What is a UPS? It is an electrical device that can provide emergency power back up to a load if there is a power failure from the mains power supply. Basically it is made up of a battery charging circuit, a battery for energy storage, and a power inverter circuit.
The charging circuit is usually frequency-modulated. It has a comparator circuit that can compare voltage differences between input and output voltages. Comparator circuit protects the battery from overcharging when it is fully charge. The battery is usually a sealed- lead acid deep cycle type. Today, some designs use lithium-ion battery, which can be charged faster, lightweight but high capacity and can last longer in operation compare to older types of rechargeable battery. An inverter is a circuit that converts a direct current to an alternating current. Older versions of UPS can provide voltage of 110 AC volts and other inverters provide 220 AC volts and frequency of 50 to 60 Hertz similar from the mains. Today, most UPS provide automatic voltage output range from 90 to 240 AC volts. Laptops, notebooks, eebooks, mobile phones, tablets, emergency lights, and some devices with built in battery, don't necessarily need a UPS. But for those data processing devices like desktop servers, personal computers, modems, routers, and DVRs that don't have batteries, will need UPS. This will prevent any unwanted interruptions like being not able to save changes of your files, network and server disconnections during mains power supply failure.
At that time before we decided to build a UPS, we tried to search a ready made one that can deliver high power output, like in kilowatts available in stores. Unfortunately, we were not able to find high power UPS, most of the devices we found designed only for a single desktop computer. So, we decided to build a homemade version.
This is the basic block diagram of our homemade version of UPS.
The UPS has:
Four 300 Watt solar panel
Three 30A 48 Volts digital automatic charger
One 60A 48 Volts solar charge controller
Four 150AH 12 Volts deep-cycle sealed battery
One 48 Volts 4000 Watt off-grid, pure sinewave inverter, 220v 60Hz
MC4 cables and connectors, thhn cables, battery cables
DC breakers, AC breakers, terminal logs
A simple pictorial diagram of the UPS we made:
We purchased a 48 dcv to 240acv, 60Hz pure sine wave, off-grid inverter. Since the input of the inverter has fixed operating voltage of 48 volts, we purchased a 48 volt charge controller, four 12v batteries connected in series to make up a 48 volts output and four 300 watt solar panels connected also in series to charge the 48 volts battery bank. This is a 48 volt setup. Most common setup are 12v, 24v, and 48v.
The frame of solar array is made up of square tubes. Unlike standard installation, we never use aluminum railings, end clamps and center clamps. The frame is then placed on an asphalt roof. A 40 cm gap between solar panels and the roof provides ventilation. Too much temperature can reduce the efficiency of the solar panels.
In connecting solar panels to charge controller, we use MC4 cables and connectors, and a double pole 30A DC switch with built-in anti spark bypass capacitors.
Male and Female MC4 Connectors
This is how a single pole dc switch and a fixed voltage charge controller look like. In the block diagram, a two pole is connected. Single pole dc switches be used in the circuit. Just choose any of the two lines, positive or negative.
The battery bank is made up of four 12V/150AH deep cycle batteries. In a 48 volt system, you will need to connect the battery terminals in series. A series circuit will give you the sum of voltage output each battery has. So, 12 volts + 12 volts + 12 volts + 12 volts = 48 volts. The total current capacity is the same as the current capacity of any of the batteries. Parallel connection of batteries will give you a total voltage output of 12v but current capacity increases to four times. We use a typical car battery cable that has 90A ampacity. We are expecting up to 4000 watt operating power. To meet the power capacity of the conductors, we use this equation:
Power capacity = Working voltage x Ampacity of conductor
Power capacity = 48 volts x 90 amperes
Power capacity = 4,320 watts
In images above, single pole dc switches are connected in the circuit. The actual dc switches we used were double pole that were connected on both positive and negative line.
Make sure the dc switches you are going to connect in the circuit are in "off" state during installation. In a direct current (DC) circuit, conductors are hotter than that are in alternating current (AC) circuit. So the shorter the cable length, the lesser the power loss and drop is.
How to start the UPS:
1. Plug the digital charger/s to the mains.
2. Switch "ON" the digital charger/s.
3. Switch "ON" the dc switch connected between digital charger/s and the battery bank. Allow the digital charger/s to charge the batteries for 5 to 10 minutes.
4. Switch "ON" the dc switch connected between the charge controller and the battery bank.
5. Switch "ON" the dc switch connected between the solar array and charge controller.
6. Switch "ON" the dc switch connected between the inverter and the battery bank.
7. Switch "ON" the power inverter.
8. Test the outlet of the inverter, either by a multimeter tester to read the voltage or by simply plugging a load.
Failure to follow those steps will indicate errors and the charge controller will not function properly.
How to shut down the UPS:
Turn off the switches by reversing the steps from step 7 to step 1.
At full sunlight, solar panels and digital chargers shared power to charge the batteries. If power from mains fails, digital charger switches off but solar panels will continue to charge the batteries. It is always better to combine a renewable dc source and dc source converted from the grid together. Aside from solar panels, it is possible to connect micro hydro generator, wind turbine generator, and other forms of renewable energy generators, as long as they generate direct current. Power inverters, digital chargers, and charge controllers nowadays, have excellent power conversion efficiency and built in protection circuits.
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