# Contactor Controller



## frodus (Apr 12, 2008)

Save up and buy a real controller. You'll save time, money and lots of head scratching.

Tapping off the pack is a bad idea because batteries become unbalanced. So either you have to rotate them (hard to judge how much each battery is cycled), or you have to replace the lowest tapped ones first. You might have to replace contactors, they're not really meant to open/close under load and have a long lifetime. They'll last, but not nearly as long as you think.

It'l be jerky, difficult to control, and you cannot limit current from the batteries. Not limiting current will surely cause your batteries to decrease in performance from the very first ride.

Only a few flavors of contactors come with diodes, look at the datasheet to check.

You can also potentially overload the contactor if the current required from the load is too high and exceeds the spec.

You could spend lots of money on the contactors, wiring, switches, etc, lots of time spent balancing your pack every month to keep the batteries from failing, and then lost of money replacing batteries because there's no current limit or because they become unbalanced.


Oh, and why do you keep starting threads basically about the same thing? Why not just keep it all in one thread?


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## major (Apr 4, 2008)

blueblizzard816 said:


> I'm looking into building a contactor controller,


blue,

If you intend to build or convert an EV intended to ever drive on the street, you really should bite the bullet and buy (or make) a suitable controller. Otherwise you will be a danger to yourself and others.

Contactors often fail closed, meaning on. And as frodus mentions, give you no current limiting. Once a failure occurs, you can only hope your brakes are strong enough to hold until the fuse blows. Or if you have the clutch, disengage it and have your motor blow up.

Yeah, this is a DoItYourself site. But we try not to be a danger 

My opinion,

major


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## DavidDymaxion (Dec 1, 2008)

His last thread was basically how to do no controller, this one is about a basic old school controller. 

As others have said, if you tap the pack you'll work your batteries unevenly. You'll also be switching under heavy currents, which could kill your contactors in 100 or so cycles.

To do series/parallel switching works the batteries evenly, a very good thing, but takes many switches: 3 for 24 to 48 V, 9 for 24 to 48 to 96 V, etc. Given that contactors are around $100 each, before you know it you have a $900 or more controller -- that's getting close to the price of a basic silicon controller.

I would humbly suggest starting small. You'll learn a ton and mistakes will be much cheaper. Get a minibike, get 2 used batteries, a used motor, etc. Switch between 12 and 24 Volts. When you do a car, you can experiment with 12V to 24V switching, or maybe 24V to 48V switching. If you conquer that, then go higher.

On my car, switching will be series parallel, so the batteries will get worked evenly. I'm also going to do my switching as I shift gears, so the sepex motor will go to idle, so the contactors will break only about 10 or 20 Amps. It would be hard to get a series motor down to 10 or 20 Amps.


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