Overview — 24V 50A Rack Mount Power Supply with N+1 and battery back-up24V 50A Rack Mount Power Supply with N+1 and battery back-up (SEC-2450BRM)
This modular 19” rack mount AC/DC power supply can provide up-to 50 Amps continuous power. The output DC voltage is highly regulated at 27.6 Volts (120 Volts AC input). An external battery can be connected for back-up and a section of the output current (up-to 8 amps) is diverted to charge the batteries when in state of discharge using the in-built battery back-up module. The battery supplies the DC load on loss of utility power. Features include N+1 redundancy, front panel analog Volt/Ampere meter display, remote monitoring and indication by 25-pin D-sub connector, fan failure warning and alarm indication. Ideal for critical power supply applications, telecommunication systems, mobile sound/audio displays, GPS receivers, security systems, CCTV systems.
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Features — 24V 50A Rack Mount Power Supply with N+1 and battery back-up24V 50A Rack Mount Power Supply with N+1 and battery back-up (SEC-2450BRM)
- Provides N + 1 redundancy
- Voltage/Ampere meter
- Module status LEDs, fault indicator LEDs and fault alarm are standard features on all models
- Highly regulated output voltage
- Short circuit protection and overload current
FAQs — 24V 50A Rack Mount Power Supply with N+1 and battery back-up24V 50A Rack Mount Power Supply with N+1 and battery back-up (SEC-2450BRM)
Q: Can I connect my SEC-1212 / SEC-1223 AC-DC power supplies in series to increase the output voltage?
Yes. However the output current rating will remain the same.
Q: Can I adjust the specified the output DC voltage of 13.8 Volts on the SEC 1212/ SEC 1223 AC-DC power supply?
The output can be adjusted by adjusting a potentiometer on the PCB inside the enclosure. The available voltage range is 12.5-18.5 Volts
Q: Can the output voltage be regulated at 12 V?
Yes this is possible by changing the zener diode in the unit. Call tech support for details.
Q: Does the unit have a fuse on the output?
No. This would have to be added externally. The unit does have a fuse on the input side.
Q: What is the minimum battery capacity required to be connected to the battery terminals of the SEC-xxxx BRM AC-DC power supply unit?
All SEC BRM AC-DC power supply units supply a maximum battery charging current of 8 A. Hence the minimum AH capacity of the battery bank connected to the battery terminals should be 54 AH (liberal estimate) to 80 AH (conservative estimate).
Q: Can the SEC-xxxx BRM AC-DC power supply be used to charge batteries in the absence of the DC load?
The SEC-xxxx BRM AC-DC power supply does not perform multi-stage charging at the battery terminals but provides a float-maintenance to the batteries with a maximum charging current of 8 Amps.
Q: Why are Switched Mode Power Supplies (SMPS) associated with radio interference?
Switched mode power supplies (SMPS) employ high frequency switching and thus, are a source of radio interference, a recipient of radio interference and a conduit of radio interference. (Older linear type transformer based power supplies do not employ high frequency switching voltages and will be quieter as compared to switching type of supplies).
Q: What are the typical sources of radio interference in a SMPS?
The primary emission sources originate in the switching devices due to their fast switching current transitions: harmonics of the switching frequency and broadband noise created by under-damped oscillations in the switching circuit. The secondary source is from the bridge rectifier, both rectifier noise and diode recovery.
The AC input rectifier / capacitor in the front end of the switching power supplies (excepting those with power factor correction) are notorious for generating power supply harmonics due to the non linear input current waveform. The noise is both conducted and radiated through the input power cord and the DC output wiring to the radio.
Q: How is a SMPS affected by received radio interference? How can the problem be solved?
Switching power supplies are also recipients of radio interference. The normal operation of the power supply can be disturbed due to RF noise getting coupled into the power supply.
Thus, the power supply may generate excessive RF noise and lose output voltage regulation due to excessive transmitter energy being coupled through the AC / DC lines to the power supply’s regulator feedback path. This may be due to antenna being too close or due to the antenna or feed system not radiating properly. First check the antenna system SWR. Then, if necessary, relocateeither the antenna or the power supply farther apart.
Q: What is the source of the "buzzing" sound heard at the receiver? How can this be avoided?
The receiver may “hear” the power supply. A slowly moving, slightly buzzing carrier heard in the receiver may be caused by the antenna being too close. As with the transmitter related noise pick up, a loose coaxial connector or a broken or a missing ground may aggravate this problem.
