How many volts can a modem handle?

[lexidobe]

Hello everyone, I am thinking about creating a UPS for my modem to keep it up during brownouts and short power outages and to offer some protection from surges. My idea is to run it off one of my extra 12V SLA batteries and use a slightly higher voltage power supply to charge the battery while it's powering the modem. The modem's power supply is rated at 12 volts and actually produces around 12.4 volts and I am wondering if the modem could handle an extra 0.8 volt or so, the battery would be getting charged at around 13.2 volts to keep it topped up at all times. I don't see that slight voltage increase as being a problem, but let me know if you think it could be. Thanks in advance!

 

If you are wondering why I am doing it this way instead of buying a low end UPS and plugging its power supply into that is, there are several reasons.

1. Better efficiency. Running the modem directly off of a battery is significantly more efficient than powering an inverter with the battery to step the voltage up and then running a power supply with it to step the voltage back down to pretty much what the battery would provide anyways. This means significantly longer run time. With a 0.5 amp draw, I would get about 10 hours of run time with this setup out of a 5 Ah battery. I wouldn't get nearly that much runtime out of a cheap (or expensive) UPS even under such a light load.

 

2. Cost. I already have a few SLA batteries laying around, so this setup would cost me nothing to build as opposed to needing to buy a UPS.

 

3. Surge protection. Since the modem would be running off the battery instead of the power supply, there is much less potential for damage if there's a power surge that kills the power supply since the battery will absorb the surge. 

[Brian Blankenship]

On the back of the modem or on the power supply, it will have a DC Volts call out. That's what you'll want to aim for and exceeding it too much will let the magic smoke out. Sometimes that call out will have a variance such as "+-5%", but if it's intended to be wall powered then I doubt that will be noted. Without knowing the variance though, you are going to need to experiment. You could, if you had another modem of the same type pump up the voltage and let it blow. Then back your voltage down to a safer level for production.

[lexidobe]

17 minutes ago, Brian Blankenship said:

On the back of the modem or on the power supply, it will have a DC Volts call out. That's what you'll want to aim for and exceeding it too much will let the magic smoke out. Sometimes that call out will have a variance such as "+-5%", but if it's intended to be wall powered then I doubt that will be noted. Without know the variance though, you are going to need to experiment. You could, if you had another modem of the same type pump up the voltage and let it blow. Then back your voltage down to a safer level for production.

Thank you. Like I said the modem's power supply is rated at 12 volts, but it actually produces around 12.4 volts. I was messing with a battery charger in a dark room and I accidentally gave it nearly 15 volts and it still works fine, so I feel a bit better about running it in the low 13 volt range now. Currently the system is running at 13.7 volts, but I am going to make adjustments to my charging system to get the voltage down to somewhere around 13.2 volts to keep my backup battery alive for as long as possible.

[panasubatomic]

I think anything over 15v would blow a fuse. Replacing a fuse on your router would not be fun. An isolation transformer may be a good idea. 

[beersykins]

What kind of loss would you have throwing in a variable voltage transformer in there?  Input V 8-30, output V 12 or similar.

 

Most electronics seem 'okay' with +-10% although the voltage sag on the bottom of that can introduce some instability.  I haven't seen consumer networking gear that fuses the input, so an increased input voltage would apply to all sub components in the device.

[lexidobe]

1 hour ago, beersykins said:

What kind of loss would you have throwing in a variable voltage transformer in there?  Input V 8-30, output V 12 or similar.

 

Most electronics seem 'okay' with +-10% although the voltage sag on the bottom of that can introduce some instability.  I haven't seen consumer networking gear that fuses the input, so an increased input voltage would apply to all sub components in the device.

Thanks for your input. Transformers won't work on DC voltage, but I might be able to get some kind of DC-DC converter to regulate the modem's input voltage to a steady 12V.

 

The modem does seem to have some type of internal regulation since the current draw drops as voltage increases, so the modem's power consumption stays approximately the same at the higher voltage. The brightness of the lights on the front doesn't seem to be affected by input voltage either.

 

I don't know how efficient a voltage converter would be, but there is another problem with that. All the converters I could find require that the converter's input voltage (or in my case battery voltage) is at least 1.5 volts higher than it's output voltage. This is a problem because the battery voltage will drop when the power goes out and will eventually go below the converter's output of 12V, so the converter would need to be able to boost the voltage as well. I don't know what kind of noise a converter could introduce into the power either.

 

Since the modem seems to have some type of internal regulation and is running fine right now on 13.2 volts, I am probably going to leave it this way.