HOW TO HANDLE THE LIMITED DC SYSTEM VOLTAGE WINDOW AN ELTEK WHITE PAPER THREE METHODS TO PROVIDE BOOST AND EQUALIZE CHARGES FOR NI-CD BATTERIES FROM DC POWER SYSTEMS WITH A LIMITED VOLTAGE WINDOW Author: Gunnar Hedlund www.eltek.com
TABLE OF CONTENT TABLE OF CONTENT... 2 INTRODUCTION... 3 CHARGE AND DISCHARGE OF A NI-CD BATTERY CONSISTING OF POCKET PLATE ELECTRODES... 3 Constant voltage charge... 3 Two level charge... 3 Single level charge... 4 Float effect / Voltage depression... 4 Charge acceptance... 4 Discharge characteristics... 5 Designing a DC system within a certain DC voltage window... 5 DIFFERENT SYSTEM DESIGN TO REDUCE THE OUTPUT VOLTAGE... 6 Drop diodes in series with the load output... 7 DC/DC converters connected in series with the load... 8 Dual output DC system divides the rectifier into two independent branches for battery and load supply... 9 CONCLUSIONS... 10 SUMMARY... 10 ELTEK`S APPROACH... 11 Eltek s Dual Output DC System... 11 REFERENCES... 12 HOW TO HANDLE THE LIMITED DC SYSTEM VOLTAGE WINDOW 2
INTRODUCTION Designing a DC system requires consideration of the ability to work within a certain DC voltage window. The maximum voltage provides the high-end of a voltage range that is available to charge the battery, whereas, the minimum voltage gives the lowest voltage within that range that is acceptable to the system so that the battery can be discharged. It is not unusual for this voltage window to be very narrow, where specifications can be as close as +/- 10% from the nominal DC voltage. This limited system voltage window can be a challenge to take into consideration when designing a DC system, especially when using it with Ni-Cd back up batteries that have a wide voltage window. What is needed is a boost or equalize charge to reach the maximum voltage that is effective for the attached batteries. This paper will present a different system designed to cope with high voltage on the load output during boost or equalize charge with a Ni-Cd battery. CHARGE AND DISCHARGE OF A NI-Cd BATTERY CONSISTING OF POCKET PLATE ELECTRODES Constant voltage charge The common method to charge a battery in stationary applications is carried out by a constant voltage system. The recommended solution is to use a two-rate type that is able to provide a constant voltage charge and a lower floating voltage or single rate floating voltage. This twolevel charger has a high voltage stage to charge the battery properly after a discharge followed by a lower voltage float level charge. This results in a quick charge of the battery and relatively low water consumption, due to the low-level float charge. Two level charge Boost charge: 1.55 1.70 V/cell Floating: 1.40 1.42 V/cell A high voltage will increase the speed and efficiency of recharging the battery. In reality, a single level charger can often be found. This is surely a compromise between a voltage high enough to charge the battery and low enough to have adequate water consumption. HOW TO HANDLE THE LIMITED DC SYSTEM VOLTAGE WINDOW 3
Single level charge 1.45 1.50 V/cell Float effect / Voltage depression When a Ni-Cd battery operates at a fixed floating voltage over a certain period of time, a decrease in the voltage level of the discharge curve occurs. It begins after one week and reaches its peak in approximately 3 months. Since this effect reduces the voltage level of the battery, it therefore reduces the performance and autonomy of the battery too. Thus, it is necessary to take this effect into consideration when sizing a Ni-Cd battery. The floating effect is a reversible effect and can only be eliminated by a full discharge / charge cycle. Please note that it cannot be prevented by just a boost charge. Charge acceptance A discharged Ni-Cd cell will take its time to reach a full state of charge. The graphs below display the time needed to achieve a certain stage of charge for a typical L- type Ni-Cd battery. A low float voltage with no possibility for a boost or equalize charge will result in a very long recharge time. A voltage window with no capability to make a boost or equalize charge will leave the Ni-Cd battery in a partial state of charge. HOW TO HANDLE THE LIMITED DC SYSTEM VOLTAGE WINDOW 4
Discharge characteristics The graphs below show a typical discharge curve for an L-type Ni-Cd battery. Depending on load, the final voltage per cell needs to be calculated within the voltage window. Typically, the final voltage can be between 1.0-1.14 V/cell depending on the number of cells included in the battery. Designing a DC system within a certain DC voltage window As described above, a Ni-Cd battery typically has a very wide voltage window, between 1.0-1.14 V/cell for final voltage and between 1.5-1.7 V/cell for boost voltage. With a limited voltage window, a single-level charge can be found, but design factors such as recharge time, state of charge, float effect and final voltage for discharge need to be taken into consideration. With an increased voltage window, the Ni-Cd battery can usually be designed more efficiently regarding recharge time, state of charge, float effect and final voltage for discharge. To increase the voltage window with no impact on the batteries, different equipment has been used, to reduce the output voltage to the batteries by installing drop diodes or DC/DC converters in series with the load output. Also, a dual output system is available where the DC charger is divided into two different branches, and each branch can independently deliver a float, boost or equalize charge with no impact on the output voltage. HOW TO HANDLE THE LIMITED DC SYSTEM VOLTAGE WINDOW 5
DIFFERENT SYSTEM DESIGN TO REDUCE THE OUTPUT VOLTAGE There are different system designs available to cope with high voltage on the load output when using a Ni-Cd battery in a DC system. The common way to increase the available voltage window for boost and equalize charge is to install equipment in series with the load to reduce the output voltage. Another method of system design is to divide the charger into two independent branches, where the battery branch can operate independently for float boost or equalize charge. A load branch supplies the load at a stabilized output voltage independently of the battery branch. Below, we will describe three different system designs to reduce the output voltage in a DC system when using Ni-Cd batteries with high boost and equalize charge. A. Drop diodes connected in series with the load during boost and equalize charge to reduce the output voltage to the batteries. B. AC/DC converters connected in series with the load to stabilize the output voltage during boost and equalize charge. C. Dual output DC system divides the rectifier into two independent branches for battery and load supply. Boost or equalize charge can be performed without any impact on the load voltage. HOW TO HANDLE THE LIMITED DC SYSTEM VOLTAGE WINDOW 6
