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.
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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.
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.
+/- 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
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.
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):
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.
On Friday, June 22, 2012 9:38:06 AM UTC-7, (unknown) wrote: [charge to a NiCd cell, circa 90 mA desired]
...
nearly extinct. I found a 400 mA adapter which could be used but then I'd need a larger capacity cell ...
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
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.
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.
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