Thursday, September 26, 2013

UPS All You Need To Know

Good afternoon everyone. Sometimes I hear people ask about UPS, it's back-up time, ratings and more questions like this. So, I'm trying to write about these stuffs. This will be a bit basic stuff. If you are into tech stuffs you might know more than this. Still hope you will enjoy reading.

UPS What is this?

UPS stands for Uninterruptible Power Supply. Your personal computer or maybe laptop or other similar electrical equipment runs from AC outlet of your home. AC power that is provided by this outlet is not stable, it may vary by small amount with load and sometimes there might be power outage. That is load-shedding or something like that, maybe a cable fault. If you are in the middle of an important work and suddenly your computer just turns off, you will loose important data or think about medical equipments that needs to be on 24/7. So, you will need an UPS to get backup to save your important data or keep life saving equipments running in power outage situation. As soon as electricity fails UPS turns on and keep your system running thus giving you the opportunity to save the important data before turning off the computer.

It has batteries inside which charges from the AC main line and with in the event of a power outage these batteries supply that necessary power.

How Does It Work?

Let's start with offline UPS(Later I have explained both of them).

UPS takes in AC power from AC outlet and simultaneously 1. Provide AC power to the output, and 2. convert it(from main's AC) to DC to charge single or multiple Lead Acid Battery and when power fails an inverter turns on taking power from the battery and provide sine wave to the output. As soon as the battery is dead UPS stop providing backup. It charges from the AC mains again when AC power comes back.

Offline UPS

This is a very basic block diagram of how UPS works. Here Red lines indicates the AC line, Blue is the DC line, and the Green is the control. As you can see it takes in AC power straight to the Transfer Switches and to the battery charger. Transfer switches are actually relay based switches. Battery charger converts AC to suitable DC levels and charges the battery. Control always monitors battery level and if battery is fully charged it sends signal to battery charger to turn of charging. When AC is on control keeps the AC flowing to the output but as soon as AC fails control sends signal to turn on the Inverter and the Inverter takes in DC power and gives off AC power. Relay switches between direct main line connector and Inverter. Thus keeping the load turned on. The whole thing happens within very small amount of time that computer doesn't turn off.

In real world things might be a bit more complex but the main working principles stays the same. An offline UPS might use the same transformer as both inverter and step down transformer for charging the battery.

Rating, No watt?

You might have noticed that UPS comes with a volt-ampere rating rather than a watt rating(which you will find in your power supply). Well we are talking about AC voltage here, In DC we just multiply voltage and current and we will get watt, but In AC there is one more term to worry about and that is power factor. Not going to get in depth of it just remember in AC Voltage*Current*Power Factor = Watt. So, if you have a 1200VA UPS with 0.6 and 0.8 power factor what will be the maximum capacity?

1200VA * 0.6 = 720W
1200VA * 0.8 = 960W

So, UPS with a higher power factor is needed here.

If you buy a 650VA UPS with a power factor of 0.6 and your system's consumption is 450W when loaded, assume you are playing Battlefield 3 or Crysis 3 which is causing the system to draw 450W power, if AC fails your computer will just turn off. It won't even give backup for even fraction of seconds. Because 650VA * 0.6 = 390W which is the UPS's capability and you are drawing 450W.  So, get an UPS with higher power factor and calculate the amount of load you are planning to put on it.

So, I get a higher VA Rating and will get higher backup?

Well, Yes and No.

UPS VA rating stands for its maximum capability not for backup time. Though higher VA usually results in longer backup time but backup time and VA is not connected that much.

Like I said earlier UPS has battery inside(or maybe outside) of it. Usually these are Sealed Lead Acid Type battery or lead acid type battery. 

650VA 800VA 1000VA UPS have one of these battery, where 1200VA 1500VA have two of these battery. So, 650VA-1000VA converts 12V to 220V and 1200VA or 1500VA converts 24V to 220V. 

Now let's look at the battery, it is a 8.2Ah 12V battery. What it means? It means if you pull 8.2Amp from this battery at 12V this battery will last for 1hour. 8.2A from 12V is 8.2*12 = 98.4W. So, If you run a 98.4W 12V load with this battery it will last for an hour.

If you pull 16.4Amp or 196.8W from this battery it will last for 30 min. And if you pull 4.1Amp that is around 50W it will run for 2 hours!

Wow my computer burns only 100W, so I will get 1 hour backup! 

Wait! You have heard half of the story!

Remember the construction, DC voltage enters into the Inverter, where it is converted to AC via Power Transistors and then fed into a Transformer to step it up to a higher voltage level like 110V/220V. This conversion method is not efficient, a lot of heat generates thus a lot of energy wastes, moreover transformers have losses, different kind of losses, so there's another power loss there. And last but not the least your system's power supply is not 100% efficient either. Maybe 60%-90% efficient. So, your system's power supply will draw more power than its providing.

So, a lot of losses decreases the backup time! Yet try to get a UPS that has higher AMP-HOUR rated battery.

Now, why multiple battery?

Multiple battery are connected in series. Hope you know what will happen if you connect it in series. Two 12V battery result in 24V. Assume two 12V 8.2Ah battery. So, total 24V 8.2Ah battery(In series current stays the same) so, now you can run 196.8W load for an hour! 

So, you see more battery more backup. It is not necessary to be with a higher VA rating. Later I will tell a bit more about it.

Although you can't just put multiple battery anywhere, the circuit has to be capable of working at that voltage.

How fast?

