Battery Backup for Cordless Telephone Base Unit




Introduction: Battery Backup for Cordless Telephone Base Unit


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

Step 1: Getting Ready

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.

Step 2: Can You Use This Design?

Can you use this design?

If you currently own a cordless phone system, you may or may not be able to use my design.  If you are shopping for a new system, you can pick out at system that will work.  First of all, you don’t have to worry about the AC adapters for the remote handset docking stations.  They are used just to keep the handsets charged.  So it doesn’t matter whether the voltage output of those AC adapters is AC or DC. 

It is the AC adapter for the base station that is important.  If the base unit has power and a phone line, it can supply dial tone to the remote units.  In order to use my design, the AC power adapter for the base unit must out DC voltage.  Otherwise you will have to hack inside the base unit itself… something that will not be covered in these instructions. 

Strangely enough, the VTech cordless system I chose, used power adapters which output AC voltage for the handsets, and a different adapter which output DC voltage for the base unit.

You need to look at the output voltage specifications on the power adapter itself or where it connects to the base unit.  If it is DC, you can continue on with this project.  When checking your base unit, you should also take note of: how many volts it requires (6VDC, 9VDC, etc…), and the polarity of the power connector (tip = +, etc…)

Step 3: Collecting the Parts

Collecting the parts

Next, you will need to know the type and size of the connector use on the base unit’s power adapter.  You will need male and female (plug and jack) versions of this connector.  You can bring your power adapter to your local electronics parts store which has “test” connectors.  You can then test fit your connector to find the proper size.  Depending on what is available, you can often find the connectors with a length of cable already attached, which makes assembly a little quicker.

You will also need a battery holder for the appropriate number of D cells required to run your base unit.  Each D cell provides 1.5 volts.  So if your base unit required 6VDC like mine, then you need a D cell holder that connects 4 D cells in series, thus providing 6VDC (4 x 1.5 = 6).  If your unit required 9VDC, then you would need a holder that connects 6 D Cells in series (6 x 1.5 = 9), and so on…

For this project I chose to use D cells, because unless your unit used an extreme amount of current, D cells should be sufficient to supply several hours of operation.  Also, to keep this project design simple and inexpensive.  So I used a relay to switch over from AC power to battery operation rather than complicating the matter by running off rechargeable batteries that were constantly trickle charged.  I used Alkaline batteries since they have a long shelf life… just sitting there waiting for a power outage.

Other items needed are a plastic project box to house the unit, a relay, wire, solder, and tools.  The relay should be SPDT; Single Pole, Double Throw.  That means it is basically a single contact that switches between one of two contacts depending on whether or not the relay’s coil is receiving power.  Choose a small low power relay that won’t draw much current.  The only other specification for choosing the relay, is to use one that operates at the same voltage as your base unit.  In my case that was a 6 volt DC relay.

Parts List
Plastic project box
* D Cell holder
* Power plug
* Power jack
* SPST DC Relay
Wire, solder, tools

* = specifics depend on your cordless phone system – see text.

Step 4: Assembly


Assembly is pretty basic if you refer to the schematic drawing.  If I had paid closer attention to the size of the battery holder vs the inside of my box, I would not have had to do some work with my dremel tool to grind down some inside corners of my box to have things fit properly.

I used a piece of double sided tape to attach the small relay to the side of the battery holder.  These two parts then fit snugly into the case.  I then soldered the rest of the wires to the exposed connections on the relay.  Your relay should come with a diagram showing which pins are for which purpose.

I then made a small notch in the side of the case for the input / output power cables to exit.  A small cable tie on the inside of the box provides strain relief for the cables.

Step 5: Testing


Once you have finished assembly and checked your wiring, you can insert the D cells.  Then you can connect the AC power adapter for your base unit.  Connect a volt meter to the output cable.  When you plug in the AC power adapter, the relay is energized, and voltage from the AC power adapter is passed through the relay’s “normally open” contact to the connector which feeds the cordless phone base unit.

Note: The voltage you measure may not be exactly what had been printed on the adapter or base unit.  Often the output of such adapters varies slightly depending upon the load placed on it.  If your unit was rated at 6VDC, you may measure anywhere from 6 to 8 volts.

To simulate a power failure, unplug the AC power adapter.  You will hear the relay click, and the power to the output cable now comes from the battery pack through the relay’s “normally closed” contact.

Although the shelf life of alkaline batteries is quite good, you may want to open up your box every six months to measure the batteries.  Some type of battery life light could have been added, but if you recall, the idea was a simple, inexpensive project.

Step 6: Summary


I sat my cordless base unit on top of the battery backup box, on a kitchen shelf, close to an AC outlet and active phone jack.  The power and phone cables run down the wall hidden by a calendar.

Doing a live test… pulling the AC adapter once the phone was connected is a good test.  After a second or so, you should be able to get dial tone on any of the handsets.  Doing this test with my phone, I found that the short amount of time it takes for the relay to de-energize at the loss of AC power, and it’s contact to switch over, was long enough for the base unit to lose its “time setting.”  This was nothing I was concerned about, and I wasn’t going to complicate the circuit by compensating.  Plus, if I happened to lose power while not home, this would be an indicator for me that it happened.

P.S.  I still keep a corded phone connected in the cellar just in case it is ever really needed.

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    5 years ago

    p.s. I'll add a voltage test button & a couple of LEDs so you can see how much charge remains.


    5 years ago

    Thanks for this. I too was considering recharging etc (lithium), but I hadn't thought of just a bunch of hefty batteries. Much easier...


    8 years ago on Introduction

    This is something that drives me crazy. A simple idea/product that would be useful and even obviously necessary, yet it does not exist.

    In the early 90's I had a Sanyo cordless phone with batteries in the base station for just this purpose. It worked like a charm- I could use it even when the power went out. Now... cordless phones have no backups and are little better than bricks when the wall power goes out. The same can be said for things like WiFi routers. Your internet is still active but the power is out. Why not have a few batteries in the thing to keep it going for a little while?

    Or why not have adaptable/adjustable backup battery packs for small items like phones, clocks, radios, and wireless routers? Exactly like this Instructable.

    This is a product I would gladly buy. Turns out it doesn't exist. Your Instructable is the closest thing to a solution I have yet found. One more reason to learn basic electronics and the use of a soldering iron.

    Thank you.

    Phil B
    Phil B

    13 years ago on Introduction

     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.


    Reply 11 years ago on Introduction

    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.


    Reply 13 years ago on Introduction

    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...........


    11 years ago on Step 4

    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.


    13 years ago on Introduction

    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 I wonder if I can implement come of the remaining parts from that......hmm this just might work.....
    Great Instructable Very informative