Help me keep my broadband on during blackouts
September 21, 2021 5:11 PM   Subscribe

If I keep my modem hooked to a UPS or battery, will the internet still work during a blackout? If so, I need help figuring out battery sizes and how long I can keep the modem on (volts, amperes, watts, watt-hours inside):

The modem is 12V 2.5A DC
The modem's AC/DC adapter is 100-240V 50/60Hz 0.8A

But the UPSs I've been looking at are rated in VA, what does that mean?

There are 800 VA, 1200 VA models etc. But is VA volt x amperes? Because that would be watts, right? That doesn't measure capacity, you would need to know watt-hours? This is confusing to me.

This model is rated 1200VA and in the customer questions area they mention that its battery is 12V 8Ah, that's 96Wh, so that would run my modem for about what, 3h?

Instead of spending R$ 528 for that, I was thinking of getting a 12V 30ah battery and running the modem straight off that, feasible? 12 x 30 = 360 Wh would give me 12h of internet, also bypassing all the ac/dc losses

R$ 225 for the battery + R$ 120 for a trickle charger = R$ 345, way cheaper for 4x the capacity. Am I missing something?
posted by Tom-B to Technology (18 answers total) 4 users marked this as a favorite
Best answer: UPSs add auto switching and an inverter for mains load. You are right that going 12v directly is significantly more efficient.

Depending on what kind of battery you buy be aware that most don't like to be used 100% [lead acid is kind of the worst case here as most dislike more than a 50% discharge]. And some don't like to be stored fully charged.
posted by Mitheral at 5:29 PM on September 21, 2021 [1 favorite]

Can your service provider continue to operate its facilities during a power outage? If so, while you may be able to power your modem, you may still not get a network connection.
posted by They sucked his brains out! at 5:39 PM on September 21, 2021 [6 favorites]

There's a decent chance that no, if your neighborhood is without power the cable/telco nodes your home is connected through will not stay online. I don't know if I'd make a serious investment in any equipment until you find out (if that's even find-outable) whether that's likely.

But yeah, a UPS is less efficient but will have the failover mechanisms.
posted by Lyn Never at 5:45 PM on September 21, 2021 [2 favorites]

It'll really depend on the nature of the electrical blackout and how far "up the chain" the failure is.

If it doesn't affect your local cell tower - you should still have cellular data; if your desktop has wifi, you can use your phone as a hotspot.

I've been in power outages where cell data was still available, and other instances where it went down too.
posted by porpoise at 5:50 PM on September 21, 2021 [3 favorites]

I was going through the same buying process several months ago. I didn't like the bulky UPS units that seemed to have batteries that would only last a couple of years, and you had to be very careful not to over discharge. Also many of the reviews spoke about loud fans that would be annoying.

In the end I chose this unit that has a 100W capacity, is about the size of my routers, uses a lithium ion battery and should power my router + fibre ONT for 6 hours or more (a comment in the reviews for the product mentioned they got 11 hours, but that seems optimistic, and would depend on the devices ratings).
I realise the link may not be useful to you, being in a South African store, but perhaps there is a local equivalent available to you.

Regarding availability during a power outage - during periods of scheduled load shedding (to reduce load on the national grid), our fibre connection has continued to work. If this applies to your situation, and the duration, would depend on the battery backups for your telco. In our case the cell service also continues to work, but the speed of the cell connections drops, I'm guessing because the towers reduce performance when running on battery power.
posted by Gomez_in_the_South at 6:07 PM on September 21, 2021 [1 favorite]

During the bizarre freeze and associated power outage in Houston in February, plugging our cable modem into a battery kept our internet available for the first half of a day, then Comcast went out altogether for another day, so our power backup no longer helped with that.
posted by metonym at 6:16 PM on September 21, 2021

I have a lot of power outages that affect my local area but do not affect my local wireless broadband node (in the next town, on a different grid). I bought one of these.

We have it set up so that router and VOIP phone are permanently plugged into it - essentially using it as a powerboard during normal operation. It runs more or less silently, but there is an annoying beep every 30 seconds to remind you that you're on battery power during an outage. There may be a way to turn this off but I prefer the reminder. It's slightly larger than a shoe box, fairly heavy, and lives under a table in the "tech corner" of the house - pretty much out of sight and mind until life gets interesting.

