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dsl modem battery backup

S

shnelson

Limp Gawd
Joined
Feb 10, 2012
Messages
145
Started tinkering around with the idea of a simple (cheap) battery backup for my dsl modem. Can anyone tell me if this is a bad idea for longevity of the device or other obvious flaw?

I have a 12v 8.0Ah rechargeable battery traditionally used for portable marine electronics, I'm feeding it with the 12v 500mA charger that came with it and tying the DSL modem DC wires to the battery terminals.

DSL modem doesn't list any power requirements other than 12vdc, but it came with a 1A brick. I am thinking the 500mA charger brick might not be enough to maintain charge on battery + power the device, but I can switch it out for the 1A if that's the case. If the modem draws 1A (which I doubt), I should see this thing run on the battery for 4 hours no problem... right?

I know the preferred method here would be plugging it into a UPS and calling it good, but I'm a tinkerer at heart and my modem sits in a telecom box on the wall - no room for ups.

20130417_082738.jpg
 
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drescherjm said:
BTW, that battery is also the most common one used in UPSs and also used in wheel chairs and security devices.

I would look into a proper SLA charging circuit. Something like this:

http://www.talkingelectronics.com/projects/BatteryCharger-12vSLA/BatteryCharger-12vSLA.html


Edit: Although something like this would be easier for most:
http://www.ebay.com/itm/12-V-Volt-S...39220?pt=Battery_Chargers&hash=item53f7677974
Click to expand...

Excellent, that's exactly what I'm looking for.

I have several of these batteries, to include one on my security system and an array of them from an APC UPS. I was thrilled to see they are the same type, I never have to buy a new one for my vexilar again :)
 
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My only concern would be that your modem will not tolerate the 13.XX volts from the battery. Note a fully charged 12V SLA battery is actually 13.75V.
 
Since you want to tinker, start by getting a basic multimeter. Make sure it has an Amps scale. Now you can find out how much current that modem really draws. Look up a 7812 voltage regulator. You will probably want a small heat sink for the 7812. You can expand your mini-UPS with other 78xxs for other wall wart driven gizmos, as long as they are all 12v or less. The battery charger you have should work since it came with the battery.

If you have dead or obsolete computer power supplies sitting around, they can be a source of heat sinks and other parts.

If you really want to tinker, look up a LM317. You can build a simple variable power supply that you can run off 2 batteries in series. The 317 can output DC from ~1.2v up to a couple volts less than the input voltage. With the variable PS, you can find out what range of voltages the modem and other gizmo's will tolerate. A lot of wall wart driven things are tolerant of under volts since wall warts are often poorly regulated. I don't recommend testing the over volt tolerance since that can be an end of life event for the gizmo.
 
Applying 500mA continuously can start to damage a car battery in about a week.

You'll probably need to apply at least 13.0V to keep the battery charged. Backup supplies seem to apply 13.6V.

Do you have an approximately 1A fuse in series with the battery, in case a short develops that causes the battery to pump out 100A?

What about isolation diodes?
 
I'm not sure about the DSL modem but pretty sure your batter won't last long in that setup (less than 2-3 months.)
 
larrymoencurly said:
Applying 500mA continuously can start to damage a car battery in about a week.

You'll probably need to apply at least 13.0V to keep the battery charged. Backup supplies seem to apply 13.6V.

Do you have an approximately 1A fuse in series with the battery, in case a short develops that causes the battery to pump out 100A?

What about isolation diodes?
Click to expand...

Back from the dead to question something several people have said - why would applying continuous charge, especially of only .5a to a 12v lead acid battery damage it? Anytime a person drives a car their alternator is continuously charging the battery to the tune of 75a, which absorbs any spikes and transfers the power to the vehicle's electrical system. Car batteries take this hours every day for years, what difference would a small regulated charge applied continuously make, as long as the charger stopped applying juice when the battery was full (which sounds like it would be never in the above scenario)?

I'm looking at doing virtually the same setup with my cable modem, 12v car battery, and 2- 3 amp scumacher "smart" charger to allow a web-based freeze/burglar/power out alarm system to notify me by text and email in the event of an outage.
 
A car is rarely charging at a 75A rate. If things are working properly, little charge goes to the battery after it recovers from the power lost while starting the car. All of the gizmos in the car are powered by the alternator. Before the age of computers in cars, a quick way to check for at least some alternator output was to start the car then disconnect the battery. If the car died, dead alternator, if it keep running, working alternator. Didn't say it was a safe test.

Keep in mind that most lead acid batteries have a full charge voltage of about 13.8V. Make sure your modem or other devices are rated for that. Most automotive electronics will specify an input of 13.8V not 12V.

Check the specs on your battery. It should list a long term charge rate.
 
Dead Parrot said:
A car is rarely charging at a 75A rate. If things are working properly, little charge goes to the battery after it recovers from the power lost while starting the car. All of the gizmos in the car are powered by the alternator. Before the age of computers in cars, a quick way to check for at least some alternator output was to start the car then disconnect the battery. If the car died, dead alternator, if it keep running, working alternator. Didn't say it was a safe test.

Keep in mind that most lead acid batteries have a full charge voltage of about 13.8V. Make sure your modem or other devices are rated for that. Most automotive electronics will specify an input of 13.8V not 12V.

Check the specs on your battery. It should list a long term charge rate.
Click to expand...

True on the car battery rarely charging at 75A, that's also the way the system the OP described would be working. The juice would only be passing through the battery to the modem, and hopefully the smart charger would down-regulate/shut off when not needed.

I have looked into the regulator you were describing earlier, although if I recall it was rated for one amp max and my (at least the current, soon to be replaced) modem requires one amp minimum so might have to size that up if it's needed. Appreciate that recommendation, modems aren't as cheap as I remember them so would rather not fry it.

Do you agree that the battery would remain unharmed by the circuit discussed?
 
Battery should be OK as long as the full charge trickle current is at or less then the battery's rated maintenance current. I think a properly heat sinked 317 is supposed to be good for 1.5A. There used to be a higher current version(337?) that went to 3A. Most of the the voltage regulators need about a 1.5V difference between input and output to work. Going from the 13.8 of the battery to the 12V of the modem should be about right.
 
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