Running on DC?

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by krauster » Sun Aug 16, 2020 1:18 pm

I have sonic fiber. I've stuck a Kill-a-Watt power meter on these two and the combination of the two are drawing about 12Watts. Assuming that the fiber would be working during a power outage if I could power these two devices and power a laptop with some form of solar panel and battery backup would that make sense?

Is anybody setting up this sort of thing for emergencies?
by dane » Sun Aug 16, 2020 1:29 pm
Yep! The easy way is just a small UPS. Find one on Amazon.
Dane Jasper
by krauster » Mon Aug 17, 2020 7:58 pm
I looked at UPS's and sent this email to tripplite:

I'm looking for a UPS that I can use to power my router/modem (measured at 12Watts) and a laptop during power outage. When I read the info about UPS the descriptions state times of a few minutes, not hours. I realize these times are given for larger power draws. And that the UPS is to help a computer shut down gracefully. I would be purchasing a device to allow me to keep internet connection up and be using the internet over a few days not minutes. Although not continuously.
So solar comes to mind. Do you make a product that can be charged with the DC output of a solar panel? or do I need to convert the DC to AC to feed the UPS?
Also, are the batteries inside the UPS replaceable with commodity motorcycle batteries?
Steve Krause

This is what they wrote back to me this morning:

Hello, Unfortunately, we do not have something like this in our offering.
Pre-Sales Application Specialist

Maybe I confused the issue by bringing up using solar, but my take on UPS supplies are that they are only meant to help you shut down. If they are rated for a lot of power I guess they can run a 12Watt draw for quite a while. Is a 1000Watt device ($150) able to power a 12Watt router/modem for 1000/12 hours? (not taking into account any power to run a laptop.)
by nwhitehorn » Tue Aug 18, 2020 6:13 am
I have a CyberPower ST625U for this, which lasted 2.5 hours the last time we had a power outage. It also has some USB ports for powering small DC things with USB charging (e.g. the Sonic ATA). Have had no trouble with it and it's cheap.
by virtualmike » Tue Aug 18, 2020 9:54 pm
That company may make that claim, but not all UPSes are so limited.

The whole point is uninterrupted power, and many are rated for much longer periods of time. Some companies' data centers have HUGE UPS farms to keep the entire data center running for days, if necessary.

Check into other manufacturers.
by paulcoldren » Sun Aug 23, 2020 12:55 pm
I run telecom equipment like this straight off of DC from time to time.

Note that the wattage rating of a commercial UPS doesn't indicate how long it will last; it indicates how much instantaneous power it can deliver based on the size of the inverter-charger (the unit that does the AC<-->DC conversion). So a "1000w" device may last for 15 minutes with a small onboard battery, or it may last for a day with a large bank (we're talking over a thousand pounds) of attached batteries. What you are looking for is the battery *capacity*, typically measured in watt-hours or a combination of amp-hours and nominal voltage.

Regardless -- a commercial UPS will waste a huge amount of its available energy running the inverter-charger circuitry. Just using some real-world numbers from equipment I have: one of my 1000w inverters will draw somewhere around 10w just as a tax for energizing the circuitry, even before you put any load on it. So if you have 12w of load, that means up to 45% of the power draw is being wasted just running the inverter circuitry. They really are designed to integrate easily with existing loads, provide bridge power until a backup supply kicks in, or provide time to gracefully shut down the equipment. They aren't optimized to run your loads as long as possible.

If your goal is extended, efficient runtime for DC-only loads, you should look into some of the portable power packs available on the market (goal zero, etc). These typically consist of a battery, some control circuitry, a few DC power outputs, a solar input, perhaps an onboard charger for recharging from wall (AC) power, and perhaps a power inverter for producing AC power from the battery.

For maximum efficiency, you'd want to use one of the straight DC power outputs, and keep the inverter turned off. Powering direct from DC eliminates the conversion losses in going from DC (battery) to AC (through power inverter) and then back to DC (through equipment power supply).

I don't know what voltage the specific gear in your photo requires. It's probably either 12vdc, 5vdc, or 24vdc. You can check on the back, it will usually say what its voltage requirements are.

You can purchase small, inexpensive DC-DC "buck converters" to convert from the nominal system voltage of your power source (most likely 12vdc) to the voltage required by the equipment, if required. If it's 12vdc, you're good, no converter required. If it's 5vdc or 24vdc, you will need a buck converter. These converters can be found for <$15.

You'll need to buy a few adapters to get the DC power into the equipment. You will need to determine the size of the barrel connector that plugs into the back of the equipment, and you'll need to buy your own compatibly-sized barrel connector that connects to the DC power output.

You'll most likely end up getting the DC power out of the power source via an automotive 12v "cigarette lighter" socket, then splicing it into a DC-DC buck converter if needed to get the proper voltage, then splicing into a barrel connector adapter to get it into the equipment.

It definitely can be done, and for the right application it makes a lot of sense. It's a bit more complex than just using a UPS, but it is what you want if your goal is extended runtime.

I have a 100ah 12v battery (about 60lbs), and even with keeping the battery above 50% to prolong its lifespan, that gives me ~600wh of usable capacity, which could run a 12w load for just over 2 days. Your runtime increases substantially when you don't waste the majority of the power running an oversized inverter and suffering double-conversion losses.
by digitalbitstream » Tue Aug 25, 2020 12:00 am
The direct current (no wall plug) UPS units last a lot longer, for these low drain devices.
There a scad of them on amazon and ebay:
These plug directly into the device, and act as the power supply during normal times.

That said I prefer models that use a lead acid battery, because such batteries have fewer problems and never overheat.
I have not found a current model (I use a discontinued model). Ideally I'd find a lead acid battery DC EnergyStar Level VI compliant UC Listed supply, but that might be asking too much.

412P Mini UPS.jpg
412P Mini UPS.jpg (108.31 KiB) Viewed 3814 times

You can also save scads of power by leaving off the router. During the power safety shutoff you'll have
access to wired Internet, without WiFi.
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