At 10:46 AM 5/18/03 -0800, you wrote:
>At 08:11 18.05.2003 -0800, you wrote:
>>I have another dumb question. This electronic stethoscope uses a 10 cell
>>nicad battery pack to generate 12 volts that is center tapped to provide
>>the bipolar supply the opamps and other chips in the thing need. The
>>"wall wart" charger has been lost. Can anybody refresh my memory about
>>what voltage and current rated wall wart would be needed to charge 10
>>nicads in series at the normal charging time. The charging circuit has a
>>current limiting resistor and a polarity protection diode in place, so I
>>assume all it wants to see is a DC wall wart capable of producing adequate
>>current.
>
>I'd like to add to the the earlier postings (rules of thumb that are all
>correct AFAIK) that you might need regulation for the charger.
>
>Check the LM317 datasheet by National semiconductor (actually, it's the 117
>datasheet, but it also covers the 317): It has an application circuit on
>page 17 for a current regulated NiCd charger. This will be the best thing
>you can make, as the current with a fixed voltage will drop fairly fast in
>the first few minutes, and then decrease slowly as the voltage on the
>batteries increases. It's only tenths of volts, but significant enough to
>give you a hard time to tell how much energy has really been pumped into
>the battery. The LM317 is a "quarter a piece" part and easy to use (just
>add one resistor for this app). It'll increase the lifetime of your
>batteries a lot: I'm still using AA cells that are more than 10 years old
>with a selfmade charger.
A voltage regulator isn't really necessary as long as you size your
current limiting resistor to limit the current to 1/10C under the worst
case conditions (fully discharged battery and high line voltage). Without a
regulator, the charge current will vary but it will always be 1/10C or
less. Charging may take longer but that's usually not an issue.
Other than abuse*, the lifetime has more to do with the quality of the
battery than anything else. I have four Japanese made Sanyos that I bought
in 1974 that are STILL good. That's nearly thirty years! They've been
charged in one of those cheapo plastic chargers with the flip open lid
(also from Sanyo). I've had MANY other NiCads fail, both when charged in
the same charger and/or in other chargers. The Mexican made Sanyos are
junk! So are the Safts, and all the no name batteries commonly found in
most consumer electronics. All of the NiCad batteries that I've seen in
Radio Shack and the cell phone stores, warehouse stores, and consumer
electronics stores are all junk. Go to a professional battery rebuilder or
battery supplier such as DigiKey and insist on the Japanese made Sanyos.
You won't regret it! (FWIW I've been told that NiCads made by Panasonic and
some of the other Japanese companies are also good but I haven't tried
them.) I used to rebuild battery packs and would buy a couple of thousand
batteries/year and I used NOTHING but the Jap Sanyos. Most of my business
was mail order and it simply cost too much to have failures and have to
replace them. I tried other brands including the Mexican made Sanyos and
none came close to the reliability or longivity of the Jap Sanyos.
*One of the most freguent abuses of NiCads is to fast charge them. There
are ways that it can be done but it adds complications such as requiring
monitoring the cell temperatures and other things and most companies try to
cut corners with their chargers and don't properly monitor the cells. Play
it safe and just charge the batteries at 1/10 C and you won't have any
problems with them. Another way to abuse them is to discharge them
completely using one of those fancy chargers before recharging them. All
you're accomplishing is using up the limited number of discharge/recharge
cycles. NiCads DO NOT develope memory except for exceptional circumstances
such as satellites in orbit that experience EXACTLY the same day/night
cycle (and therefore charge/discharge cycle). There are some companies
that will try to tell you otherwise but they're usually just trying to sell
expensive chargers. Fully discharging cells not only shortens their life
but it can also cause cell voltage reversal if you charge the cells in
series. In other words if you apply a charging voltage to more than one
cell at a time, the sum of the voltages on several cells in series can over
power one cell in the set and can drive a negative voltage into it. I won't
go into the details of what that does but needless to say it really screws
it up inside! It's best if you recharge them when they drop to ~ 1
volt/cell. OCCASIONALLY deep discharging the cells seems to help with some
old batteries but it should never be done on every charge/discharge cycle.
Joe
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Author: Joe
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