Normally these noises will be below the background or “band” noise. Increase the separation between the power supply and the receiving antenna. Use an outdoor antenna. This will reduce the amount of signal picked up from the power supply and also increase the amount of the desired signal.
Q: What are the standards pertaining to RF noise in SMPS?
The conducted and radiated noises are limited as per the applicable national / international standards. In North America, the applicable standard is as per FCC Part 15(B) for Class “B” digital devices.
The European standard is as per EN55022, Class “B” & EN610000-3-2, 3. Thus, the RF interference is limited but not entirely eliminated.
Q: How can conducted RF noise be limited in a SMPS?
The conducted RF noise from these power supplies is limited to the maximum allowable levels by internal filtration. The filtered RF noise currents (normally < 5mA) are bypassed to the chassis of the power supply. The chassis is, in turn, connected to the earth ground pin of the AC input power cord (for Class 1 units).
Thus, the filtered noise currents are intentionally leaked to the earth ground. This is termed as the “Earth Leakage Current”. For safety against electric shock, this earth leakage current is also required to be limited. It will be seen that these two requirements are conflicting.
NOTE: In some cases, to prevent electric shock hazard due to abnormal leakage current (like in marinas, spas, hot tubs, wet spaces etc.), the AC outlet circuits / receptacles in these areas are served through a GFCI (Ground Fault Circuit Interrupter).
This GFCI is normally set to trip when it senses an earth leakage current > 5 mA. A single GFCI may be serving multiple AC outlet circuits / receptacles and therefore, will be sensing the sum of all the leakage currents of the devices connected to these. As the switching power supplies have intentional leakage current as explained above, it may trip a GFCI feeding multiple AC outlet circuits / receptacles. In such cases, disconnect devices connected to the other AC outlet circuits / receptacles served by this GFCI.
Q: What additional steps can be taken to reduce the effects of RF noise?
Following additional guidelines may be followed to reduce the effects of RF noise:
- Use additional appropriate AC radio frequency interference (RFI) power line filter immediately before the ac input of the power supply. Recommended: Corcom Inc. ( www.cor.com ) “Q” series. Filtered, ferrite coated cord set (www.emceupen.com ) is another choice. These cord sets, with integral line interference filters, reduce common and differential mode interferences over a wide frequency range. Because they are shielded, they are also effective against radiated interferences. In addition to the built-in filter networks, the cable conductors are coated with an RF absorbing ferrite compound. This provides additional attenuation at high frequencies that is lacking in most regular LC filters. The RF absorption of the ferrite-coated cable avoids resonance’s at high frequencies, reducing the conducted and radiated RF noises even further
- Use additional appropriate DC radio frequency interference (RFI) power line filter immediately after the DC output of the power supply. Recommended: Corcom Inc.(www.cor.com ) “DA” / “DC” series
- Twist the positive and negative wires from the output of the power supply to the radio
- The DC side positive and negative outputs of these power supplies are isolated from the chassis. As explained at paragraph 6 above, the noise currents are filtered to the chassis ground and the chassis ground is connected to the earth ground through the earth ground pin of the AC power outlet receptacle. Avoid connecting (referencing) the DC negative output terminal of the power supply to the earth ground
- Connect a 1/4″ wave length of wire on the negative terminal of the power supply. Connect one end of the wire to the negative terminal and leave the other end free. The wave length corresponds to the wave length of the interfering frequency. (May not be practical for long wave lengths)
[ Formula: Wave length (Meters) = 300 / frequency in MHz ]
Q: What is Photo Voltaic (PV)?
The word photo-voltaic is derived from two different words; the word photos, from the Greek, meaning light and the word ‘voltaic’ developed from the name of the Italian scientist, Volta, who studied electricity. This explains what a PV system does: it converts light energy from the sun into electrical energy.
Q: What is in a photovoltaic (PV) system?
Non Grid-tied PV / Solar System
Above diagram shows a typical non-grid tied Photovoltaic (PV) System with its main components. It consists of a PV / Solar Panel (Module), Charge Controller, Batteries and Power Inverter. The PV / Solar Panel (Module) converts the solar light energy into DC electrical energy. The Charge Controller conditions the DC electrical voltage and current produced by the PV / Solar Panel (Module) to charge a battery. The battery stores the DC electrical energy so that it can be used when there is no solar energy available (night time, cloudy days etc). DC loads can be powered directly from the PV / Solar Panel (Module) / Battery. The inverter converts the DC power produced by the PV / Solar Panel (Module) / stored in the battery into AC power to enable powering of AC loads.