Drop diodes in series with the load output Normal operation During normal operation when batteries are in float, the contactor is closed and output drop diodes are shorted. Boost or equalize operation During boost or equalize charge, the contactor is open and a voltage drop over the diodes is maintained according to the customer s specification. During boost or equalize charge, all load current is flowing through the drop diodes and creates a power loss, which will generate heat. Depending upon load output and design of voltage drop over the diodes, this power loss can exceed several kw. The design of such a system incorporates heat dissipation, additional power from the rectifier for power loss over drop diodes and bulky drop diodes mounted on cooling plates. Many drop diodes in series will also increase the risk of a single point of failure. HOW TO HANDLE THE LIMITED DC SYSTEM VOLTAGE WINDOW 7
DC/DC converters connected in series with the load To stabilize the output voltage during boost or equalize charge, a DC/DC converter can be connected in series with the load output. The DC input of the DC/DC converters has a very wide voltage window to accept the high output voltage from the rectifiers during boost and equalize charge. The output voltages from the DC/DC converters are stabilized during boost and equalize charge. The design of such a system incorporates rectifying the DC output voltage in two steps, which could be an expensive solution in large systems, but could be a good solution for systems with low power on the load output. As DC/DC converters are usually current-limited, maximum current for fuse tripping needs to be included in the total power calculation for the DC/DC converters. HOW TO HANDLE THE LIMITED DC SYSTEM VOLTAGE WINDOW 8
Dual output DC system divides the rectifier into two independent branches for battery and load supply Normal operation (float, boost or equalize) The battery branch can operate independently for float, boost or equalize charge. The load branch supplies a stabilized output voltage, independently of the battery branch. Operation in case of mains or rectifier failure Mains or rectifier failure of the load branch will automatically trigger a sequence to immediately connect the batteries to the load with no interruption. An inter-cell tap connection prevents a drop in output voltage, before the contactor to the full battery voltage is closed. If the battery branch is in boost or equalize charge, as in the case of a blackout of the load branch, the boost or equalize charge will be inhibited before the contactor is closed, to prevent high output voltage. Any situation where current flows thru the diode will trigger this operation. Total rectifier capacity No additional power for heat loss in the drop diode needs to be added to the rectifier capacity. Load branch capacity is calculated according to needed load, and battery branch is calculated according to battery recharge time. In total, the capacity of the two branches will be less than a system with drop diodes where additional power for voltage drop needs to be included into the total rectifier capacity. HOW TO HANDLE THE LIMITED DC SYSTEM VOLTAGE WINDOW 9
CONCLUSIONS Ni-Cd batteries are widely used in the industry and are one of the most reliable systems that can be found in the market. They are often the only reasonable choice when operating under extreme environmental conditions, especially in high temperatures. When designing a system such as one with a wide voltage window between final voltage and boost charge, charge acceptance, float effect and discharge characteristics need to be taken into consideration. With a limited voltage window, a single level charge can be found, but with an increased voltage window, the Ni-Cd battery can usually be designed more efficiently regarding recharge time, state of charge, float effect and final voltage for discharge. Different system designs are available to cope with high voltage on the load output when using a Ni-Cd battery in a DC system. The common way to increase the available voltage window for boost and equalize charge is to install equipment in series with the load to reduce the output voltage. Another method of system design is to divide the charger into two independent branches, where the battery branch can operate independently for float boost or equalize charge and a load branch supplies the load at a stabilized output voltage independently of the battery branch. SUMMARY Drop diodes in series with the load are one of the solutions used to cope with high voltage on the output. Power loss over the drop diodes can exceed several kw, where heat dissipation and additional power from rectifiers need to be included in the design of such system. A DC/DC converter can be connected in series with the load output. Rectifying the DC output voltage in two steps could be an expensive solution in large systems, but could be a good solution for systems with low power on the load output. Maximum current for fuse tripping needs to be included in calculations. Dividing the charger into two independent branches where the battery branch can operate independently for float boost or equalize charge is a design that eliminates heat loss in drop diodes and is very cost efficient in a large system with high power on the load output. HOW TO HANDLE THE LIMITED DC SYSTEM VOLTAGE WINDOW 10
ELTEK`S APPROACH Eltek can supply DC power systems with all mentioned designs like drop diode on output, DC/DC converters in series with the load output and dual output DC systems. Our Smartpack2 controller has the ability to control the contactor switch in systems with drop diodes, as well as dual output systems with individual voltage control and individual currentsharing between the battery and load branch. Eltek s Dual Output DC System Delivers boost and equalize charges with no impact on load voltage No heat is generated during boost or equalize charge There is no impact on efficiency of power system No additional power for heat loss in drop diodes Battery power available for fuse tripping Less cost than drop diode unit No diode in series with load during equalize or boost charge Space saving Up to 300 A output for load and battery power HOW TO HANDLE THE LIMITED DC SYSTEM VOLTAGE WINDOW 11
REFERENCES 1 Technical Manual EN-GAZ-TMSR-001 2 Saft Industrial battery group Doc no:21541-2-0407 3 HBL NiCad batteries (UK) Ltd. NCCP single tech HOW TO HANDLE THE LIMITED DC SYSTEM VOLTAGE WINDOW 12