This page discusses the traction circuit, which is the BIG circuit that connects the EV Batteries, Curtis Controller, and EV Motor together. My traction circuit is fairly typical of a DC-based EV conversion, except that my car has an electronic reverse since my EV Transmission Modification included removal of the mechanical reverse gear.

Layout and Construction

This circuit is Big. The Wires are big, and the Motor, EV Traction Battery, and Controller are all big. Placement of components is critical, but largely dictated by various necessities:

  • The motor attached to the (slightly relocated) transmission in place of the engine.
  • Batteries go whereever they would fit; while keeping the weight distribution of the car even. In my case, ten in the engine compartment and seven in the front compartment. Chassis Modifications, EV Battery Racks and Custom Motor Mounts were allpart of the job of fitting the batteries.
  • The controller went in the engine bay below the batteries, ahead of the motor. This location allows decent protection from road spray and rocks, but provides plenty of cooling air movement.
  • The reversing contactor goes in a convenient spot in the engine compartment next to the motor.
  • The circuit breaker went between the driver's and passenger seat, to allow easy reach.
  • The main contactor goes in a convenient spot in the front compartment, at the most positive point on the battery pack.
  • The Ammeter shunt goes in a convenient spot in the tunnel area; doubling as one half of a fuse bracket.
  • Three fuses went in where they would fit; one in front battery bank, one in back, and one between the two battery areas.

Major Components

advanced DC 9 inch motor
advanced DC 9 inch motor
motor and controller installed in the car
motor and controller installed in the car
controller with wiring attached
controller with wiring attached
rear compartment with batteries and cables installed
rear compartment with batteries and cables installed

The major components of the traction circuit are the batteries, motor, and controller. The batteries store energy, the motor converts electrical power into mechanical power, and the controller modulates power to the motor.

Safety and Control

The circuit must contain several components to allow control and provide safety features.

Contactors

main contactor (white case) and kill switch contactor (clear case)
main contactor (white case) and kill switch contactor (clear case)
main contactor with battery cable installed
main contactor with battery cable installed

Main Contactor

This is basically the big on/off switch for the motor and controller. It should be wired to open up under zero throttle (not just when the ignition is off) This way, if for some reason (it is known to happen) the controller shorts out, you can still cut power by letting off the throttle completely. Mine is a Tyco Electronics 250A rated contactor, but it will break 2000A if it needs to (but should probably be replaced if it actually does do this). Under normal circumstances, it will switch under no load so the contacts should last a long time.

Kill Switch Contactor

This is part of the EV Control Electronics. It is an off-the-shelf from fry's electronics 12v coil, SPST relay with ratings sufficient to handle 100+VDC at a couple of amps. It opens and closes in parallel with the main contactor, and also switches full traction battery voltage, to be supplied to the logic circuit of the controller. It is separate from the main contactor for a reason. Should the main contactor fail to open, this one will still open, and when it does, it cuts power to the controller logic circuit which causes it to shut down.

Circuit Breaker

Location identified for main breaker.  Easy reach for both passengers.
Location identified for main breaker. Easy reach for both passengers.
Hole cut, brackets made for breaker.
Hole cut, brackets made for breaker.
Breaker in place and shifter linking installed around it.
Breaker in place and shifter linking installed around it.
Interior console (with section cut away to allow breaker access) back in place.  Missing trim piece will be fabricated later.
Interior console (with section cut away to allow breaker access) back in place. Missing trim piece will be fabricated later.

Another safety power cutoff. My Heinemann 250A DC circuit breaker is reachable by the driver and passenger. It allows you to cut power in the event of a runaway controller, or a stuck throttle, or whatever. I also turn it off when the car is parked. It will pop automatically if current exceeds its rating for too long. It will go minutes at 300A to 400A, but will open within seconds once the current goes into the 1000s of amps, such as what would happen in a short circuit situation. It is also useful for cutting off the battery when working on the car. Electrically, mine is located between the most positive battery and the positive terminal of the controller, which means that openning it cuts power to the motor and controller but doesn't cut power to the DC-to-DC converter, heater, or disconnect the charger. A regular 20A fuse does that job on my car.

Fuses

fuses for the traction battery
fuses for the traction battery
front compartment fuse holder
front compartment fuse holder
fuse holder mounted in front compartment
fuse holder mounted in front compartment
fuse (and ammeter shunt) in tunnel area.  Undoubtedly the one to blow first.
fuse (and ammeter shunt) in tunnel area. Undoubtedly the one to blow first.

There should be several fuses in series with the battery pack. Mine came from Discount Fuse's Ebay store. These will blow if for some reason the circuit breaker does not, or if somebody drops a tool and shorts out only a portion of the battery pack. They should be as evenly distributed between the individual batteries as possible. In my case, I have three fuses, and there are no more than 5 batteries (30v) in series between any pair of fuses. The circuit breaker divides the most positive and least positive set of batteries. My fuses are rated for 500A (probably should be 600, as the controller itself is rated for 500) but so far I have been unable to pull more than 400A for more than a second or two anyway, so it will probably be fine.

Reversing contactor

Albright Reversing Contactor Mounted
Albright Reversing Contactor Mounted
Reversing Contactor with wiring installed.
Reversing Contactor with wiring installed.

