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Charging a 1.2V, 450mAh NiCad cell with 18mA ?

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treez

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Hi,

I’ve just left a “2/3AA” 1.2V, 450mAh Ni-Cd cell charging with a 18mA charge current.

(I put the Nicad in series with a reverse polarity diode and a 160R resistor…then put this across 5VDC.
There is also a red led in series with 100R across the Nicad.)

The cell voltage was 1.24V at the start of charge. (I read that Nicad can be at 1.2V and be 99% discharged?)
I presume it will take 25 hours to get fully charged?
Also, is it ok to leave it charging with 18mA indefinitely?
 

If you leave indefinitely charging at 18mA rate you will overcharge and destroy battery. Trickle charge should be no more tan C/10 ie. 4.5mA
 

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Thanks, i am using 18mA , so thats less than C/20. I am wondering if somehow low charge currents just dont cut it?
 

C/20 trickle charge of NiCd and NiMH battery is basically safe, lifetime can be nevertheless reduced to a few years. I remember that an application note suggested timer controlled C/10 to C/20, restarted by a voltage comparator.
 
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The charge rate won't do any damage but it isn't ideal. However, I can't work out what the purpose of the LED is. It will never light up under normal charging. It might do if the battery is removed but I've never seen a charger with an indicator to say the battery is missing!

Brian.
 
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I can't work out what the purpose of the LED is. It will never light up under normal charging. It might do if the battery is removed but I've never seen a charger with an indicator to say the battery is missing!
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Thanks, thats interesting , this is actually a hair trimmer which i bought.....it had a dodgy looking charger circuit which needed connection to 110VAC...so i had to hack the little circuit board to be charged from a 5v wall wart instead. I must admit i thought the led was a bit questionable on a nicad.
 

I guess they only put the red led there as a "failed battery" warning.......but i reckon the very slight leakage current through the led will discharge the battery over time, so i think i will remove the led. Would you agree?
 

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