I get about two hours out of it which handles most of the normal issues (branches on lines). I also have a generator for longer outages, and found that this, running off the generator, sort of cleans up the generators slightly jerky load balancing. In fact, during the normal day, I've found that the router is much more stable than if it's plugged directly into the wall - I can only assume that being out in the countryside power is perhaps not as stable as it could be. There are bigger units out there, but this does fine for me so far and was cheap enough that I'm not going to cry if I have to replace it in a couple of years.
posted by ninazer0 at 8:02 PM on September 21, 2021

Another alternate item to consider is a 4G cellular modem with Ethernet fall over, if you want to keep your internet up over a wireless network.
posted by nickggully at 8:25 PM on September 21, 2021

Best answer: Connecting a modem designed to run off a 12V wall wart directly to a nominally 12V battery risks both damage and non-functionality, since a nominally 12V battery can have anything from 10V to 14.4V across its terminals depending on its state of charge and whether or not it's being actively charged.

Something like this little buck/boost DC-DC converter between a float-charging 12V battery and the modem would keep the modem's internal power supply within the usual 5% voltage tolerance range for these things.

Lead-acid batteries are much happier if kept fully charged, so trickle or float chargers work well for them. You'd want your charger to be able to provide more current than your modem load draws, obviously.
posted by flabdablet at 9:28 PM on September 21, 2021 [1 favorite]

Best answer: The main things you get from a proper UPS that you don't get from a simple trickle charged battery setup are (a) power output compatible with your local AC mains supply and (b) the ability for connected equipment to interrogate the UPS about its state of charge and organize a clean shutdown before the UPS goes completely flat and withdraws supply, possibly in an unhappiness-inducing brown-out kind of way.

The 12V buck-boost converter I linked above should be able to keep the modem supplied with a steady 12V until the input voltage from the battery gets so low that it needs to draw more than 3A from the battery in order to maintain the modem's power draw at 12V, at which point it will probably start to cycle between complete shutdown and providing 12V that collapses as soon as the modem draws power again. So the modem might end up seeing rapidly repeated power cycling once the battery is flat; that won't be good for it.

On the other hand, running a 12V lead-acid battery down to the kind of discharge levels that would make the DC-DC converter do that is terrible for the battery as well. So unless you're going to engineer some kind of battery voltage monitor that enforces a clean shutdown when the battery drops below say 11V and refuses to start again until it's charged up to say 12V, you should upsize your battery so that it won't ever go below say 50% charge during any reasonably likely blackout.
posted by flabdablet at 9:39 PM on September 21, 2021 [1 favorite]

The rational behind the plan: sometimes a power outage is limited to your immediate neighbourhood or corner. In such case a UPS will be helpful to maintain internet connection. However if the outage is wide spread, just powering the router/modem will not help.

However, there is an alternative: using your cellphone as modem (or hotspot) for internet connection. Cell towers keep working for limited time during outages.
posted by oberon_1 at 9:52 PM on September 21, 2021 [1 favorite]

"It depends", mainly on does your outage affects your local ISP "node" that relays the signal to you, sometimes called "the final mile". If that' node is down, then no amount of power will keep your Internet on. And that, obviously, depends on your ISP.

Keeping the power on to the modem is not a problem. A normal UPS can handle that. Calculating the power use of a modem/router however, is a different problem. And a UPS can keep your PC running as well. But generally speaking, a UPS is to give you a limited amount of time to save your work so you don't lose progress, rather than let you keep working.

To keep working, you need a standby generator, and that's a whole different beast altogether. And that goes back to the first question... Is there any point in keeping the modem on if the upstream node is down? The answer is no. Unfortunately, there's no way to test this... until you have a power outage.

You can switch to wireless data / phone hotspot for your Internet, but that's a whole different problem.
posted by kschang at 10:14 PM on September 21, 2021

Best answer: Keeping the power on to the modem is not a problem. A normal UPS can handle that. Calculating the power use of a modem/router however, is a different problem.

Not really. Just take the output power (volts * amps) of the wall wart, which would be the upper bound of what the modem needs. In the situation here that's 12 * 2.5 = 30W. The UPS suggested by the OP is supposed to be 1200VA, which would extremely theoretically provide 40 times the power the modem needs, but that's not giving you 40 times the (vaguely specified) nominal runtime. Which it won't, plus there's always conversion losses and its internal circuitry needing power as well. Furthermore, that UPS is fitted with a single 12V 7Ah SLA battery. so 84Wh, which would manage about 2.5 hours at 30W delivered if you discount any conversion losses, not depleting until fully flat (= dead), early shutdown because of deterioration of the battery, etc.