My car is somewhat unique in that it has an electronic reverse. The Albright reversing contactor is basically a huge DPDT switch, wired to control the polarity of the field winding of the motor relative to the armature. It does this by energizing one of the two coils on the contactor to close the proper contact to make normal polarity. Energizing it the opposite way will reverse the polarity of the field winding. Having this contactor adds another safety dimension: I wired it such that it will cut off the motor from the controller when the ignition is turned off. Therefore, even if both my main contactor shorts, and my controller shorts, shutting off the ignition will still open this contactor, shutting off the juice. Note that it is also possible to wire it such that while it still works under normal circumstances, cutting off power to its coils will short out the output of the controller instead of opening it up. The motor would stop running, but it would still burn something up. The correct way to wire it is to have the common side of the DPDT switch on the field winding of the motor. See the EV Wiring Schematics page.

Cables and Insulation

Battery cable lugs and terminals.
Battery cable lugs and terminals.
Raw materials for battery and motor cables.
Raw materials for battery and motor cables.
Battery cables with insulating boots on old batteries for a test fit. (Rear compartment arrangement)
Battery cables with insulating boots on old batteries for a test fit. (Rear compartment arrangement)
Battery cables in the tunnel section under the car.  Breaker and Ammeter shunt visible.
Battery cables in the tunnel section under the car. Breaker and Ammeter shunt visible.

Battery and motor cabling must be heavy wire to handle the current loads of several hundred amps. It must have robust insulation to resist wear, rubbing, and other abuse. It should be flexible. Welding cable (available in bulk from welding and industrial supply outfits) is ideal for this purpose. For my car (and any non-drag-racer EV) 2/0 sized cable is fine, and that is what I used. I used red cabling, but I really wanted UL orange, as EMT and other first responders are trained to recognize orange wiring as dangerous. Red was the closest I could get.

The cabling needs to have connecters installed. I used regular automotive style terminals for the battery connections, and lug type terminals for everything else. The lug type terminals are intended to go over a post or be held down by a bolt. I used a hammer type crimping tool to attach the connectors to the wire, and some large heat shrink tubing to help seal the connection. I also used copious amounts of anti-corrosive compound on the crimped connections.

In some of the more exposed areas of the car, or where the battery or motor cabling would run close to other things or make contact, I added one (or in a couple cases) two layers of corrugated plastic cable sheathing as an extra layer of armor for the cabling. Anywhere a cable went through a metal bulkhead, I made sure there was a rubber grommet lining the hole.

Often overlooked or ignored on a lot of cars I have seen is a good effort to insulate the battery cables. Battery cables and any other high-current cables (especially those easily reached) should be as fully inslated as possible. You can buy rubber boots that fully cover terminals of a battery after the cable is hooked up. This way, as long as all the rubber boots are in place there is no exposed metal to be shorted out by a dropped tool, etc. Also it reduces dirt and corrosion buildup. I admit I did not put rubber boots on most of my motor and controller cables as they are located under the car where it is impossible to drop something on them, and because they don't have any voltage on them except when the car is running.

Smoke Testing

This circuit can provide enough power to cause serious damage, and serious injury, so it must be treated with respect. Therefore, I took several precautions when powering it up for the first time.

  • First, I hooked up batteries into several short strings, and then after testing each isolated string for their expected voltage and polarity, I hooked them all up into larger strings. This way I made sure I was getting things right, and I kept the voltages I was working with as low as possible for as long as possible.
  • As recommended in all the EV conversion books I have read, I made a couple of "battery wrenches" by simply fully insulating the handles of a 1/2 inch box and a 1/2 open end wrench so that it is impossible to drop them across a pair of battery terminals and cause a short. All other tools I work with I try to keep shorter than the distance between the terminals of a battery. I try not to lay tools on top of batteries while I am working.
  • Before I hooked up the final large cable (the most negative one in the car in my case) I jumpered it with a much smaller test lead. This allowed me to power up the traction circuit with its full voltage, but "fused" and current limited by the very small test lead. I was able to run the heater and check voltages for expected values at the charger plug, DC-DC converter input, etc. without risking hundreds of amps flowing unexpectedly. I was also able to check the polarity of the ammeter shunt (predictably, backwards) before hooking up the final big cable. The only thing that I was unable to try in this configuration is actually running the motor.
  • Since the test lead in the last step did not burn up, I decided it was safe to hook up the final wire. Once I did this, and then turned on the circuit breaker (with the transmission in neutral and my foot on the brake, and ready to instantly turn the breaker back off again if something unexpected happened) the traction circuit was live. I pushed down the throttle just barely enough to close the main contactor and kill switch contactor. I heard the controller whine, which was a good sign. A tiny bit more throttle and the motor spun up. Very good sign. Had the controller been wired wrong, it would either have not worked, had the motor run away, or burned up.
  • Since I have an electronic reverse, there was still the question of whether or not I got the polarity of the reversing contactor right. I put the car in gear, and very gingerly applied throttle until the car jerked forward slightly. As it turned out, I did have the polarity right. Had I not, I would only have had to reverse the two wires that supply power to the contactor coils, which are easily reachable in my front battery compartment. With foot off throttle and other foot on clutch to engage the right interlock switches, I put in in reverse, and applying throttle miraculously made it go the opposite direction.
  • The next major smoke test involved driving around the block at about 15 miles per hour, listening, watching and sniffing for anything funny. Once I got around, parked, cut power with the breaker, and I reached under the car and felt the controller. No heat. Good. As I have progressively driven farther, and up hills, I have been periodically doing the same thing: looking for overheating components.
  • With all this done, it basically passed the smoke test. I will still check things periodically of course, but as my confidence grows and the car proves itself, I won't have to do this sort of thing as often.