Instructables

Battery Backup for Cordless Telephone Base Unit

Picture of Battery Backup for Cordless Telephone Base Unit

Introduction

Make a battery backup for a cordless phone base unit, to allow all handsets to work during a power outage.
 

 
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Step 1: Getting Ready

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Getting Ready

I recently started getting telephone service from my cable TV / Internet provider.  One of the things to be aware of is the fact that if you lose AC power to your home, you would lose phone service.  The cable provider gives some protection for such occurrences by providing about 8 hours of phone service by a backup battery pack inside their phone / internet interface box (known as an EMTA). 

I kept that in mind as I was shopping for a new cordless phone system.  I wanted a base unit (where the phone line connects), and a number of handsets that only required their AC power dock for keeping them charged.  With cordless phones you are always encouraged to keep at least one corded phone around to provide service in case of a power outage. 

I thought that it would be useful feature for the cordless phone to have a backup battery pack (like the EMTA), so that all the cordless phones could still make calls in the event of a power outage.  There may be some such models out there, but I couldn’t find any.  Anyhow, the set I liked had a lot of other attractive features, so I decided to make my own battery backup for the cordless base unit.

The idea is that this battery backup box will connect between your base unit’s AC power adapter and the base unit itself.  When you have power to your house, power from the base unit’s AC power adapter is passed through to the base unit.  When you lose AC power, a relay is released, and power to the base unit is then provided by batteries.

Although these instructions will tell how I made my unit, you will have to do some technical homework and minor changes for your project.

 

Phil B4 years ago
 Thank you for a good and useful Instructable.  I had wanted to do something like this on an older digital alarm clock.  But, I was pretty certain the time lag for the relay to engage would still cause the display to lose its setting.  I have wondered if a capacitor across the leads would supply enough power until the relay engaged.  I have also thought about wiring batteries across the power leads.  The AC supply would keep them charged.  If the AC went out, the batteries are immediately on-line.  But, we no longer have that clock.
I would like to see a diagram and possibly a model of that very set up you just described. I'm looking to power our battery opperated keyed deadbolts with AC and turning the batteries into the backup.
solarmaze (author)  Phil B4 years ago

Thank you for your comments.

If you look at the circuit drawing, putting a capacitor across the cable to the base unit would not help since when the relay is energized (that is, AC power available), the capacitor is ALSO across the relay coil, and the input voltage, so when the input voltage goes away, the capacitor would not only be supplying power to the base unit, but also be holding the relay on for a short time... then when the capacitor drains, the relay clicks off, and you still end up with a short lag time as the contact switches to the battery pack.  This is the problem with this simple "external" solution.  You'd have to do some hacking inside the base unit, or...

Change the whole design... keeping a rechargable battery across the input power, (as you mentioned),... along with the proper electronics to regulate the trickle charge to the batteries.  This is what is usually done in commercially produced products that incorporate automatice battery backup.  But once again, the key here was simple and inexpensive.
 

I have had this Idea for about 15 years and decided to Invent it,,so,,i thought to do a search to see if it was out there ,,and low and behold,,here it is being discussed,,well,,Dont know what to do with this idea now,,seems everytime i think i have an invention or idea,,its out there,,i actually was the man who come up with the Voice Activated trolling motor,,long story short,,2 yrs passed,,so i lost all rights,,did not have the money to start it and the Company i submitted the idea to took it and patented the idea and today,,it is a big deal with serious pro fishermen,,What do i Do ???,,i have so many idea's like this,,i dont sleep at night and that is when all these invention's or idea's come to me,,,Can anyone help ??? everytime i find a place ,,they want an outrageous price for the Patent,,and i honestly dont have the money to back me up...Thanks for any help you may have...........
Exiumind3 years ago
nice and simple ideia, how could i miss that =P
but in my case, the circuit must recharge the batteries automatically when not in use (Li-ION) hmmmm..
but for now i will stick to your ideia, thanks alot for sharing this m8 =D
I like this, gives me some ideas for other things. I normally have my phone hooked to my computers battery backup but this is just cool. It would be even cooler if they were rechargable batteries that charged by the wall outlet.
bwpatton14 years ago

This is a great idea, during IKE last year we had to use a corded phone to get be able to use our home phone (as the cell phones are crap in emergencies), I just had a battery back up die......so I wonder if I can implement come of the remaining parts from that......hmm this just might work.....
Great Instructable Very informative

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