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Well, I've been stuck at home and since I've run out of things to do, I've started thinking about creating a simple cheap backup for my modem, router, and desktop fan.

Since I've got several lithium powerbanks that output at 5v 2.1A (max) I was interested to see if I would be able to use these to backup the modem and router. A few videos on youtube show that this can be done as long as the amperage of modem or router is really low. My router is rated 9v at 0.6A. If this is legitimate can I just connect a 5.5mm x 2.1 DC plug to the end of a USB cable to connect the power bank to the router?

I also read that a better way to accomplish this would be to use a 5V to 9V step-up adapter which will supposedly allow me to power slightly higher amperage devices which might be better as a backup for my modem (12V at 1A).

Another option I have to maybe power the modem is using my portable 20000mAh 12V lithium battery jump starter. It has a type C USB (5v/3A) output and a DC 15V/10A outout. It came with a 5.5mm to female cigarette lighter adapter for use with the 15V/10A output.

My first question related to this device is why the unit is listed as 15V and not 12V which is the type of battery it is designed to jump start? Secondly, because of this 15V spec, would it be safe to still use this to power the modem?

If it is safe, can I just make a male 5.5mm to male 5.5mm cable by joining two 5.5mm cable ends and use this to go from the jump starter pack direct to the modem?

Another option would be for me to buy a 200W or 300W inverter (200W is under $20) then could connect the jump starter power bank to the inverter with the cigarette lighter adapter. Then all I would need to do is plug the modem and the router into the inverter. I understand this option is not very efficient as I'd be losing energy converting to AC and then back to DC.... but since the combined power requirements of the modem and router is low, this doesn't appear to be an issue?

Lastly, I would like a backup option that would run my AC 120V 0.5A desk fan for a few hours. It is variable speed so I think it draws far less than the 0.5A at the low setting which is all I would need. By my calculation this means it is using 60W at most. My jump starter outputs 12V at a max of 10A, which means it should be capable of powering a 120W device so even if I lose some energy through an inverter, I should still be able to power the fan with the power bank - does this sound right?

I have also read that a 300W inverter with a cigarette adapter is limited to 150W because of its wiring. So should I just buy a 150W or 200W inverter instead? The problem with some of the 150W adapters is that they most have one AC plug so maybe its best to stick with the 300W and just be careful to keep total watts low?

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  • \$\begingroup\$ Welcome to EE.SE, You seem to have a lot of questions. It is virtually impossible to answer them within a single thread here. Please break it up into multiple posts, otherwise it will not be very useful for later readers. Furthermore be aware, that questions on the use of electronics are off-topic, if you cannot point out the engineering task involved. \$\endgroup\$ – Ariser Apr 19 '20 at 11:35
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I've got several lithium powerbanks that output at 5v 2.1A... My router is rated 9v at 0.6A. If this is legitimate can I just connect a 5.5mm x 2.1 DC plug to the end of a USB cable to connect the power bank to the router?

If the router is designed for 9 V then 5 V may not be enough, particularly if it has some circuitry that needs 5 V (because its on-board regulator will have some voltage drop).

I also read that a better way to accomplish this would be to use a 5V to 9V step-up adapter which will supposedly allow me to power slightly higher amperage devices which might be better as a backup for my modem (12V at 1A).

Yes, matching voltage to the device specification is more likely to work. But remember that as the voltage is stepped up so the input current must go up to provide the required power (watts = volts x amps), and the voltage booster is less than 100% efficient so even more input current is required. For example if the converter is 75% efficient then to get 12 V at 1 A it must draw (12 V x 1 A / 75%) / 5 V = 3.2 A. Your 5 V 2.1 A supply will not be sufficient.

My first question related to this device is why the unit is listed as 15V and not 12V which is the type of battery it is designed to jump start?

The jump starter needs to overcome resistance of the jumper leads and connections. At several hundred amps it doesn't take much resistance to cause a large voltage drop. The battery in the jumper starter is probably a high current lithium type, which puts out higher voltage than a 12 V lead-acid battery. This is good because unlike jump starting from another vehicle you don't have the option of running the engine to increase voltage (typically 14-15 V from the alternator).

Secondly, because of this 15V spec, would it be safe to still use this to power the modem?

Perhaps, perhaps not. If the original power supply is unregulated (ie. a simple transformer and rectifier) then it may normally put out quite a bit more than 12 V. On the other hand, if the power supply is a switch-mode regulated type then the modem may be expecting no more than 12 V. Older modems often used an AC supply (ie. just a transformer) with the rectifier built into the modem. Powering such a modem from DC may either not work (if it derives positive and negative voltages from the AC voltage) or could overheat the half of the rectifier that is passing the DC current.

So again, if you don't know exactly what the device can handle then matching the original supply specification is the only way to guarantee success.

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  • \$\begingroup\$ Great thanks for the clarification. Do you think the 200w inverter option work for both if I power the inverter with the jump starter - I guess I may still have issues if the jump starter is supplying more than 12V to the inverter. \$\endgroup\$ – McMath Apr 18 '20 at 21:54
  • \$\begingroup\$ Most AC mains power supplies will work off a properly designed inverter, since that is what inverters are designed for. The higher voltage from the jump starter should not be problem, since '12V' inverters are usually designed to be powered from a vehicle even when the engine is running. The inverter's specs should tell you for sure (look for eg. 'input voltage 9.8-16V' or 'over-voltage protection 16.5V'). If the fully charged no-load battery voltage is too high then you might need to discharge it a bit to get within the inverter's input voltage range. \$\endgroup\$ – Bruce Abbott Apr 18 '20 at 22:13
  • \$\begingroup\$ Excellent, so it seems that the best option might be a low cost inverter powered by the jump starter pack. From what I've read a modified sine wave inverter will be fine for the modem and router. Any idea such an inverter will also work with the fan I referenced? thx \$\endgroup\$ – McMath Apr 18 '20 at 22:21

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