7 3/8 x 9 1/4 T echnical / Build Your Own Electric Vehicle / Leitman / 373-2 / Chapter 8
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Build Your Own Elec tric Vehicle play two rounds of golf. So this figure tells you how long your batteries will power your golf cart: two rounds, three rounds, etc. • Three-Hour Reserve Capacity—This is the BCI standard currently coming into vogue covering EV users. It is defined as 74 percent of the 20-hour rate. Because three hours translates to the average amount of time an EV might be in daily use, commuting, shopping, etc.: 3-Hr Reserve Capacity 5 0.74 3 20-Hr Reserve Capacity
The Gentle Art of Battery Recharging The objective with batteries is to maintain a balance. How fast batteries are filled and emptied are critical factors determining both their immediate efficiency and ultimate longevity. Where the batteries are filled and emptied, relative to their state of charge, are equally critical factors. Because urban driving patterns for EVs are highly intermittent, battery discharge rates will vary all over the map. While energy is drawn out of your battery pack a lot harder than C/20 on startup and acceleration, you’re only doing this momentarily, and the urban driving cycle usually implies that an EV’s battery pack is given a certain amount of “rest” between discharge requests. The bottom line is • Avoid placing continuous, heavy, C/1-type loads on your batteries anywhere in their state-of-charge cycle. A battery pack that can deliver 100 percent of its capacity when discharged in X time might only deliver 50 percent of its capacity when discharged in X/3 time. Remember the example of the water flowing out of the jug—the faster you take it out, the less pressure there is to push out the remaining amount. • Avoid over-discharging your batteries when they’re below 20 percent state-ofcharge. High-rate discharging below the 20 percent state-of-charge can greatly reduce battery life or even destroy them. • Unlike with discharging, you can control the destiny of your batteries during the charging process. In fact, it’s vital that you do, because both overcharging and undercharging shorten battery life. Continually overcharged or too rapidly charged batteries can be destroyed; constantly undercharged batteries become sulfated and inefficient. Chapter 9 covers modern battery rechargers that can help you. The top of Figure 8-2 shows the ideal battery charging curve. • Confine heavy charging within the 20 percent to 90 percent of the state-ofcharge range, because a lead-acid battery’s ability to store energy is reduced when almost full or nearly empty. Below 20 percent and above 90 percent, C/20 is the most efficient rate (divide the capacity of your battery in ampere-hours by 20) to charge your batteries. In the 20–90 percent range, C/10 delivers the fastest rate at which it’s efficient to charge a lead-acid battery; it wastes more heat than at the C/20 rate, but saves time. Below 90 percent, control charging by limiting the current so as not to charge nearly empty batteries too rapidly. Above 90 percent, limit voltage so as not to overcharge the batteries (or possibly damage other attached electronic devices).
