On Wed, 24 Feb 2016 08:12:11 +0000, Woody wrote:

"Johnny B Good" wrote in message
...
On Sat, 20 Feb 2016 04:40:14 -0800, Phil Allison wrote:

RJH wrote:


Phil is right but I would wait until you have extracted it before
buying a replacement as it could be Ni-Cad or Ni-MH depending on
the age of the tuner, and they require slightly different charging
regimes. Equally it could be 3V, 3.6V or 4.8V although I would
admit that the size suggests the latter.


** AFAIK, Quad FM4s only ever used 4.8V NiCd battery packs with four
cells specially made for memory back-up. It was trickle charged at a
few mA whenever the tuner was powered up.

I doubt any re-engineering is needed to employ a four cell NiMH pack
instead.


Agreed, In fact, when I was looking for a replacement 3 cell NiCd for a
Potterton 2000 CH program controller, they'd changed to a larger
capacity NiMH version. Oddly, the tagless drop in batteries used by
this programmer were over twice the price of the solder tagged ones.

Naturally, I bought the cheaper tagged battery and pulled the tags off
and dressed the 'pips' with a fine file to recreate the plug in version
at less than half price for less than ten minutes of D-I-Y activity.
:-)

The 4.8 volts seems unusually high for battery backed memory though.
The more usual with static cmos ram being 3.6 volts. CMOS sram is
guaranteed to retain data integrity right down to the 2 volt point -
and that includes the RTCs with their 70 8 bit registers 'going spare'
as used by IBM in their AT PCs first marketed back in August 1984).

--

Quad list it as a 4V battery - quite how they achieve that is another
question altogether!

It might be the charger's nominal upper voltage limit for a 3 cell NiCd
(blue) or 3 cell NiMH (green) battery. Constant current charging of
either type results in a 'resting voltage' immediately after going off
charge in the region of 1.4 volts per cell, usually dropping to 1.36
volts after a day or two. The 1.2 volts rating for these cells is the
minimum voltage just prior to becoming exhausted of useful charge.

It's quite likely that a very simple blocking diode and current limiting
resistor charging circuit fed off the 5v logic supply is all that's being
used to keep the battery charged up just like the arrangement used on
early pre-Pentium PC motherboards to maintain the RTC battery backup
(typically a 3 cell 60mAH NiCd soldered onto the board).

If needs must, you could replace it with a lithium primary cell (3.1v)
and snip the resistor lead to disable the charging. Those CR2032 180mAH
coin cells now commonly used in PCs will keep the RTC running for
anywhere from 18 months in the case of the ****e PC Chips branded
motherboards right up to 5 years or more on the better quality brands of
motherboard. If you wire in one of those "Half AA" Lithium 'batteries' so
favoured by the manufacturers of smoke detectors guaranteed to last ten
years on their soldered in battery, I should imagine it will last a
couple of decades at least.

--
Johnny B Good