Grid-tied PV / Solar System
Above diagram shows a Grid-tied PV / Solar System. In this system, the Solar Panels (Modules) / Arrays directly feed to an inverter and the inverter is connected to an Electricity Transmission and Distribution System (referred to as the Electricity Grid) such that the system can draw on the Grid’s reserve capacity in times of need, and feed electricity back into the Grid during times of excess production. In order to safely transmit electricity to your loads and to comply with your power provider’s grid-connection requirements, you may need the following additional items:
- Power conditioning equipment
- Safety equipment
- Meters and instrumentation
Q: What are solar (PV) cell modules and arrays?
The basic element of a PV System is the photovoltaic (PV) cell, also called a Solar Cell. An example of a PV / Solar Cell made of Mono-crystalline Silicon is shown below. This single PV / Solar Cell is like a square but with its four corners missing (it is made this way!).
Theory Behind Operation of a PV/Solar Cell
A PV / Solar Cell is a semiconductor device that can convert solar energy into DC electricity through the Photovoltaic Effect -conversion of solar light energy into electrical energy. When light shines on a PV / Solar Cell, it may be reflected, absorbed, or passes right through. But only the absorbed light generates electricity.
PV Module / Panel and PV Array
To increase their utility, a number of individual PV cells are interconnected together in a sealed, weatherproof package called a Panel (Module). For example, a 12 V Panel (Module) will have 36 cells connected in series and a 24 V Panel (Module) will have 72 PV Cells connected in series
To achieve the desired voltage and current, Modules are wired in series and parallel into what is called a PV Array. The flexibility of the modular PV system allows designers to create solar power systems that can meet a wide variety of electrical needs. Below is a schematic of a PV cell, Panel (Module) and Array.
The cells are very thin and fragile so they are sandwiched between a transparent front sheet, usually glass, and a backing sheet, usually glass or a type of tough plastic. This protects them from breakage and from the weather. An aluminum frame is fitted around the module to enable easy fixing to a support structure. The picture below shows a small part of a Module with cells in it. It has a glass front, a backing plate and a frame around it.
As mentioned, PV / Solar cells are wired in series and in parallel to form a PV / Solar Panel (Module). The number of series cells indicates the voltage of the Panel (Module), whereas the number of parallel cells indicates the current. If many cells are connected in series, shading of individual cells can lead to the destruction of the shaded cell or of the lamination material, so the Panel (Module) may blister and burst. To avoid such an operational condition, Bypass Diodes are connected anti-parallel to the solar cells as in the diagram below. As a consequence, larger voltage differences cannot arise in the reverse-current direction of the solar cells. In practice, it is sufficient to connect one bypass diode for every 15-20 cells. Bypass diodes also allow current to flow through the PV module when it is partially shaded, even if at a reduced voltage and power. Bypass diodes do not cause any losses, because under normal operation, current does not flow through them.
Q: What are Current, Voltage and Power Curves of a Solar (PV) Panel?
A Current (I) versus Voltage (V) Curve of a PV / Solar Module shows the possible combinations of its current and voltage outputs. A typical I-V curve for a 12 V Module is shown above.
The power in a DC electrical circuit is the product of the voltage and the current. Mathematically,
Power (P) in Watts (W) = The Current (I) in Amperes (A) X the Voltage (V) in Volts (V) i.e. W = V X A
A Solar (PV) Cell or a Panel / Module produces its maximum current when there is no resistance in the circuit, i.e. when there is a short circuit between its Positive and Negative terminals. This maximum current is known as the Short Circuit Current and is abbreviated as Isc. When the Cell / Panel (Module) is shorted, the voltage in the circuit is zero.
Conversely, the maximum voltage occurs when there is a break in the circuit. This is called the Open Circuit Voltage (Voc). Under this condition, the resistance is infinitely high and there is no current, since the circuit is incomplete. Typical value of the open-circuit voltage is located about 0.5 -0.6 V for Crystalline Cells and 0.6 -0.9 V for Amorphous Cells. These two extremes in load resistance, and the whole range of conditions in between them, are depicted on the I-V Curve. Current, expressed in Amps, is on the vertical Y-axis. Voltage, in Volts, is on the horizontal X-axis.
