Make a battery backup for a cordless phone base unit, to allow all handsets to work during a power outage.
Step 1: 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.
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.