Battery for power -- cable modem and wifi router during outage

[Adrian B]

Let me start by saying that the point of this question is to figure out if I can do this with items on hand instead of buying a UPS.

I have a cable modem and wireless router which require 12V 750mA and 12V 1000mA respectively. In the event of a power outage, I'd like to be able to manually disconnect them from their wall warts and then connect both (in parallel) to one battery which will have been kept on a "battery tender" up to that point. In very short tests, each device seems to work fine when powered individually from a 12V sealed AGM battery that I have on hand.

I'm assuming that the modem and the router have their own internal regulators to some working voltage lower than 12V, but I don't really want to crack them open to see. What's the worst case scenario if I just hook the two devices to the 12V battery in parallel and let them run until one or both stop working? Maybe each device will turn off when the battery voltage falls below the device's regulator's cut off voltage? Maybe cut off voltages will never be reached and the battery will fully discharge? Is there significant risk that either end would damage the devices, or will their (assumed) regulators handle the situation gracefully?

I understand that full discharge isn't ideal for the battery but one website said an AGM will still get something like 150 cycles even when each cycle is a 100% discharge. Power outages aren't frequent, so 150 cycles is plenty.

Is there anything else I should be considering? Thanks!

 

[mfb]

Damage from voltages below the design is unlikely. I cannot rule it out but it would surprise me.
The most likely result: the devices will operate for some time, then one device stops working, the other one keeps working a bit more until it stops, too, then the battery slowly continues discharging until it is fully depleted. Put them back to grid power and they will work again. If the battery is too weak, the devices might not work at all, or just one of them for a while. If the battery is good, it might be able to provide 12V (or nearly 12 V) for a long time.

 

[Windadct]

There is nothing wrong with your plan... however you may want to consider permanently connecting the load ( modem and router) to the battery - perhaps with a voltage regulator ( LM2940) - this simple and easy to implement device will provide 12V out over a wide range of input - to allow for the case when the battery is charging (13.6V for a lead acid battery) and down to below 6V as the battery discharges.

 

[davenn]

There is nothing wrong with your plan... however you may want to consider permanently connecting the load ( modem and router) to the battery -

Yes .. my suggestion too ... run them off a say ... 12V 7.2AH SLA ( sealed Lead Acid) and have the SLA on permanent trickle/float charge
That way you don't have to mess about with any switching. Then your wifi and router units continue running till 1) they battery goes flat 2) mains power is restored

Dave

 

[Adrian B]

Thanks for the feedback, everybody. I posted this same question on electronics.stackexchange.com and received no comments or answers, so +1 for Physics Forums!

Since I'm the only user of this equipment and I wouldn't want charge wasted if the power should happen to fail while I'm not present. So I'd actually prefer to manually switch to the battery.

Windadct: Out of curiosity, I checked out the LM2940 data sheet. Doesn't section 6.7 say that the minimum input is 13.6V if I want 12V output? If so, I don't think I understand your comment about a wide range of input from 13.6V down to below 6V.

 

[davenn]

Since I'm the only user of this equipment and I wouldn't want charge wasted if the power should happen to fail while I'm not present. So I'd actually prefer to manually switch to the battery.

doing it the way Windadct and I suggest means there is no drop out of service ... you are in the middle of something important ...
a video call, downloading a file etc. The big reason that way is preferred ;)

Out of curiosity, I checked out the LM2940 data sheet. Doesn't section 6.7 say that the minimum input is 13.6V if I want 12V output?

Yes, That is correct :) so obviously you are not going to get that from a 12V battery

I don't see the need for a voltage regulator anyway

Dave

 

[NascentOxygen]

You may well find that in practice the devices are happy to work from 10v, or even 9v.

An alternative to a lead-acid battery would be a dozen or so C or sub-C NiMH cells. The latter can be had in 6AH. The caveat in using a string of cells is to not prolong discharge until any cell becomes reverse-charged. You intend being present while the backup supply is in use, so switch off after a duration appropriate to the cell's AH.

 

[meBigGuy]

I guess you are trying to access the internet with battery operated devices.

I am surprised that your cable system works when power is down.

 

[Adrian B]

I am surprised that your cable system works when power is down.

Had a power failure about an hour ago and I can verify that the cable system (internet, at least) continued to work.