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I'm in a place where power cuts are very common for around 5 hours a day. I have a DSL internet connection and a PC. I recently purchased a new laptop which easily gives me 4 hours of battery backup.

So, if power goes off, the ADSL modem goes off and I'm no longer online. The ADSL wifi modem only needs very little DC power through an AC to DC adapter.

I want to be online with laptop and small power supply for modem. So, i purchased this INTEX UPS which has 12V 7.5A-H lead acid battery. My idea was to use this UPS with modem alone so I can easily have 4 hours of power supply to modem. But when I use it with DSL modem alone, the UPS only works for 5 minutes and then shuts down. It is not that the UPS bettery is nill, but may be it is due to UPS thinking that there is no output device due to very low power need of modem.

If i use the UPS with PC, it gives the regular 10-15 minutes backup. How can i use the UPS with modem alone?

What is the minimum output wattage required for the UPS to not shut down.

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I would design a UPS to shut down on power input and not on power output. – Buscar웃 Apr 26 '13 at 21:45
What type ADSL and what are the V and A on it. – Buscar웃 Apr 26 '13 at 21:51
teracom ADSL2+ wifi modem. It needs 12V 500mA DC power. – bharat Apr 26 '13 at 21:56
So you have a 90 Watt UPS. You could add a car bulb (lets say 10 Watts). – Buscar웃 Apr 26 '13 at 22:07
@Buscar웃 Math says it should power that modem for about 15 hours, minus the losses to convert power. So at least 14 hours. – Darth Android Apr 26 '13 at 22:15

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