1
\$\begingroup\$

I have a modem which runs at 12V 1A, also I have an UPS battery of 12V 7Ah. I need to design a circuit so that during power outages my modem works from the battery.

Also, a circuit to charge the 12V 7Ah battery with full charge cut off so that I can connect these circuits so that there is no need to look after it after connecting everything like need to recharge the battery every day. So please help me to set up a circuit.

  1. To connect the 12V 7Ah battery to the modem 12V 1A safely without causing damage to the modem.
  2. A circuit to charge the 12V 7Ah battery with auto cut off on full charge.
  3. Automatically change over to battery so that the modem works on power outage with out disconnection.

Also one thing, the battery can't be connected directly to the modem because the modem is 12V 1A and the battery is 12V 7Ah; also the battery when fully charged will show 13.4V on multimeter, the battery is of type lead acid.

sample battery image http://img3.tradeee.com/up/lilian2010/74108.jpg

\$\endgroup\$
5
  • 1
    \$\begingroup\$ What chemistry is the battery? Lead-acid? \$\endgroup\$
    – markrages
    Commented Jul 8, 2014 at 17:26
  • \$\begingroup\$ Hi guys can anybody point to me a designed circuit so that i can develop this , i have googled a lot and can't find attached solution of (1+2+3) , the charging circuits i found wont have auto cut off on full charge.... so i would like to build a circuit ..... \$\endgroup\$
    – cc4re
    Commented Jul 8, 2014 at 18:07
  • \$\begingroup\$ "Ah" is not the same thing as "A". "12V 1A" means that if you force 12V across the modem's power input terminals, it will not draw more than one Ampere from the supply. I don't know exactly what "12V 7Ah" means, but it's something pretty close to, "If you start with a full charge on a battery in like-new condition, and you draw a constant one Ampere from it, you can do that for about seven hours before the battery is damaged. "Ah" is a product so, half an Amp, 14 hours, 100 mA 70 hours, etc. \$\endgroup\$ Commented Mar 30, 2015 at 19:47
  • \$\begingroup\$ + 1 for this question, Extensive Googling and I too have not found much the closest thing is this mini-box.com/… . I also want a low voltage disconnect on the battery (if the modem uses too much of the battery)... Surely this circuit cannot be too hard! but there is nothing out there! \$\endgroup\$
    – Hightower
    Commented May 15, 2015 at 17:47
  • \$\begingroup\$ found a solution to your requirements. homemade-circuits.com/2013/04/automatic-micro-ups-circuit.html \$\endgroup\$
    – Hightower
    Commented May 15, 2015 at 17:52

2 Answers 2

1
\$\begingroup\$

You can charge the battery using an Arduino as the controller.

Sense the voltage of the battery with the ADC, making sure you scale the twelve volts down with a voltage divider as the Arduino will only handle an input of 5V on the inputs. From this you can work out the duty cycle of the signal to send to the battery to charge.

Once the battery is fully charged voltage, stop the input signal.

What are you going to use for an input, as you may have to get a AC to 12V DC power supply?

Do a search for Arduino PWM charger for more details.

This is all dependant on if you want to build not buy, buying will be easier but where is the sense of achievement in that:>)

\$\endgroup\$
0
\$\begingroup\$

I would suggest using an off the shelf module. Googling "12V battery charger for [your type of battery]" would be a good start. I can't link you because I do not know what type of battery, I can assume lead acid though, which is good as these are fairly common circuits.

As for the 1st point, connecting a 12V battery to the modem will not be damaging. The modem will only use as much current as it needs. Just because the battery can supply larger currents won't cause harm unless the modem is defective. Most of the chargers are designed to switch over to battery power if no alternative source is supplied. You can get prebuilt units from Amazon, or individual IC chips from a supplier like digikey depending on your EE knowledge. If it is little, I'd go with the former.

\$\endgroup\$
2
  • \$\begingroup\$ Expanding up on this since my answer would generally be the same; many batteries have an integrated circuit board that will protect the battery from over charge and under-voltage lockout. Obviously the depends on the battery but it is something to keep in mind. \$\endgroup\$
    – Funkyguy
    Commented Jul 8, 2014 at 17:36
  • \$\begingroup\$ "connecting a 12V battery to the modem will not be damaging." I would not make that promise. The label on the modem says 12V. How much over or under voltage will it tolerate? A "12V" lead-acid battery charger will apply close to 15V to the battery at some point in the charging cycle. With charger disconnected, the terminal voltage will vary from 13.something down to 10.something. Most likely, the modem runs on lower voltage and has an internal regulator, but unless you've reverse engineered the same model or tried it yourself, then you don't have much on which to base your promise. \$\endgroup\$ Commented Mar 30, 2015 at 19:37

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.