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AC charger for NiCd AA battery?

Started by Borrall Wonnell June 22, 2012
Hi all,

I have a device which is powered by a solitary 1.2V 800 mA NiCd cell.
An AC wall charger (5VAC @ 180 mA) is used to charge the cell.  This
is done via a simple half-wave rectifier that is built in to a small
circuit board inside the device. The battery sees about 90 mA (true
RMS).

It seems strange to use a hard-to-find AC power source rather than a
commonly available DC wall adapter to do this job.  Particularly since
the  built-in circuit simply rectifies the signal anyway.  Any
thoughts on why a product would use this approach?



In article 
<32c633ba-a958-4a5d-b6c4-48fea52c38ff@n16g2000vbn.googlegroups.com>,
 Borrall Wonnell <dbonnell@gmail.com> wrote:

> Hi all, > > I have a device which is powered by a solitary 1.2V 800 mA NiCd cell. > An AC wall charger (5VAC @ 180 mA) is used to charge the cell. This > is done via a simple half-wave rectifier that is built in to a small > circuit board inside the device. The battery sees about 90 mA (true > RMS). > > It seems strange to use a hard-to-find AC power source rather than a > commonly available DC wall adapter to do this job. Particularly since > the built-in circuit simply rectifies the signal anyway. Any > thoughts on why a product would use this approach?
A half-wave rectifier would provide a crude form of "pulse charging" which (at least in more refined forms, so perhaps it helps here) appears to be better for NiCad lifespan than a straight DC charge. If they wanted that and are nickel and penny pinching like Jeorg ;^) an "easy for them to find since they buy large piles of them" AC supply and a diode might be cheaper than a DC charger & parts to pulse charge from DC. -- Cats, coffee, chocolate...vices to live by Please don't feed the trolls. Killfile and ignore them so they will go away.
> A half-wave rectifier would provide a crude form of "pulse charging"=20 > which (at least in more refined forms, so perhaps it helps here) appears=
=20
> to be better for NiCad lifespan than a straight DC charge. >=20
Very crude. Unfortunately the product was discontinued and repair parts ar= e nearly extinct. I found a 400 mA adapter which could be used but then I'= d need a larger capacity cell to maintain C/10 charge rate. No problem for= NiMH but impossible in NiCd world. The stock battery has welded tabs with crimped ends that slide over rigid += /- terminals on the circuit board, holding the battery in place. I haven't = seen this before and I haven't found any batteries with welded crimp-style = tabs. Could be a pain to find a replacement, so I may bypass the battery a= ltogether and power directly at 1.5V. Will be ugly because the existing te= rminals are button contacts that mate with the charging dock.
On 22 Jun., 18:38, dbonn...@gmail.com wrote:
> > A half-wave rectifier would provide a crude form of "pulse charging" > > which (at least in more refined forms, so perhaps it helps here) appear=
s
> > to be better for NiCad lifespan than a straight DC charge. > > Very crude. =A0Unfortunately the product was discontinued and repair part=
s are nearly extinct. =A0I found a 400 mA adapter which could be used but t= hen I'd need a larger capacity cell to maintain C/10 charge rate. =A0No pro= blem for NiMH but impossible in NiCd world.
> > The stock battery has welded tabs with crimped ends that slide over rigid=
+/- terminals on the circuit board, holding the battery in place. I haven'= t seen this before and I haven't found any batteries with welded crimp-styl= e tabs. =A0Could be a pain to find a replacement, so I may bypass the batte= ry altogether and power directly at 1.5V. =A0Will be ugly because the exist= ing terminals are button contacts that mate with the charging dock. could find room inside for a bit of electronics? you can get many different small flat lipo batteries and single chip chargers that just need 5v in but that is 3.7V nom. so you need a bit more than just that -Lasse
dbonnell@gmail.com wrote:
>> A half-wave rectifier would provide a crude form of "pulse charging" >> which (at least in more refined forms, so perhaps it helps here) appears >> to be better for NiCad lifespan than a straight DC charge. >> > > Very crude. Unfortunately the product was discontinued and repair parts are nearly extinct. I found a 400 mA adapter which could be used but then I'd need a larger capacity cell to maintain C/10 charge rate. No problem for NiMH but impossible in NiCd world. > > The stock battery has welded tabs with crimped ends that slide over rigid +/- terminals on the circuit board, holding the battery in place. I haven't seen this before and I haven't found any batteries with welded crimp-style tabs. Could be a pain to find a replacement, so I may bypass the battery altogether and power directly at 1.5V. Will be ugly because the existing terminals are button contacts that mate with the charging dock.
