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186 Build Your Own Electric Vehicleplay two rounds of golf. So this figure tells you how long your batteries willpower your golf cart: two rounds, three rounds, etc.• Three-Hour Reserve Capacity—This is the BCI standard currently coming intovogue covering EV users. It is defined as 74 percent of the 20-hour rate. Becausethree hours translates to the average amount of time an EV might be in dailyuse, commuting, shopping, etc.:3-Hr Reserve Capacity 5 0.74 3 20-Hr Reserve CapacityThe Gentle Art of Battery RechargingThe objective with batteries is to maintain a balance. How fast batteries are filled andemptied are critical factors determining both their immediate efficiency and ultimatelongevity. Where the batteries are filled and emptied, relative to their state of charge, areequally critical factors.Because urban driving patterns for EVs are highly intermittent, battery dischargerates will vary all over the map. While energy is drawn out of your battery pack a lotharder than C/20 on startup and acceleration, you’re only doing this momentarily, andthe urban driving cycle usually implies that an EV’s battery pack is given a certainamount of “rest” between discharge requests. The bottom line is• Avoid placing continuous, heavy, C/1-type loads on your batteries anywhere intheir state-of-charge cycle. A battery pack that can deliver 100 percent of itscapacity when discharged in X time might only deliver 50 percent of its capacitywhen discharged in X/3 time. Remember the example of the water flowing outof the jug—the faster you take it out, the less pressure there is to push out theremaining 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 greatlyreduce battery life or even destroy them.• Unlike with discharging, you can control the destiny of your batteries duringthe charging process. In fact, it’s vital that you do, because both overchargingand undercharging shorten battery life. Continually overcharged or too rapidlycharged batteries can be destroyed; constantly undercharged batteries becomesulfated and inefficient. Chapter 9 covers modern battery rechargers that canhelp 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-ofchargerange, because a lead-acid battery’s ability to store energy is reducedwhen almost full or nearly empty. Below 20 percent and above 90 percent, C/20is the most efficient rate (divide the capacity of your battery in ampere-hours by20) to charge your batteries. In the 20–90 percent range, C/10 delivers the fastestrate at which it’s efficient to charge a lead-acid battery; it wastes more heat thanat the C/20 rate, but saves time. Below 90 percent, control charging by limitingthe current so as not to charge nearly empty batteries too rapidly. Above 90percent, limit voltage so as not to overcharge the batteries (or possibly damageother attached electronic devices).