(Lots of my posts from last Wednesday got lost in limbo here for a few
days. Catching up.)
Samuel W. Heywood wrote:
> On Tue, 29 Aug 2000 03:00:29 -0700 Mark Willis <[EMAIL PROTECTED]> wrote:
> > Hi, all. In talking with some people about replacing Real Time / CMOS
> > batteries, I have had some ideas on "Improving" these.
>
> > Wanted to ask: Has anyone here soldered a Cordless Telephone battery
> > pack in place, to replace the standard 3V/60R NiCad battery pack?
> > (Disadvantage: It'd take "forever" to charge off the motherboard.
> > Advantage: It'd run your CMOS / Real Time Clock, for a LONG time.)
>
> I've done that. It works. I charged the battery *before* soldering
> it onto the motherboard.
Yep <G> I planned on that (Remove old batt. from cordless phone, snip
wires, solder, install replacement battery in cordless phone <EG>
Since then I've found that the local Radio Shack will let me check their
NiCad recycle box for connectors and possibly batteries; Well, mostly
connectors <G>
> > Just a semi-random thought, that I may try (Sometimes can get 3-AA NiCad
> > packs for $2 or so, this'd be interesting for some machines that are
> > normally powered off, though I may have to add a power charging circuit
> > to charge this quicker when the machine's powered on!) So many machines
> > here are usually off, only occasionally powered up for a few hours, and
> > that's rough on CMOS batteries. I've thought of putting a 117V to 5V
> > trickle charger in those machines.
>
> I know from personal experience and observation that a machine can be
> shut down, powered off, and remain unplugged for a couple of years and
> usually still conserve the CMOS settings and the correct time/date. It
> only takes a few micro-amperes to hold the time and CMOS settings. That is
> not at all rough on the batteries. Installing a trickle-charger would be a
> waste of time and money.
I've had less luck with long term storage; I grok BIOS settings pretty
well so this isn't a bad problem, and learned long ago to do everything
the same on EVERY computer, and to write HDD parameters where they can
be seen on older machines <G>
Scary numbers: With 6mA charging a 360mAh NiCad pack, it'd take about
60 hours for a full charge. Probably longer, all things considered.
<G>
> > I am imagining a quite slow voltage-limited, current-limited trickle
> > charge for these batteries (Probably just a diode from 5V then a 120 ohm
> > resistor to current limit this to about 6mA so a 60mAh pack charges at
> > 1/10C or so?)
>
> If you should decide to build a trickle charger for a 60mAh battery pack,
> then I agree that the current should be limited to about 6mA. However, I
> disagree with the value you have calculated for your resistor. Using the
> Ohm's Law formula,
> R = E / I, and substituting we get R = 5 / .006 or 833 ohms.
Nope. Your number's right for a resistor directly across 5V - you're
forgetting the voltages across all the parts here. The resistor's
actually across about 0.7V; I do this for a living - so I'm glad I
didn't mess up:
5 Volts - 0.7 Volts (Forward drop on the Diode!) - 3.6 volts (battery
voltage, roughly) = about 0.7 volts across the resistor.
6mA, 0.7V, Ohm's Law says 116.666 ohms, so I picked 120 ohms as it's a
standard value. You'd get a little more current when the battery's
first turned on after a long absence, no big deal though, maybe 8mA
max? (Consider that some newer Fast NiCad charge controllers can charge
a NiCad at not 0.1C but a whopping 4C - so a Max713 charger would dump
up to 240mA into that same battery if you used one! <G> I prefer slow
chargers, myself.)
(For those not knowing: The diode's pretty mandatory, it's so you don't
discharge the battery to all the 5V powered stuff when the power supply
on your computer's turned off! That'd be "bad". <G>)
> All the best,
>
> Sam Heywood
Enjoy!
(If you go, while there at Radio Shack, if you see a "CueCat" bar code
scanner, know that while free, using them tells them which scanner
you're using, as they're internally serial numbered in the electronics -
and tells you which catalog or magazine or device you own, so they can
do market research for free, once they have you using this device. I
think my privacy's worth something, so if I use those for anything,
it'll be for parts <G>)
Mark
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