Build a simple current limiter to charge at C/10 (80 mA): ------- +6 ---Vin| LM317 |Vout---+ ------- | Adj [16R] | | +-----------+ | + [Batt] | - Gnd ---------------------+ Note that Vin is _6_ volts in the above. With a 16 ohm resistor, you'll get a bit over 78.125 mA constant current. The resistor will dissipate ~102 mW, so a 1/4 watt gives a good margin. The circuit does not "care" that there is a diode in series with the battery inside the device, it still produces ~ 80 mA constant current. If you drop Vin down to 5 volts, that internal diode may become a problem as the 317 could become "headroom starved". You could use a higher Vin - if you do, more heat will be dissipated in the 317. Dissipation in the 317 is P = (Vin - Vout) * .08 and Vout = 1.28 + Vbatt Dissipation in the resistor is constant regardless of Vin. Ed
On Friday, June 22, 2012 9:38:06 AM UTC-7, (unknown) wrote:
[charge to a NiCd cell, circa 90 mA desired]
> > A half-wave rectifier would provide a crude form of "pulse charging"
...
> Very crude. Unfortunately the product was discontinued and repair parts are nearly extinct. I found a 400 mA adapter which could be used but then I'd need a larger capacity cell ...
> The stock battery has welded tabs...
Two solutions: 1>Find a smart charger for an AA NiCad cell and clip it onto the battery whenever you need it charged ( this will require you to bypass the rectifier and/or use other connections than the socket on the device). Beware, many chargers only charge TWO CELLS IN SERIES. 2>Use any-old-AC low voltage transformer, preferably in conjunction with a timer (10 hours charge, then shut it off) with a series limit resistor sized so it gives you a 90 mA charge.
On Jun 23, 6:38=A0pm, whit3rd <whit...@gmail.com> wrote:
> On Friday, June 22, 2012 9:38:06 AM UTC-7, (unknown) wrote: > > [charge to a NiCd cell, circa 90 mA desired] > > > > A half-wave rectifier would provide a crude form of "pulse charging" > ... > > Very crude. =A0Unfortunately the product was discontinued and repair pa=
rts are nearly extinct. =A0I found a 400 mA adapter which could be used but= then I'd need a larger capacity cell ...
> > The stock battery has welded tabs... > > Two solutions: > 1>Find a smart charger for an AA NiCad cell and clip it onto the battery =
whenever you need it charged
> ( this will require =A0you to bypass the rectifier and/or use other conne=
ctions than the socket
> on the device). =A0 Beware, many chargers only charge TWO CELLS IN SERIES=
.
> > 2>Use any-old-AC low voltage transformer, preferably in conjunction with =
a timer (10 hours
> charge, then shut it off) with a series limit resistor sized so it gives =
=A0you a 90 mA charge. or a dc one, doesnt matter which walwart & resistor is the sensible option. NT
On Saturday, June 23, 2012 3:08:27 PM UTC-2:30, whit3rd wrote:
> 2>Use any-old-AC low voltage transformer, preferably in conjunction with =
a timer (10 hours
> charge, then shut it off) with a series limit resistor sized so it gives =
you a 90 mA charge. Won't using a series resistor also reduce the voltage? For now I'm just going to replace the battery using one of the suggested id= eas. At some point I'd like to do one of two things: 1) Convert the device to use external power (no battery). Run at about 1.5= VDC internally after using a regulator to drop from 5V DC external input. 2) Dual use...allow the device to run on external power or on battery power= . When connected to external power, battery is being charged. The only ci= rcuits I've seen for this use diodes which would probably mean sub-par oper= ation due to voltage drop. There's very little room inside the device, ext= ra circuitry is probably only a pipe dream.
On Sunday, June 24, 2012 4:27:07 PM UTC-7, (unknown) wrote:
> On Saturday, June 23, 2012 3:08:27 PM UTC-2:30, whit3rd wrote: > > 2>Use any-old-AC low voltage transformer, preferably in conjunction with a timer (10 hours > > charge, then shut it off) with a series limit resistor sized so it gives you a 90 mA charge. > > Won't using a series resistor also reduce the voltage?
Yes, of course. The implication of using any-old-wallwart is that it has to be greater voltage than the battery, you MUST reduce the voltage or your battery will not just charge, but charge, overheat, explode.