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I get a backup of only about 10 minutes when my FTTH ONU and WiFi router are connected on the 220V ports of my desktop UPS. So, in order to avoid the conversion loss, I want to install two 12V DC jacks to connect the ONU and router directly to the 12V 7Ah (Sealed Maintenance Free) battery of the UPS. From this DIY post (about connecting a WiFi router to the car battery through the cig lighter port), I learned that a 12V voltage regulator IC needs to be connected to ensure that the output voltage stays 12V when the UPS battery is charging. So I want to get suggestions from the electronics DIY community on the following circuit.Circuit diagram of connecting 12 volts DC jacks to desktop UPS


EDIT: Adding more details for clarification.

Battery: Amaron Quanta. New. Installed 4 days ago. Manufacturing month: August 2021.

The place where I live get frequent power cuts; sometimes lasting > 4 hours. So my objective from this project is to get a backup of at least 4+ hours for the ONU and router.

I won't be connecting anything to the 220 V outlets.

I'm willing to replace the linear regulator in the circuit with a switching one if it can give me longer backup. It would help me greatly if someone can give a circuit diagram for the same or direct me to a DIY guide.

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    \$\begingroup\$ You'll be bypassing any power monitoring that the UPS does, which may make it stop working right if it doesn't know what the battery state of charge is. Unless it's designed to have a 12 V output, anyway. \$\endgroup\$
    – Hearth
    Dec 11, 2021 at 19:44
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    \$\begingroup\$ You should ask a question. As a suggestion, I would recommend not to modify hardware that has potentially lethal voltages, large currents that can melt wires, and a battery you don't want to accidentally damage inside it. \$\endgroup\$
    – Justme
    Dec 11, 2021 at 19:44
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    \$\begingroup\$ A linear regulator such as a 7812 is likely to have significantly greater losses than the convertor in the UPS. Also, the circuit as shown will allow the battery to be discharged to the point where the battery is destroyed. The solution is to get a UPS with a higher capacity. (Or a generator.) \$\endgroup\$ Dec 11, 2021 at 21:01
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    \$\begingroup\$ What is your design objective, anyway, are you looking for, say, 20 minutes of runtime so you can sync and quit? Or hours so you can use it normally? Does the UPS power anything besides these 12V devices? What is the manufacture date on the battery? \$\endgroup\$ Dec 12, 2021 at 7:01
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    \$\begingroup\$ @AdityaNaique As Jasen's answer answer indicates, my comment suggesting a 170 Ah battery seems to be off by an order of magnitude or so because I based it on the runtime you've seen (10 minutes). Are you sure that the new battery you've installed has had a chance to be fully charged? \$\endgroup\$ Dec 12, 2021 at 16:34

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7812 will not work here, it needs 14 V input to function correctly.

The battery in your UPS may not be isolated from the mains supply.

Perhaps start with a 12 V battery charger supply like used for alarms and add a switching 12 V regulator (eg: QSKJ QS-1212CBDE-20W)

You should get more than 6 hours of run-time from a 7 Ah battery as your loads are less than 1.25 A total.

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  • \$\begingroup\$ Thanks so much for guiding me in the right direction by mentioning the name of the switching regulator module (it is also popularly called XL6009). I found some DIY videos such as this one youtube.com/watch?v=JYI-tOnpGw8 that uses this module to make a UPS system with a power bank. The maker of that video also recommends adding two numbers of 4700uF, 25V capacitors in parallel to the output of this module if the router resets when the battery switches from backup to charging. I'll try doing it and if all goes well, I'll come back and update my answer with the new circuit diagram. \$\endgroup\$ Dec 13, 2021 at 7:18

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