Electricity is dangerous

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Life is dangerous. Sticking your finger in a light socket is dangerous, but you probably continue to change light bulbs anyway, taking the necessary precautions. And driving a car is even more dangerous, regardless what kind of car it is; simply because you are sharing the road with millions of other humans, a certain percentage of whom are idiots, some of whom are drunk at any given time, some of whom are subject to seizures, some of whom cannot react quickly, and so on.

It takes a lot of energy to push a car 300 miles at highway speeds. No matter how that energy is stored, suddenly releasing all that energy can do a lot of damage.

Riding around with a tank full of gasoline right behind you, is kindof like carrying a bomb with you everywhere. Get rear-ended very hard, and that 300 miles worth of go-juice is liable to burn up all at once, with disastrous consequences. Electric cars do not carry that kind of hazard; batteries do not generally explode. (It's possible on a small scale, because charging them can electrolyze the water and generate hydrogen. But it's a small amount of hydrogen, and hydrogen is much lighter than air so it tends to get out of the way pretty fast rather than spilling all over the ground. And you will not be generating hydrogen while going down the road, anyway, since you are not charging the batteries then.)

As for the electrocution hazard, there are some important rules you need to follow when building your own car. The most important, is that the wires which go from the battery pack to the controller and thence to the motor, are very well insulated, and not connected to anything else. Certainly not to the frame of the car! You can have a negative ground for your 12V-powered accessories, but the traction pack must be completely isolated.

Beyond that, it is important to have some respect for your batteries, when you are doing maintenance on them, or connecting and disconnecting battery terminals. If one hand is touching a battery terminal, the other should be in your pocket. And you should be wearing rubber gloves on both hands besides.

There must be a large fuse which can break the circuit if a short somehow develops. There must also be a manual emergency disconnect, which you can reach from your driving position behind the wheel. It should be capable of breaking a current of a few thousand amps, and extinguishing the arc.

The pack is not continuously connected to the motor controller; rather, there is a main contactor, a large relay which is energized when you "start" the car and disconnected when you are parked.

There should also be an inertia switch, which will automatically disconnect the power from the traction pack in case of a hard impact. This is much like the kind which fuel-injected cars have, to shut down the fuel pump automatically in case of an accident. Since this kind of switch usually cannot break a large current, you may simply use it to break the circuit controlling the main contactor, so that the impact will cause the contactor to open up and prevent further driving until it is manually reset.