UPS into home UPS. If you own a desktop computer, you may have a UPS
that can power your computer for 10-15 minutes after power failure.
By Girish Radhakrishnan
The purpose of the UPS is to save your work and shutdown your computer properly to avoid potential data loss and hardware loss such as Hard disk of your computer.
Most of us always underestimate the potential of the computer UPS that sitting beside your computer. The average computer UPS can deliver around 600VA, which is enough to power your low power appliances such as fan, tube light, computer, television, etc.
If you own more powerful UPS such as 1KVA you can power your home appliances more.
So how much can my computer UPS can provide? 600VA means the apparent power, but the real power is 60% of specified value. In other words it provides 60% of the VA rating.
If you have 600VA UPS then 600VA x 0.6 = 360W maximum output.
If you have a 1KVA UPS then 1000VA x 0.6 = 600W maximum output.
If my computer UPS can provide this much power, then why computer UPS only power 10-15 min?
This is because most of the computer UPS is only powered by 12V 7AH battery which is sitting inside the UPS.
To increase its backup time we need to connect several numbers of batteries with same specifications in parallel. The motto of this article is to make a cost effective home UPS from a computer UPS.
The procedure explained in this article is not suitable for beginners in electronics.
The whole UPS consists of an internal battery and several external batteries, which are crucial part of the UPS. The internal battery which is charged by internal circuitry of the UPS.
No external battery must be connected to charging circuitry during charging period. This is because the UPS is only capable of charging single SLA battery.
Exceeding more than one may overload the charging circuit and may lead to physical damage to UPS such as fried circuitry or may even cause fire. This is exactly opposite during discharge. All the batteries are connected in parallel including internal battery, injecting the power to UPS.
The external charging circuit consists of voltage regulator LM317 and op-amp comparator circuit for full battery cut-off. The voltage regulator gives out 13.75v for charging which is healthy amount of voltage for charging all kind of 12V SLA batteries.
When the battery reaches full battery voltage, relay cut-off the batteries from charging circuit and gives float charging to the battery via 150 ohm/5 WATT current limiting resistor. The relay triggered by op-amp comparator circuit.
There are two transformer one for charging which is 5A or more, the other transformer (500mA) for sensing the presence or absence of mains power.
If the mains are present relays are activated and connected to charger. If mains are absent relays are deactivated and batteries are connected to UPS. The 5A charging transformer may be replaced with SMPS.
The 100 ohm/5 watt resistor for quick discharge of 1000uf capacitor, so that relay can be deactivated instantly during power failure.
During power failure all the batteries connect automatically in parallel powering the UPS. When the batteries are at low state the UPS automatically disconnect its batteries and shutoff itself.
There is always low battery cut-off circuitry in the UPS. Most of the computer UPS gives out modified sine wave during power failure, and mains sine wave during normal state. This is suitable for most of home appliances.
Do not omit the internal battery, this plays an important role in giving
uninterrupted power output and UPS circuitry gets unstable without the internal
It is not recommended to connect more than 5 external batteries.
Do not connect this UPS to mains as what we do when we have genuine
manufactured home UPS [IMPORTANT].
Use a branded computer UPS to proceed this project.
Never overload the UPS during normal/backup state.
sure that all internal and external batteries are with same capacity (AH) and
Do not connect any inductive loads other than table fan.
Place the whole setup at well ventilated area and don’t allow water to contact
Author’s prototype which shows how he could convert his computer UPS to home UPS:
I used two external batteries and charging circuit is embedded inside old DVD player chassis.This is running since 3 years, and absolutely error free.