ADSL router battery backup

[Stephen Howard]

I have a need to organize some kind of battery backup power supply for my adsl router ( 12v dc ) and the simplest option available to me is to use a spare UPS box. However, this strikes me as being a bit OTT as the power supply for the router will have to convert the mains output from the UPS in to 12v dc. A simpler solution, perhaps, would be to connect the router directly to a 12v battery ( such as the small 7ah batteries used in the UPS ) and then connect a trickle charger to the battery - thus saving space and doing away with the need for all the switching and power conversion gubbins.

Does anyone here know if this is a reasonable proposition, and if so what would be the requirements for the trickle charger?

Regards,

[Grunff]

Yes, it's a very sensible solution, and will work fine.

For charging, just buy a trickle charger, like:

will need to build a voltage regulator to supply your router. Most routers are 5V, 7.5V or 9V (although I have seen the odd 12V one). Alternatively you can just buy one like this:

[Grunff]

The battery will chuck out 12V (or near enough) just fine, but what voltage is your router?

[Stephen Howard]

chuck out a straight 12 volts and wouldn't require any regulation...or is there something I need to know!?

Regards,

[Grunff]

The biggest argument for doing it using the proposed method is run time. With a UPS > Inverter > Wall Wart > Router setup, you'll lose perhaps

20W in the setup, when the router only requires perhaps 5W.

With the Battery > Regulator > Router setup, you'll lose 1-2W max. This means you'll get 4 times the run time from the same capacity battery.

[Peter Scott]

UPSs are cheap now in eBuyer or Misco, about 30 pounds. The very lowest capacity one will do for this. The time it will take to set up a similar function yourself hardly seems worth it.

Peter Scott

[Grunff]

No, I wouldn't bother with anything between the battery and the router (apart from a fuse). The battery voltage will vary between around 13.2V and 10.8V, but the router will easily take care of this.

[Stephen Howard]

It's 12 volt...so there'd be no need for a step-down. I just wondered if you meant there was a need for some kind of current protection that I wasn't aware of.

Regards,

[Stephen Howard]

That's what I was thinking...the business of converting 12v to 240v, then back again ( via the router power supply ) to 12v has got to cost something.

Regards,

[Andy Hall]

much the same shopping list.

The idea was to have something that would power a couple of Cisco 800 series routers and a PABX for a couple of hours. I didn't care about having signaling that the power had failed because there isn't an action to take such as server shutdown.

I found that I could buy a cheap 500VA UPS for about £40 so it didn't seem worth the effort in the end.

The router and PABX PSUs are all switch mode, so are quite efficient anyway.

[Peter Crosland]

It will also eliminate the possibility that the battery/trickle charge bodge will introduce noise to the ADSL line and impair the performance.

Peter Crosland

[Dave Plowman (News)]

Good point. Battery chargers ain't usually smoothed and filtered to electronics standards.

[Peter Scott]

In fact we don't know how long the op wants to run the device for. I assumed it would be relatively short, perhaps to finish a download and check email, or at most until a laptop battery or desktop UPS runs down. If he wants many hours then a bigger battery would be needed. If he only wants an hour or two, then a standard UPS should do fine. Your projected

25 W consumption should give plenty of time on a 350 VA UPS even assuming considerable inefficiency.

Peter Scott

[Andy Hall]

Entry level routers aren't the paragon of well designed electronics either.

[Stephen Howard]

Good points. Currently it looks like I can set the knock-up system up for around a tenner - and can revert to a proper UPS if I notice anything untoward regarding line quality...and I can always make use of a 12v charger wart elsewhere.

Regards,

[Dave Plowman (News)]

Dunno whether it is an 'entry level' router. But even if so the comment is perhaps even more valid.

[Andy Wade]

Two points:

1) For float charging an SLA battery you need a regulated and temperature-compensated constant-voltage charger. Follow the battery manufacturer's recommendations for the float voltage. Using a dirty transformer-and-rectifier-only charger will knacker the battery in no time at all (but you will not discover that it has until you really need that battery...).

2) I doubt that any small charger (regulated or not) will differ much from the supplied wall-wart PSU in terms of common-mode mains-borne noise it injects into the line.

[Harry Bloomfield]

On charge your batteries volatge will be around 13.8v. Off charge, but fully charged it could be any where between 12.5 to 12.8v. So whether some sort of voltage regualtor might be needed, depends on how tollerant the router is of input voltage, assuming it indicates 12v as the input voltage. If it does state 12v and the existing PSU is unregulated, then it will most likely be fine fed with anything from 11.5 up to 14v.

So far as the charger goes, look for one which is a controlled type, intended to be left on charge all the time without damage to the battery.

[fred]

You have it the wrong way up, a battery & charger typically involves a transformer, ancillaries and a linear regulator - no noise. A UPS involves a larger version of the same with a beefy switch mode inverter bolted on the end producing (generally) a dirty pseudo mains output - lots of noise.

In this case the UPS is the inefficient OTT bodge and a 30quid one a particularly bad one.

[Andy Hall]

Whichever way, it is assumed that the power when it reaches the router electronics is reasonably clean. Medium sized routers and large telco ones tend to have built in SMPS and filtering if they are going onto an AC supply, or special filtered DC supplies if they are going onto 50v DC services used in telephone exchanges.

With consumer grade products, the emphasis is on cost and being able to pass the EMC requirements with the least done. In some cases there are filter components in the wall wart or other PSU, in others it's in the router itself.

Given that DSL involves very low signal levels, and if there is indeed filtering in its intended power supply, then removing that may have an influence on how well the router performs, or it might not.

This might be an interesting experiment if one doesn't mind ending up with a battery charger, battery etc. if it doesn't work. OTOH, if the objective is to provide some runtime for the router when the power fails, then a UPS is a lower risk option for around the same cost as well as being ready to go in a box.