Powering the modem from a 12V mini-UPS as Gomez_in_the_South suggests is a much neater option. They're basically a 12V version of the power bank for your phone/tablet. Couple of years back I used one like that for a while for an intermittently connected data logger as it's a neatly packed battery/output voltage regulator/trickle charger package, and it worked fine. AliExpress offers hundreds of them (which are just half a dozen or so rebranded by different vendors); this is one of them. There are larger ones and it would certainly be possible to recharge them from a 12V car battery if necessary.
posted by Stoneshop at 11:50 PM on September 21, 2021 [2 favorites]

Side note from someone who has a ups on his modem:
Cheap UPS don't have very large batteries, and even expensive ups that are as heavy as a cinder block don't necessarily have a ton of efficiency or amp hours, especially down at the bottom of their wattage range. The APC smt1500, which is about as big and beefy as you can get for a household power outlet, will only last 5 or 6 hours under low loads. It does keep the internet on here, while it lasts.
posted by wotsac at 7:52 AM on September 22, 2021

Best answer: I use an 845 MCA deep cycle battery from Walmart with a 6 watt solar panel from Coleman to keep it charged, I am running a rectifier & freezer 24 / 7 , a modem somewhat similar load. Simple system.No worries.
posted by hortense at 11:47 AM on September 22, 2021 [1 favorite]

The APC smt1500, which is about as big and beefy as you can get for a household power outlet, will only last 5 or 6 hours under low loads.

Another datapoint: the APC Smart-UPS 1000 I have powering my network, 3 switches with some PoE gear attached, plus a small server manages a little over 3 hours with an estimated 100W load. Which is fairly well in line with the installed battery capacity: 4x 12V 8Ah, so 384Wh. Those batteries had been lightly used before I installed them, and as said, the UPS isn't going to drain them flat.
posted by Stoneshop at 12:04 AM on September 23, 2021

Best answer: I was thinking of getting a 12V 30ah battery

Just don't get a car battery. Car batteries are optimized for providing extremely high current in extremely short bursts, because that's what starter motors require. They're not super good on round trip efficiency, and because they spend most of their working lives being topped up at potentially very high currents from beefy car alternators they're pretty much the worst kind of rechargeable battery at dealing with slow charging and deep discharging.

Running a small load off a car battery for an extended period of time is using it in a way that's about as far from what it's optimized for as you can possibly get. I'm sure you know from experience that it doesn't take very many episodes of accidentally leaving an interior light on all night to damage a car battery, and that's pretty much what the battery is going to see if it has to run your modem during an extended blackout.

If you're going to go lead-acid (which is still the chemistry that gives you the most bang for your buck, especially for applications where the battery isn't going to be moved about) then go for an absorbent glass mat (AGM) type with nice thick lead plates to let it repeatedly discharge at moderate current to 50% state-of-charge without taking damage.

The fact that fully half of a lead-acid battery's capacity is best devoted to keeping the battery itself in good condition means that when you're sizing such a battery for any given application you should pick one with twice the nameplate capacity you get by multiplying the current you need to draw from it by the time for which you anticipate needing to do that.

UPS manufacturers typically don't do that; they will generally design in the smallest battery that will allow them to supply their instantaneous VA rating and not give two hoots about how long the battery can keep doing that for. The assumption is that the UPS is really only there to give connected equipment enough time either to shut itself down in an orderly fashion or get the backup generator cranked up. Commercial UPS gear is not designed to be the sole power source for the connected equipment for more than a few minutes at a time.

My own networking and home server gear runs off a commercial UPS, but I use that with its case opened and the internal battery cables wired to an external AGM deep cycle battery with twenty times the capacity of the 9AH internal SLA that originally came with it. I'm in the process of replacing wall warts with DC-DC converters like the one I linked above, and once that's done I'll ditch the UPS altogether and charge the big battery off a 170 watt solar panel instead.
posted by flabdablet at 4:53 AM on September 23, 2021 [1 favorite]

Best answer: There's a lot of unnecessary worrying in this thread.

Connecting a modem designed to run off a 12V wall wart directly to a nominally 12V battery risks both damage and non-functionality, since a nominally 12V battery can have anything from 10V to 14.4V across its terminals depending on its state of charge and whether or not it's being actively charged.

This isn't actually a problem. Nothing in a broadband router actually runs off the input voltage. The 12V input will be stepped down internally to 3.3V, and maybe some 1.8V and 5V. A router with 12V input will run just fine off anything between 10-15V, probably 7-20V too. I have been intimately familiar with the circuit boards of dozens of these things, and have designed similar hardware.

Also, your router will not actually draw 2.5A. Not even close. That's the specification it requires for the supply - i.e. it wants to be able to draw a peak of 2.5A without problems if it needs to. In practice I would expect a device like this to draw around 1A. Easy to measure if you want to check.

A 12V 30Ah battery and a trickle charger is a perfectly good solution here, and will probably keep the router up for at least a day. If you want to be nice to the battery, don't take it below about 50% charge, but you'll get something like 12-15 hours out of it even with that constraint.
posted by automatronic at 3:49 PM on September 23, 2021 [2 favorites]

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