Subject: cordless phone battery display? - (NiMh) long
Date: Fri, 3 Jan 2003 18:03:14 +0000
NNTP-Posting-Date: Fri, 3 Jan 2003 18:10:08 +0000 (UTC)
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I am trying to understand the display on a cordless phone with
rechargeable (AAA format) cells. It is stated as needing NiMh type
(650mAh), and these are being used.
The reason for the need to understand is that I have been having a *lot*
of trouble with the phone system, and two handsets, and after more than
one replacement (for various reasons) I do not really trust the product,
but when it works it is really good, and is exactly what I want - and
the handbook is unhelpful in the possibly important matter of battery
management. And I need to decide if it is worth yet another round of
complaints to the manufacturer.
The panel display includes a battery status icon which shows
three bars when fully charged, then two, then one, then no bars, when
If run when discharged, the phone stops operating, with no display of
any kind indicated. Presumably a voltage sensed event?
If fully charged batteries are inserted, (and the charging cradle is
then not used), the phone operates well for the expected (full charge)
period, but the battery status always shows no bars. This suggests that
the status display is derived from something other than battery voltage
- not surprising for a rechargeable perhaps, although I do not know how
battery status indicators are designed.
If either fully charged, or discharged batteries, are inserted, and then
the phone is placed onto its own charging cradle, it indicates
*charging* (battery icon flashes, with or without bars). This continues
for a long period (12 hours??) and the status indicator gradually
increases bars ending with three bars, which remain. With off-cradle
use, the bars reduce in number as might be expected.
If the phone remains *on* the cradle for the rest of its life, except
for phone calls etc, I notice that there are occasions when it appears
to *discharge*, then charge again, on the cradle - at least, battery
icon reduces in bars, and increases again. When this happens, and at the
indicated 'discharged' state, it really is fully discharged because I
have sometimes tried to use the handset then, and it soon shuts down.
With batteries (been in use many months) which have a reduced capacity
(say approximately 450 mAh), the self discharge/charge behaviour seems
to get quite frequent if the phone is always left on the cradle. This
becomes quite inconvenient to the point of becoming unusable - it spends
so long apparently discharging that there is a good chance of
interrupted calls from insufficient charge.
After reading information about NiMh rechargeables, I wondered if the
phone system is less clever than I thought it was, and batteries are
deteriorating because of constant charging/overcharging. A comment from
an experienced phone shop employee suggested that this general type of
phone was not really designed for constant charging, but for a lot of
off-cradle use (it does have a belt hook, very useful in the garden).
(The handbook does say that the battery status indicator is not relevant
until after the first charge cycle).
Open circuit voltage at the cradle terminals is 7.4 volts rms, my guess
is that the cradle contains simply a transformer.
Current entering the phone when on cradle is 20mA rms (measured across a
series 120ohm resistor). I do not know if this changes according to
I would appreciate comments about this situation ?
- the cause of the apparent on-cradle self discharge etc
- relationship between battery status icon and actual charge in battery
- possibility of battery health/damage with the apparent ongoing
charging for a life in the cradle
Alan Cocks Berkshire UK
For Prostatitis Support in UK - see www.bps-assoc.org.uk