UPS has a very fast switching speed. It can switch between AC main line and Inverter within millisecond so that Computer won't turn off. Each computer power supply has a Hold up time. This is the amount of time Computer's power supply can provide power(from the bulk capacitors) to keep the system running even if the AC power is out. You will need a UPS which has a speed faster than this Hold up time. Within the hold up time if power fails and inverter start providing backup computer won't feel anything. But if, lets say the computer or any other device has a hold up time of 15ms and the UPS needs 18ms to provide power, computer or that device will simply turn off. Luckily most consumers UPS can provide power within 5ms.

Types of UPS

There are two major types of UPS available in the market. 

Offline UPS
Online UPS

Offline UPS:


Diagram that I have shown on top of this post is actually of an Offline UPS. In this type Inverter only turns on when AC fails. So, rest of the time AC supply will go directly to load or maybe via Automatic Voltage Regulator or Surge Protection System. But if these protections are not available surge might damage the load. But most modern UPS has these protections so don't worry. Offline UPS is the cheaper one and less reliable one. Although it is more efficient than the online counterparts.

Online UPS

In online UPS Inverter always stays on. So, there is no break. This type of UPS takes in AC voltage, step it down with a transformer then converts it to DC and use that DC to both Charge the battery and to feed the inverter circuitry. As soon as AC fails battery starts providing the voltage required for inverter. So, this circuit produces more heat than offline UPS. But as output is totally isolated from the input AC, the output power quality is higher. Different types of protective measures can be taken here. This units cost a bit more than offline one.

You might sometimes see that this type of UPS is marked as double conversion UPS, the reason is it actually has two conversion units running simultaneously. One converts the AC to DC and the other converts the DC to AC and drives the load. As the inverter has to always stay on the heat generation is also high. Moreover inverters are not that efficient so the overall efficiency of online UPS drops to 80-90%. 

Online UPS may employ power factor correction circuitry and frequency regulation circuit which makes the efficiency and quality of power a bit better.

Schematic Of An Online UPS

As you can see online UPS has a totally isolated output. It is a bit more reliable than offline UPS. Because of the isolation it is easier to suppress the AC noise and protect the load from ripple, transient voltage or surge and brownouts. Better protective measures like low voltage or high voltage protection and surge protection can be implemented easily.

Why Back Up Time Decreases Overtime?

As you might have already faced backup time decreases overtime. And with frequent using backup time decreases even more. That's because of the characteristics of sealed lead acid batteries being used in this devices. 

From the graph shown above its clear that if you use 100% of its capacity everytime you will only get about 200 cycles. This number will change from battery to battery but the thing is higher the discharge depth is lower the number of usable cycle is. 

If you use a battery everyday to 100% and fully charge it back again, it will only last around 200 days, not even a year. Now you can do your calculation.

If capacity drops significantly you can replace existing batteries with similar type of battery.

You can read this manual for more info.


1. Don't use it in a very hot place. As you can see from the graph below that with higher temperature service life reduces drastically.


2. Don't leave it discharged for long time. As it will damage the battery even more. Charge it regularly.

 As you can see with time the battery self discharge and looses its capacity if not charged after reaching 60% of its capacity.

3. Don't overload it. 

4. Keep it in a dry place.

5. Visually inspect for any mechanical failure or check for any smell that indicates burning of something.

6. Regularly clean it, you can use a blower to do that.


Assume you have a 1000VA single 12V battery UPS which has a power factor of 0.8. And the battery is dead. You want to replace it. You can replace it with a higher capacity battery even with a car battery for that matters. Say you had a 12V 9AHr battery earlier. You can easily replace that with a 12AHr battery which will increase the back up time slightly.

Connect battery with the proper polarity. Positive with positive and negative with negative. You can use a multimeter to determine the polarity. And batteries will usually comes with polarity marked on them. Another thing you can look up is, usually the black wire is the ground wire.

What if a car battery is used? Like a 12V 100AHr battery? well it will give back up but the charging will be a bit tricky. Lead acids battery needs to be charged at 1/10th of its current capacity. So if a UPS has 9/10AHr battery the charging circuit is providing only 1Amp of current. But a 100AHr car battery requires 10Amp of current to charge, charging it at 1Amp will need days to charge it properly.

One way to solve this problem is using a different charging circuit for the batteries, connecting with a relay based switch. The way it will work is when the AC power is available the load will run from the AC and the battery will be charged from a different charging circuit, as soon as the AC is gone the relays will shift their switches in such a manner that battery terminals get to contact with the UPS actual battery terminal thus providing back up.

I'm adding a small schematic to clear this up a bit.

UPS Hack

Only two items are needed, one a simple relay that is connected with the Charging circuit, battery and the battery terminal of the UPS. Relay's common terminal is connected to the battery and the normally connected terminal is connected to the battery terminal and the normally open is connected to the high current charging circuit. Relay can be powered directly from the AC main line via a step down transformer, AC to DC converter and voltage regulator. 

When the AC is available the load is being fed from the AC main line via the Transfer Switches, and that AC is keeping the relay On and the charging circuit On. The way the relay is connected if the AC power is On, the Common terminal will touch the Normally open terminal thus charging the battery. As soon as the power fails, relay's common terminal touches the normally connected terminal thus providing power to the UPS circuit and the inverter. So the load will keep on running. As relays are very fast we don't need to worry much about the load's hold up time.

Some smart UPS might not be happy with no battery on its battery terminal but some of them just work fine.

When powering up the relay make sure to use proper transformer. You want to select such a transformer that will provide such a voltage that won't be able to drive the relay at AC input 190V or lower.  

High voltage application, be cautious.


Some more info:

AC to DC conversion.
Voltage Regulating.
How Relays Work.

Read my other posts here.

Have a nice day.

1 comment:

  1. As you've stated, an UPS is only good when its new but as time passes and overusing of it power would result to a shorter life span. The promised life span however is still commendable and integration of an uninterruptible power supply is still the wise move.