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I have an ZTE zxhn h108n router, which needs an 12 V input.

I want to make it work when the electricity goes off.

An example photo of it

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    Does your cable/DSL/... modem also work without an AC input? Even if it has an internal battery, it may shut down the cable/DSL/... parts and only run the phone jacks to conserve battery charge during a power outage. – ThreePhaseEel Nov 25 '18 at 17:20
  • Hello, and welcome to Stack Exchange. You need some sort of battery backup at least (and @ThreePhaseEel's concern is valid). That said, "shopping" questions are off-topic here. – Daniel Griscom Nov 25 '18 at 17:24
  • @DanielGriscom it's not an shopping question ... i want to make it at home ... "modem also work without an AC input?" i don't no i didn't try that !! "it may shut down the cable/DSL/... parts" why would it do that ... the Land line Phone works fine when electricity goes off ... . maybe because like what you said "only run the phone jacks to conserve battery charge during a power outage" "Even if it has an internal battery" i don't think that – mina nageh Nov 25 '18 at 18:23
  • There are a number of different configurations, some of which (POTS w/DSL, FIOS w/Telco battery backup) will result in your phones working even when nothing else works - that is very common. – manassehkatz Nov 25 '18 at 18:44
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    Best practice would probably be to blank out the MAC and serial numbers of your device when you post pictures, lest you attract the attention of somebody looking for vulnerable hardware. – sh1 Nov 26 '18 at 20:07
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Get a UPS

No, not that UPS. An uninterruptible power supply, also known as a battery backup. This is a box that takes normal AC (e.g., 110-120V in the US) and uses it to charge a battery and power your devices. If power goes out (technically: voltage out of range) then it uses an inverter to power your devices from the battery. All automatically.

You typically want to connect:

  • Cable or DSL or FIOS modem
  • Router
  • Computer if you are using a desktop computer
  • Any other small but important devices - e.g., if you have an Ethernet switch then connect it so if you have a laptop or other device on that switch (instead of WiFi) it will continue to have a wired internet connection

Sizes and prices vary, primarily based on the size of the battery which, combined with the size of the load, determines how long it will run when AC is out.

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    Not worth DIY for a one-time project. Remember that you are dealing with dangerous AC (to charge your battery pack) on one side and need to provide clean consistent DC on the other side. An off-the-shelf UPS has the big advantage of letting you plug in different things - e.g., if you get a new router that uses 18V or 5V instead of 12V you just plug it in. – manassehkatz Nov 25 '18 at 18:40
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    @minanageh Not sure why you'd want to hack together a dangerous device when a UPS already safely does exactly what you want it to do. That product you listed looks like more of a hassle than it's worth and wouldn't be useful if you also need to power on your modem along with your router. – HazardousGlitch Nov 25 '18 at 18:55
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    OK...... I agree with you – mina nageh Nov 25 '18 at 18:55
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    But there's nothing dangerous here. Obviously you'll just use a UL listed COTS DC supply, that plugs in and is safe. (Why would you not?) The problem with a UPS is the utterly needless double conversion loss and they are designed for one thing, giving 5-10 minutes to orderly shut down a PC: commit DB writes, flush caches, etc. they're not made for extended or continuous use, that would require ugly hacks. DC isn't hacky, Google does it in their datacenters. Ask the off-grid solar communtiy about the gold standard for LVDC work in homes. – Harper Nov 25 '18 at 22:43
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    On the flip side, I do use a UPS and as wasteful as AC may be it's the only common standard between all the things it has to power -- including a NAS box and a 120W PoE+ injector (which is necessary to provide UPS power to the WiFi access points and cameras in other parts of the house). – sh1 Nov 25 '18 at 22:48
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Yes, that's fine

You can connect a battery to the DC side of the NAT router directly and have that be its primary power supply. You would discontinue use of the router's own power block, and use an appropriate off-the-shelf battery charger for that battery type. This battery charger will be perfectly safe if UL listed, and will simply plug into the wall. The AC side will be protected and you'll have access to the safe low voltage side only.

Of course I note this router does not have a phone/line or cable input; that means there is also a cable modem or DSL modem somewhere. If this loses power, the Internet will be inaccessible. Fortunately most such devices also take 12VDC power (check this), and likewise can be run off the same battery.

The market is thick with routers and modems that run on 12VDC; it's easy to buy ones that do.

By the way, the more common standard these days is a combo modem/router that does both the modem part and the NAT router in one unit. The downside to this is less control over features and config. The upside may be less power consumed, which means longer runtime or a smaller battery.

Of course, if the provider-side equipment loses power, you again have no Internet. You need to see how your cable provider responds to loss of power, a lot of them have poletop devices which require power from the nearby electrical wires. I can tell you the phone companies are very, very good at it; with large submarine-sized batteries in the basement of every central office. And generators either onsite or trailered around to switch centers, as needed to top up batteries. Landline phones stay up indefinitely in extended outages.

Your battery system can also top up from solar panels.

It can also power things like iPad or phone chargers.

With an inverter it could also power a refrigerator. However you don't want to leave an inverter running any moment it doesn't have to; they waste a lot of power in conversion losses, even when idling. Powering a natively 12VDC device through an inverter and then wall-wart isn't even stupid; you pay a double conversion loss for no useful gain.

All these factor in to how large the battery has to be, based on the runtime you desire.

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    I think a lot has to do with frequency (pun intended) of the AC/DC conversion. If you have a few, mostly short, outages a year (my most recent that triggered my UPS was when I cut power to wire up a new light in the same room - that doesn't even really count), a decent UPS will take care of things quickly & easily. If you have long and/or frequent outages then setting up some real DC power infrastructure makes sense. – manassehkatz Nov 25 '18 at 22:56
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    With a deep cycle 12 V sealed battery (such as a wheelchair battery), a battery box, a battery tender (as opposed to a regular battery charger), and some cables and connectors, you can keep your network running for a long time. If you want to charge laptops, phones, etc., use car chargers because they tend to be more efficient than an inverter plus wall warts. – mrog Nov 27 '18 at 19:15

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