same

So if I had a 500 amp controller (I have a prototype Paul & Sabrina 1000 amp unit) and one of your kickdowns

I could - take off at full throttle - once I was rolling hit the kickdown and get an extra boost
Up at about 60kph the 500 amp controller would be 100% and the kickdown would no longer help

At anything over the 60kph I could open the "kickdown" - while maintaining full throttle - without damaging the contactor

You would need to know the back EMF characteristics of your motor and to have a relatively low voltage system to do that safely

And that still give you a problem if you need to shut down at a lower speed


----------



## dcb (Dec 5, 2009)

fwiw, I recall reading about the evolution of the white zombie, and at some point he rigged the series/parallel setup so that parallel bypassed the controller, and he sort-of knew what he was doing.

but series/parallel seems to be a thing of the past, as are such bypasses, since large semiconductors are readily available, and the mechanical switches are a constant source of failure/trouble.


----------

