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Cordless phone handset battery, usage?

Discussion in 'Electronic Repair' started by Terry, Mar 30, 2005.

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  1. Terry

    Terry Guest

    My neighbour has a four year old 3.6 volt 850 milliamp/hr cordless telephone
    handset battery which is used frequently and often for long periods; i.e.
    talking continuously for several hours. Removed from the handset and with no
    load the open circuit voltage of the three cell battery is 4.02 volts.

    She feels battery is defective; I feel it is overused between charges!

    Can anyone suggest please;
    a) How long, continuously, can such a battery, when fully charged,
    typically, operate the handset before requiring to be placed back onto the
    main unit to be replenished?
    b) What would be a typical or designed 'duty cycle' for a cordless phone.
    i.e. what percentage of use away from the main unit versus placed on the
    main unit for recharging?

    It would seem that the designers must base the battery capacity of some sort
    of average expectation of the amount of handset use away from the main unit.

    Similarly; we recently mislaid our 900 mhz cordless handset, for nearly a
    week. When found its battery was 'flat'. It took several days for the
    battery to recharge to operate 'normally'.

    Comments welcomed. TIA
  2. harrogate2

    harrogate2 Guest

    Would that all questions were so simple.

    A rechargeable battery, whether Ni-Cad or Ni-MH, can be recharged
    typically 500 times, give or take a bit.

    If the battery is four years old and it has been caned, as your
    description indicates, it is likely well past its sell-by date and
    needs replacing - they don't cost the earth, assuming you can still
    get one!
  3. Ross Herbert

    Ross Herbert Guest

    You don't say whether you measure 4.02V after the battery has been
    flattened during use in a normal day or whether it is measured
    directly after recharging. It is highly unlikely that you would
    measure this voltage if it had been flattened.

    The nominal voltage of a Ni-Cd or Ni-MH cell is 1.2V, but when new and
    fully charged and left to settle they will usually measure around
    1.25V. Multiplying 3 x 1.25 gives 3.75V.
    There is no such thing as "overuse between charges" for any type of
    battery. They simply expend their storage capacity during use. The
    length of use which can be expected is variable depending upon the
    rated capacity, whether the battery is new or old, and the proportion
    of time the handset is spent in receive mode as compared to transmit
    mode. The battery will be more heavily loaded during transmission than
    when receiving and the length of operating time while off the base
    station will be reduced. The best guide as to the length of time for
    "normal" handset operation will probably be in the user guide obtained
    with the cordless phone. The more time the handset battery is
    flattened, the more charging cycles it will be subjected to. The more
    charging cycles a battery is subjected to the greater is the reduction
    in the rated capacity, and eventually the battery will not hold a
    charge for any length of time to provide its normal length of

    Since the phone is only a few years old there will be no problem
    obtaining a new battery.
  4. Kim Clay

    Kim Clay Guest

    Four years of hard use & its time for a new battery! With a typical
    lifetime of 500-1000 cycles (under very good operating conditions) &
    figure maybe 250 discharges a year & you get a 2-4 year life.
    If its got less talk time between recharges than it did when new, it
    _is_ probably defective...
    Its only overused if it is used after the batteries are discharged :)

    If the built-in battery management if working correctly, using the
    handset until it cuts off from low battery should not hurt in - BUT - it
    should be promptly recharged. Actually nicads like being used &
    generally give very good life. End-of-life is a gradual reduction in
    capacity (as neighbor may have noticed).
    Usually "the designers" spend a few extra $ to let the consumer know
    these things & publish the document as an instruction manual, which the
    consumer thinks is extra packing only to throw away...
    If it really took "several days" to recharge something is probably
    defective. 10-14 hours (overnight) is what I see most...
    Get the information from the neighbors manual (hahaha) or look up the
    model # on the web & d/l it.

    I have found most phone manuals available on the manufactures web
    site. Below is part of a pdf on a Sony 900MHz from ~1999. The handset
    battery referred to as BP-T23 is: 3.6V, 600mAH (3 std. AA cells).

    Quote from pdf...

    Battery duration
    A fully charged battery pack lasts for about:
    Approx. 7 hours when you use the handset continuously
    Approx. 6 days when the handset is in standby mode.

    The battery pack will gradually discharge over a long period of time,
    even if not in use.

    If you leave the battery pack in the handset without charging it, the
    battery pack will be completely discharged.

    It may require several times of charging to recover its full capacity.

    To obtain the best performance from the battery :
    Do not place the handset on the base unit after each call. The battery
    works best if the handset is returned to the base unit after two or
    three calls. However, do not leave the handset off the base unit for a
    long period of time as this will completely discharge the battery pack.

    When to purchase a new battery pack:

    If the battery lasts only a few minutes even after 10 hours of
    charging, the usable life of the battery has expired and needs
    Contact your local Sony authorized dealer or service center, and ask for
    a Sony BP-T23 rechargeable battery pack.

    Note : Battery life may vary depending on usage condition and ambient

    If you do not use the handset for a long period of time, remove the
    battery pack after charging for more than 10 hours. This maintains
    battery life.

    Specifications - Handset
    Power source
    Rechargeable battery pack BP-T23
    Battery charging time
    Approx. 10 hours
    Battery life
    Standby: Approx. 6 days
    Talk: Approx. 7 hours
  5. These phones charge very slowly. It might be that she is leaving
    it off the hook a greater percentage of the time than the system
    can handle.

    It is not unusual for NiCads in this kind of service to last a lot
    longer than 4 years.

    Best regards

    Mark W. Lund, PhD ** Battery Chargers
    CEO ** Bulk Cells and Custom Battery Packs
    PowerStream Technology ** Custom Power Supplies
    140 S. Mountainway Drive ** DC/DC Converters
    Orem Utah 84058 ** Custom UPS ** Engineering, manufacturing, consulting
  6. Terry

    Terry Guest

    Many thanks for the various comments. Terry.
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