I Have A Battery Backing Up My Cordless Phone


Cordless Phones have always been susceptible to power cuts, since they don’t generally come with a battery backing up the Base Unit. It’s expected that users provide a basic phone which can be plugged into the line in an emergency. In the past this phone would have been powered by the 50V exchange battery (which is always on) fed down its copper line.

We are now moving into a time when that type of phone connection is being superseded by landlines being fed over fibre broadband. This requires all customer equipment to be powered locally. During normal times this power would come from a local (the primary) mains supply, but during an emergency, like a power cut, a secondary supply from a generator or a battery is required.

For long term power cuts, a generator would be required but mains power cuts in the UK are rare and usually only occur during a fault. However, during the 2022-23 winter it has been suggested that generating capacity may fall short of what is needed and any power cuts, as a result, could last up to three hours.

I have taken heed of this warning and already provided a Battery Backup for my Broadband Hub so I can keep my Wi-Fi on to get good mobile phone access. Now I have taken steps to keep my landline working too.


Although this work involves low voltages these can drive huge currents through low resistances when they are fed from a high energy low impedance source, such as Lead-acid or Lithium-ion (Li-ion) batteries. This can cause things to get very hot. In particular take care playing with Li-ion batteries which can get explosively hot when shorted out. The battery I have used here has got short circuit protection, but I wouldn’t short it out just to prove it works.

Preparing For A Battery Backing Up My Phone

My first course of action was to determine what power would be required by the Cordless Phone System Base Unit. I did this by reading the label on the power unit provided with the Cordless Phone System which suggested:

  • D.C. 7.5V 0.3A

I don’t know what actual current the base unit requires but it will either be equal to or less than this value. A later observation determined this must be very little when the phone is dormant.

Of the various 12V batteries on the market made to supply such things as LED Strips, Tape Lights, CCTV Cameras, etc., I am using a 12V 6,500mAH battery Model: YSD-12650 sold via Amazon by Safari Drives. It’s a super polymer li-ion battery made in China. I bought it originally for my other project, “I’ve Given My Broadband Hub A Backup Battery“, for which I eventually got a more powerful 8,300mAH battery.

The YSD-12650 physical dimensions are: 12.5 x 7.5 x 3cm; 220 Grams. You can see it below:

The YSD-12650 12V 6,500mAH Li-ion Battery Backing Up My Phone
The YSD-12650 12V 6,500mAH Li-ion Battery Backing Up My Phone

It is supplied with a 12V 1.5A D.C. power unit which has to be connected to the 12V socket to charge it. This has a red light on it during charging. That light turns green when fully charged. The battery itself has a red light which comes on when the power switch is ON.

A simple but informative instruction sheet is provided.

Installing The Battery Backing Up My Phone

Because my Cordless Phone power supply is 7.5V I have to use a voltage regulating device (D.C. to D.C. converter) to reduce the voltage from 12V. I got the one shown below and mounted it in a small plastic box as the output connections are delicate.

12V to 7.5V D.C. to D.C. Converter
12V to 7.5V D.C. to D.C. Converter

The wires of the thin phone power cord are carefully soldered to the more substantial ones coming out of the D.C. to D.C. converter and insulated with sleeving. The input wires are substantial and so I terminated them in a 5.5mm x 2.1mm CO-AX Receptacle. The converter is stuck to the box lid with Gorilla tape. See the opened up box below:

To connect the battery so as to give the equipment an uninterruptable supply the D.C. to D.C. converter, Battery and the Power Unit must all have their 12V power lines connected together. To do this the following must be possible:

  • The D.C. to D.C. converter must have power input tolerances which can cope with an input voltage as high as 12.6V at least. This will be present when the battery is fully charged.
  • The D.C. to D.C. converter must continue to deliver 7.5V when the battery is discharged after a useful time. In practice down to 11V or less after approximately three hours.
  • Disconnection of the Battery must not let the supply voltage rise to a dangerously high level. When connected it clamps the voltage down to ≤12.6V.

The Phone’s Power Connections

Schematic of Battery Backing Up My Cordless Phone
Schematic of Battery Backing Up My Cordless Phone

Key to CO-AX Power Connectors

  • Co-ax Jack Jack: . . . . . same as receptacle but mounted on equipment.
  • CO-AX Power Plug Plug: . . . . . fits in a Receptacle or Jack.
  • CO-AX Power Receptacle Receptacle: . . . . . like a Jack but connected inline to a cable.

The diagram shows all plugs disconnected just to make clear which are plugs and which are receptacles/jacks. To work correctly all plugs must be inserted.

Measuring The Supply Voltage

If an extra splitting connector with two plugs and one receptacle is inserted in the power line, then a voltmeter can be connected via one of its plugs thus (see schematic below):

Schematic of Battery Backing Up My Cordless Phone Voltage Test
Schematic of Battery Backing Up My Cordless Phone Voltage Test

Then the following measurements can be taken:

  • With all items connected the working voltage can be measured.
  • If the power unit alone is disconnected the voltage can be measured when only the battery is used to supply the equipment,
  • If the battery alone is disconnected (or switched off) the voltage can be measured when only the power unit is used to supply the equipment.
  • If the Cordless Base Unit is disconnected the battery voltage can be measured while it is charging.
  • If the Cordless Base Unit and Power Unit are disconnected the battery voltage can be measured while it has no load and is not being charged.

CAUTION: Poking voltmeter probes into Receptacles or Jack sockets is a recipe for shorting out the Battery/Power Unit or both. It’s much safer to connect probes to the CO-AX plugs.

Backup Battery Usability Test & Conclusions

After I had fully implemented the Backup Battery, I conducted a Usability Test. This involved disconnecting the Power Unit from the mains and observing how long the equipment continued functioning satisfactorily. I also measured the voltage from time to time as the Backup Battery discharged. It never failed over a 3-hour period after which the Backup Battery had discharged by much less than 1V and was still above 12V.

There was one thing I had observed and experimented with. When I used the mains adapter supplied to charge the battery I heard mains hum when using the phone. When using the battery alone there was no mains hum. So I decided to try another mains adapter. I chose the one supplied by my broadband provider to feed the fibre interface (ONT) as it wasn’t being used with the battery backup arrangement. This proved to be ideal from the mains hum point of view. There was none. However it is only rated at 12V 0.5A. It doesn’t keep the battery at such a high voltage. However I ran it like that for a while. Then I went back to using the original battery charger and tolerating the mains hum. I thought I might eventually experiment with a dropping resister and smoothing capacitor in the power connection since there are a few volts to play with between 7.5 and 12.

Connecting To My Broadband Backup Battery

After a couple of weeks using the backup system I concluded that with such little current being drawn when the power is off I would try connecting it to the same Battery Backup system I was using for my broadband equipment. I have done this very successfully to date. It works well and there is hardly any mains hum to be heard with that battery’s power supply.

I have been able to tidy the part of the living room involved with all these backup components. I removed a two socket mains extension cable, a battery and a power unit. The power lead from the cordless phone base unit runs neatly into the corner of the room to connect to the one backup 8,500mAh battery now used for everything.




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