7 3/8 x 9 1/4 T echnical / Build Your Own Electric Vehicle / Leitman / 373-2 / Chapter 4
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Build Your Own Elec tric Vehicle
Your Batteries Make a Difference Of all the pickup truck advantages, its extra room makes the biggest difference because it makes more space available to mount batteries. With car conversions, you must either choose a larger chassis or go to 8- or 12-volt batteries. With a pickup truck chassis, you can use more powerful 6-volt batteries to produce 120 volts and more. Both the 6- and 8-volt (more energy storage) and 12-volt (less total weight) options are available with the pickup conversion. More batteries means higher voltage, which dramatically improves your performance (see Figure 4-14). Vintage EV designs from the 1970s found 72 volts acceptable. Today, you would be unhappy with anything less than 96 volts, and 120 volts—the setup used for Chapter 10’s conversion—is even better. Beyond that, a 144volt battery setup is still better for motors designed to use the extra voltage. Remember, you need to have enough voltage to push the current you need to get the torque you need to go the speed you want. It is usually more efficient to run higher RPMs than higher current to get the power you need. Both maximum RPM and maximum current must be kept within the motor’s design spec. When the battery pack is more than 80 percent discharged is when you are most grateful for extra voltage. A 120-volt battery string comprising 20 6-volt batteries (about 1,200 lbs.) typically delivers a top speed of 60 mph or more and a 60-mile range (at reduced speed) in a 3,000-lb. curb-weight pickup truck with 4-speed transmission. You might get more or less depending on your design and components. Jim Harris’ Ford Ranger pickup conversion covered in Chapter 10 goes 75 mph, and his range was still increasing at press time. Increasing voltage will increase maximum motor speed and that equates to being able to drive faster. Increasing battery amp-hr will extend range. Adding batteries will add voltage and weight. But it is possible to increase voltage without changing weight by changing to a different battery.
Figure 4-14 More batteries are alw ays better than less—up to a point.
