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Green Battery Charging

Save energy by choosing the correct charger for your customer.

The greening of America, record high oil prices, global warming and rising energy costs are all reasons why your customers are looking for ways to reduce their energy consumption and, ultimately, lower their utility bills.

Electric lift truck fleet managers now have many different ways to charge their fleets while still sticking with the tried-and-true lead-acid battery. A battery charger is basically a rectifier, changing AC voltage to regulated DC voltage. The three most popular topologies in use today are ferroresonant, silicon controlled rectifier (SCR) and switch mode power supply (high-frequency). Each topology has its own unique set of advantages and disadvantages, but for the purpose of this article, we will concern ourselves with energy efficiency and the associated cost of operation.

Ferroresonant Chargers
Taper chargers utilizing ferroresonant transformers are still the most popular type of chargers sold in the United States today. They are the simplest and most reliable type of chargers, yet they have inherent- ly strong power characteristics. A typical well-designed ferroresonant battery charger with an output voltage of 36 volts will have a charge cycle efficiency of 85 percent to 87 percent and a power factor above 0.9. This translates to low infrastructure installation costs and good energy efficiency.

SCR Chargers
Chargers utilizing SCRs offer more flexibility than ferroresonant ones, but their electrical characteristics are not quite as strong. Typically, an SCR charger will have about the same energy efficiency as a ferroresonant charger, but the power factor will be much lower. This means increased installation costs as a result of higher input current.

High-Frequency Chargers
Over the last few years, the industry has experienced rapid growth in demand for high-frequency chargers. This growth was initially driven by the demand for rapid chargers but is now becoming popular for conventional charging as well. The interest in high-frequency is due to the excellent electrical characteristics that are available in this type of charger. The energy efficiency of a well-designed high-frequency charger can exceed 90 percent with power factors equivalent to the ferroresonant. This combination typically makes high-frequency chargers a good candidate to reduce energy consumption.

Determining Energy Savings
Charger manufacturers provide other methods to reduce energy consumption and/or costs without having to invest in new chargers. Most manufacturers provide electronic charge controls with energy-saving features, such as automatic termination, delayed start, time-of-day start and time-of-day block out.

There are many termination methods (which are primarily variations of the same method). The charger control monitors the battery during the final portion of the charge cycle for changes in voltage, current or resistance. When these values stop changing, this is an indication that the battery is fully charged and the charger will shut off, minimizing the amount of overcharge and thereby minimizing the amount of energy consumed to charge the battery.

Utility companies charge for energy consumption in a variety of ways. Charging for peak demand typically means that the utility monitors the customer’s energy consumption in 15-minute increments, and the highest 15-minute-period demand determines the customer’s demand rate charge. To help control peak demand, customers can use the charge controls to program start delays so that a bank of chargers won’t all be running at maximum power requirements.

Many utility companies also charge more for energy during certain times of the day, usually when the demand is highest. Some high-level controls give the customer the option of programming specific start times so that their chargers will always start at the same time of day, avoiding peak periods. Another method offered by some controls is time-of-day block out. Just like it sounds, when this option is selected, the charger begins charging just seconds after the battery is connected. If the charger is still charging when the beginning of the block-out time is reached, the charger will shut off and will remain off until the end of the programmed time period is reached. Again, this option can help the customer avoid charging during peak demand periods.

Table 1      
  Ferroresonant SCR High-Frequency
Energy usage per day (kWH)       62.40      61.80    58.40
Power demand (kW)         6.80         6.10       5.80
kVA demand (kVA)         7.30         8.80       6.10
Monthly usage charge    $47.34    $46.79   $44.22
Monthly demand charge    $67.66    $61.41   $58.04
Total monthly charge $115.00 $108.20 $102.26

Table 1 shows typical energy consumption data comparing ferroresonant, SCR and high-frequency chargers. All data assume charging a 750AH, 36-volt flooded lead-acid battery; operating two eight-hour shifts per day for five days per week; and utilizing conventional charging methods. Calculations use $0.035 per kW for energy charges and $10.00 per kW as a demand charge. Note that the high-frequency charger has the lowest total monthly charge.

Because there are so many variables in applications, you should consult your battery and charger provider to determine which charging method will work best for your customer.

Material Handling Equipment Distributors Association

Jim Keyser Meet the Author
Jim Keyser is business manager at AMETEK Prestolite Power, located in Troy, Ohio, and on the Web at www.prestolitepower.com.

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