Maximum Power Point and Rated Power of Solar (PV) Panel
There is a point on the knee of the I-V Curve where the maximum power output is located and this point is called the Maximum Power Point (MPP). The voltage and current at this Maximum Power Point are designated as Vmp and Imp.
The values of Vmp and Imp can be estimated from Voc and Isc as follows: Vmp = (0.75 to 0.9) Voc Imp = (0.85 to 0.95) Isc.
The rated power of the PV / Solar Module in Watts (Pmax) is derived from the above values of voltage Vmp and current Imp at this Maximum Power Point (MPP):
Rated power in Watts, Pmax = Vmp x Imp
Example of I-V Curve and Ratings of a 12 V Solar (PV) Panel
This Maximum Power Point in the example curve given above is where Vmp is 17 Volts, and the current Imp is 2.5 amps. Therefore, the rated or the maximum power Wmax in watts is 17 Volts times 2.5 Amps, or 42.5 Watts.
Standard Test Conditions (STC) for Specifying PV / Solar Modules
The I-V curve is also used to compare the performance of PV / Solar Modules. The curve is, therefore generated based on the performance under Standard Test Conditions (STC) of sunlight and device temperature of 25°C. It assumes there is no shading on the device. Standard sunlight conditions on a clear day are assumed to be 1,000 Watts of solar energy per square meter (1000 W/m2 or 1 kW/m2). This is sometimes called one sun, or a peak sun. Less than one sun will reduce the current output of the PV device by a proportional amount. For example, if only one-half sun (500 W/m2) is available, the amount of output current is roughly cut in half.
Q: What are types of batteries for solar systems?
There are several different types of battery chemistry including liquid lead-acid, nickel-iron (NiFe), nickel-cadmium (NiCad), alkaline, and gel-cell. Batteries are either sealed or vented. Simply, there are only two principal types of batteries: starting and deep-cycle.
Starting batteries are designed for high cranking power, but not for deep cycling. Used as energy storage, they will not last long in a deep cycle application. Starting batteries use lots of thin plates to maximize the surface area of the battery. This allows very high starting current but lets the plates warp when the battery is cycled. This type of battery is not recommended for the storage of energy in hybrid system. However, they are recommended as starting battery for the back-up generator.
Deep Cycle Batteries
Deep cycle batteries are the type of battery best suited for use with inverters. The physical dimension of the plates is thicker and the active material that holds the charge is denser to increase cycle life. The deep cycle type of battery is designed to have the majority of their capacity used before being recharged. They are available in many sizes and in either non-sealed or sealed types.
Usual battery inverters are optimized for use with lead acid batteries that have a nominal voltage of 2.0 volts per cell. NiCad/NiFe batteries (also called alkaline batteries) have a nominal cell voltage of 1.2 volts per cell. The nominal voltage of a NiCad / NiFe battery bank can be made the same as a lead acid bank just by juggling the number of cells (10 cells for 12 volts, 20 cells for 24 volts and 40 cells for 48 volt systems). However, the NiCad/NiFe battery bank must be charged to a higher voltage to fully recharge and will drop to a lower voltage during discharging compared to a similarly sized lead acid type battery.
Q: What are some requirements for solar batteries?
Typical requirements for the battery to be used in long term storage are:
- low specific kWh-cost, i.e. the stored kWh during the whole life of the battery
- long lifetime
- high overall efficiency
- very low self-discharge
- low maintenance cost
- easy installation and operation
- high power
Usually it refers to a sum of investment and operation costs of the battery divided by the stored kWh (kWh) during its whole life. This cost is thus influenced by the battery’s lifetime.
The lifetime of the battery should be long, especially in order to keep the specific kWh-cost and the installation cost low, particularly in remote areas.
nE = nI n nV
The battery will usually need more charge than was taken out to fill it back up to its starting point. Typical average efficiencies are 80 to 85 % for stand-alone PV systems.
The voltage efficiency is determined by the average discharge voltage (VD) and average charging voltage (VC). VC is lower than VD particularly by internal resistance of the battery.
nV = VD / VC
The overall efficiency should be as high as possible, to be able to pass the biggest proportion of the energy in the battery, which is generated by the PV generator.
The battery discharges itself even without load connected. This effect is caused by secondary reactions at its electrodes and proceeds faster with higher temperature or in older batteries. Thermodynamic instability of the active materials and electrolytes as well as internal- and external short-circuits lead to capacity losses, which are defined as self-discharge. This loss should be small, particularly in respect of annual storage.
