The Real-World Battery Charger

10 amps (C/20) for 5 hours 5 50 AH 20 amps (C/10) for 7 hours 5 140 AH 10 amps (C/20) for 1 hour 5 10 AH Totals: 13 hours 5 200 AH

This approach requires 13 hours to charge a 200 ampere-hour capacity battery. Provided you don’t exceed battery temperature, you could charge at the C/5 rate during the middle of the cycle (40 amps for 3.5 hours) and reduce total time to 9.5 hours.

The Real-World Battery Charger

The starting battery in your internal combustion engine vehicle is recharged by its engine-driven alternator, whose output is controlled by a voltage regulator. The starting battery is discharged less than 1 percent in its typical automotive role. The entire rated output power of the alternator is placed across it, and the voltage regulator makes sure voltage doesn’t climb above 13.8 volts—simple. Why not use the same setup to recharge your deep-cycle EV batteries?

The answer is voltage, current, and “boom.” This approach is not used because the voltage of your EV’s deep-cycle battery pack string is probably 96 volts or more, but the alternator is typically set up for driving a 12-volt starting battery. Assuming you could adjust your alternator or buy the correct voltage model, and had a suitable electric motor around to turn it, the full alternator output applied to a completely discharged deepcycle battery pack would deliver too much current and charge it far too quickly. You’d damage and/or destroy your batteries in short order—that’s the “boom” part. The voltage regulator would not only be useless in stopping this, but would prevent raising the voltage to 2.58 volts per cell for the final 10 percent of the cycle, and would not allow the voltage to be further raised to 2.75 volts for the equalizing charge process.

The solution is AC power, a transformer, a rectifier, a regulator of either the “electronically smart” or “manually adjustable” variety, and a timer: an accurate description of today’s EV battery charger.

The x pattern in the graph at the lower right in Figure 9-3 shows what most actual battery chargers deliver. Using a variation or combination of constant-current, constantvoltage, tapering, and end-of-charge voltage versus time methods, all battery chargers arrive at a method of current reduction during the charging cycle as the cell voltage rises. Fortunately, you can buy something off the shelf to take care of your needs. But you have to investigate before buying to make sure a given battery charger does what you want it to do.

Battery chargers are “sized” using the formula: Charging Current 5 (Battery Capacity 3 115 Percent)/(Time) 1 DC Load

In this equation—very similar to the equation earlier in the chapter—the charging current determines the size charger you need, the 115 percent is an efficiency factor to take losses into account, and the DC load is whatever else is attached to the battery (this is zero, assuming you disconnect your batteries from your EV’s electrical system while recharging). You’re already familiar with battery capacity and time. You can plug chargers up to 20 amps into your standard household 120-volt AC outlet. Higher current capacity chargers require a dedicated 240-volt AC circuit—the kind that drives your