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Secrets of the box

September 21, 2014

Promises were made for information. Here is the delivery. I've been looking at a lot of other people's EV conversions, particularily under the hood. In most cases The required componentry and cables are displayed proudly, often with flashy lighting and LED displays. Kinda going the other way here. More marine grade plywood put to use, as was used in the battery boxes. The squigly bits now reside in a box reminiscent of a tool box one may have found in a 1937 Chevy in its hayday. 

An electrical circuit has 4 primary parts; A power source, a switch, a load, and a protection device. In this case the power source is our high voltage battery pack, the load is the motor, but what about the other two? Not so obvious answers. Protection devices are in the form of high voltage fuses. Seen on the left side of the box connected to the red cable. Shockingly (he he) enough this is the positive feed. A 500 amp fuse is installed for the motor as the Curtis motor controller tops out at 500 amps. At 150 Volts this gives us our 75 kw rating. The switching is accomplished in a little more round about manner. That round thingy in there about the size of a beer can, it's a contactor. Basically a solenoid switch that is contolled by low power ( and voltage in this case ) that switches high power. The low voltage coil requires 12 volts, just like a normal car. When it is energized it turns on the big power for the motor.

So where do we get 12 volts from? You don't see a good old fashioned lead car battry under there do ya?

Well that other fuse in the box there ( the black one ) rated at a mere 100 amps powers  a DC to DC converter. This handy dandy device  shown below converts our 144v nominal battery to 12v nominal (actually about 13.5v) to run all the normal car things like lights and windshield wipers and of course the ignition switch that turns all this on and powers up the contactor so the motor will make the car go. Simple right? 

To the left is a shunt, connected to the negative feed. This device has a specific resistance, so that when 600 amps are flowing thru it, it has a voltage drop of exactly 75mv. This allows a remotely mounted ammeter to display the current flow in the system without having to run said massive amounts of power thru the dash. Connections are also made here to display the battery voltage inside the car.

That grey box may be the most interesting part. It is a clever little control box is supplied by EVTV. with their 3000 watt charger shown in the top right. Most EV battery chargers are preset at the factory for a specific voltage battery pack. This charger which is controlled by a CAN bus is adjustable. This is handy for guys like me, experimenting with custom battery packs. This CAN bus ( Car Area Network ) has to be supplied by a controller and this little grey guy does that. It is configured with a laptop and when it is powered up with 12v it tells the charger to charge to yur chosen setting. in this case about 160v.

 

Next time I'll show ya what's connected to the other end of those wires attached to the shunt.

Laters

 

Chris

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