The maintenance, e.g. water refilling in case of lead-acid batteries, should be kept as low as possible.
In special cases, a battery must be highly loadable for a short time, e.g. at the start of diesel generators or in case of momentary power extension of PV systems. There are many types of batteries potentially available for use in stand-alone PV systems. Useful data of available batteries given in Table 1 shows approximated values and are provided as a guideline.
Table 1. Comparison between selection criteria of available batteries
|Type||Cycle life until 80%, DOD||Efficiency||Temp. range, °C|
|Lead Acid||500 – 1500||80||-15° – .+50°C|
|NiCd||1500 – 3000||71||– 40° – +45°C|
|NiFe||3000||55||0 – +40°C|
Since many values are dependent on charge and discharge conditions, they have not been standardized for PV applications and for test purposes until now. Therefore, the comparison between batteries and selection of the most suitable one for each application are not easy. Due to particular operating conditions with PV applications in practical operation, the cycle life given by manufacturer (and in Table 1) for cycling load can be reduced more than half.
According to Table 1 above, it follows that in most cases the lead-acid batteries would be the best choices for PV applications. The selection of suitable choices should be based on specific application.
Q: What are characteristics of lead acid batteries?
A basic equivalent circuit of the lead-acid battery is modeled by a voltage source with an equilibrium voltage (VE) in series with an internal resistor (Rin)(see below). It must be noted here that this configuration can describe only a current state because the magnitude of VE and Rin are not actually constant, but is function of many parameters such as state of charge (SOC), temperature, current density, and aging of the battery.
Furthermore, it is to consider that these parameters depend also on the current direction (charging or discharge). When the battery were at rest or under open-circuit condition VB = VE. When current is drawn from the battery, the voltage will be lower than VE. When current is flowing into the battery, the terminal voltage will be higher than VE.
Higher discharge current results in reduction of the terminal voltage. Therefore, to specify the state of the battery by the battery voltage, discharge current should be also measured.
In case of discharge, the minimum voltage level acceptable for a lead-acid battery is defined as discharge voltage threshold. Falling below this threshold is called deep discharge, with which the battery may suffer damage. In case that the battery is left longer after deep discharge, lead of the support structure is converted to lead-sulphate in rough-crystalline form, which during charging can be only bad or cannot be converted again anymore. As a result, the battery loses a part of its storage capacity; besides loss of support structure arises as well.
In practice, harmful deep discharge is to be prevented: the loads will be compulsory disconnected from battery as soon as the discharge voltage threshold is reached i.e. with the help of a so-called deep discharge protection (DDP). This threshold is basically given in the data sheets by the manufacturer for different discharge currents. Preferably, the value of this threshold should depend on the discharge current. The relation between the discharge current and the voltage during discharge for the lead-acid battery is presented below.
The above graph shows the discharge profile of a typical battery type at several constant current rates. The typical end-of-discharge voltage at these discharge rates can also be noticed where the voltage starts to drop steeply. Moreover, the end-of-discharge voltage varies between 1.75-1.9 V, depending on the battery type and the discharge current. Higher service capacity is obtained at the lower discharge rates. At higher discharge rates, the electrolyte in the pore structure of the plate becomes depleted, and it cannot diffuse rapidly enough to maintain the cell voltage. However, intermittent discharge, which allows time for electrolyte diffusion will improve the performance under high discharge rates.
With 2.3 V and 2.4 V, gas is developed at the electrodes in the battery, by which the water is decomposed into hydrogen and oxygen. Both gases mix together in the battery providing detonating gas (explosive!) and escape through ventilation opening in the vent plug. With the gassing, the battery loses also water, which must be refilled according to maintenance within regular intervals. The gas is the unwelcome secondary reaction of the chemical conversion during charging because current is consumed for the electrolysis and therefore the storage efficiency of the battery is made worse unnecessarily. After the gassing voltage is exceeded, voltage stays approximately constant. The whole charging current during this period results in H2 and O2, which is defined as loss.
Freezing of electrolyte
For applications with low ambient temperature, the lead-acid battery must also be protected against freezing of electrolyte. The risk of freezing depends on the state of charge.
The graph below illustrates the freezing limit as a function of the state of charge.
Cycle life of lead acid batteries
Q: How a RV solar system works?
Download a schematic of how a RV solar system works with Samlex products and your RV.