# Simple solar internet?August 7, 2019 8:36 AM   Subscribe

Say you have a cellular modem that runs off a 12V power supply. Say you'd like to run it off a 12V battery bank instead of a wall wart. The battery bank's output varies from ~12V to ~13.5V depending on sunshine and what else is plugged in/running at the moment.

What is the voltage tolerance for the modem likely to be? I'm considering just cutting the outlet cord off the wall wart and connecting it, with an inline 2A fuse, directly to the main bus. Is this going to be a problem? If so, why? And what's the simplest solution?
posted by bricoleur to Technology (9 answers total) 1 user marked this as a favorite

I would be shocked (no pun intended) if the modem was unhappy running on 13.5V but it's not really possible to say with 100% certainty without knowing more about the circuitry inside. If it were my equipment, I wouldn't hesitate to hook it up as you describe after confirming polarity and assuming adequate current delivery capacity from the battery bank.
posted by exogenous at 8:45 AM on August 7, 2019

There's no chance that 13.5V will hurt your device. Your typical cheapo 5V regulator can take up to 35V... 13.5 is not even going to hurt its feelings.
posted by ftm at 8:56 AM on August 7, 2019

It likely has tolerance, as voltage regulation usually is vague in the wall wart and specific in the modem (as in, the wall wart supplies 12V-ish and the modem likely regulates this to 5V for most of what it actually does). Based on this theory (emphasis: theory) you might find that the regulator in the modem runs a tiny bit warmer when the voltage is slightly high because it's dropping more voltage but the current it draws is probably under 500mA so there's probably less than a watt in it.

Also, if you find a modem that runs in a car it will have a built-in tolerance because car voltages vary from about 11-14V depending on what's happening at the time.

Again, no promises here, but that is probably a gamble I would take (with full awareness that if the modem goes pop I did that to myself and have no one else to blame).
posted by How much is that froggie in the window at 8:56 AM on August 7, 2019

If I was you I'd get a cheap 12V voltage regulated car adapter (and probably a corresponding socket), just to be sure. It even has a fuse!

But also the voltage tolerance should be in the specs for the modem somewhere. What modem is it?
posted by neckro23 at 9:22 AM on August 7, 2019

It's a Huawei B310s-518.
posted by bricoleur at 9:29 AM on August 7, 2019

I did exactly this with an lte/wifi router on my boat. Our batteries typically run 12.5-14.5v depending on charge/discharge state, and are charged either via solar panel regulator or shore power converter. I cut the end off the wall wart and put a 3A fuse (based on maximum draw for this model) in between. Been running smoothly for two weeks now.
posted by kyliej at 10:05 AM on August 7, 2019

So far, so good. No "pop!" And no more listening to that inverter fan, either. Thanks!
posted by bricoleur at 11:26 AM on August 7, 2019

Something you didn’t ask about: how efficiently the device will run at that voltage. Many consumer devices still use pretty inefficient voltage converters, or run them at an inefficient voltage inside their range. I’ve found that using a switching dc-dc step down converter (\$10 ish online ) to drop 13.5 down to 7 volts, and putting 7 into the wireless device that used 5 volts internally was useful. the total watts that come out of my battery dropped by a third and the pricy radio runs much cooler. Not as big a gain as turning off the inverter, but possibly useful if you want to leave the WiFi on longer.
posted by unknown knowns at 6:24 PM on August 7, 2019

> "There's no chance that 13.5V will hurt your device. Your typical cheapo 5V regulator can take up to 35V... 13.5 is not even going to hurt its feelings.
Not really a good assumption to make. Yeah, most of the common jellybean regulators hobbyists use are good to 35V~40V - but that doesn't mean most equipment manufactured these days uses them.

There's an awful lot of 5v and even 12v regulators in cheaply made equipment (like cellular modems, wireless gateways, etc) with max 20V, 15V (e.g. LM1117 or, for higher currents, the near-ubiquitous REG1xx series & clones), or even lower inputs. And that's before you even factor in the thermal requirements - a 5V linear regulator circuit that's designed for 12V input is going to see ~20% increase in power dissipation at 13.5V.

Will the OP's modem be fine at 13.5V? Yeah, probably.

Would I do it myself? Yeah, probably, though I'd probably check the regulator used & keep an eye on things for at least a while.

Would I give a blanket "no chance that 13.5V will hurt your device … not even going to hurt its feelings" answer? Nope, that's just irresponsible.
posted by Pinback at 7:33 PM on August 7, 2019

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