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Author Topic: using a resistor to discharge a NiCd batt.  (Read 1794 times)

PMV

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using a resistor to discharge a NiCd batt.
« on: September 30, 2003, 01:30:55 PM »

I use an FM assisting device at school (yes Im hearing impaired), some days its used for 1hr, some days it needs to go for 10 hrs. When I charge it, the battery started to lose charge quicker. My teacher gave me advice to discharge the battery with a low ohm high wattage resistor.

I using a 5.5ohm, 10 watt resistor now, and my dmm I have hooked to the battery (only wanna find how long it takes) seems to stay at 1.2v for the last long while.

Do I need a higher wattage resistor or am I checking it the wrong way? or does this way just take forever and if then, is there some way I can speed it up? (other then a sledge on the battery :P)

Never did this way before
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BiGReD

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using a resistor to discharge a NiCd batt.
« Reply #1 on: September 30, 2003, 02:05:50 PM »

1.2v eh... P=i^2*R

Find out the current going through that puppy, then we can calculate watts/hour, and determine an appropriate resistor.  You could always use LED's to discharge the battery... It would even tell you when it was done discharging.
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viridius

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using a resistor to discharge a NiCd batt.
« Reply #2 on: September 30, 2003, 05:36:22 PM »

Using an LED would create the risk of either damaging the battery from shorting it or burning out the LED.  I think the 5.5 ohm resistor is your best bet.  You could get a lower ohm, higher watt resistor if you like.
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ClearCaseMan

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using a resistor to discharge a NiCd batt.
« Reply #3 on: September 30, 2003, 05:42:44 PM »

I wouldnt use the resistor if the resistor does burn out it will do one of 2 things 1.burn and go open 2. burn and go dead short.

use a small dc motor or something to drain the batt. a led doesnt draw much current it will drain very slowly, you could just leave the device turned on to drain it faster
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I LOVE ED

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using a resistor to discharge a NiCd batt.
« Reply #4 on: September 30, 2003, 06:10:57 PM »

I have a Tamiya nicd battery discharger.  Maybe I'll open it up and see what's inside.  It has an LED indicator and it's rather light, so i would guess there aren't much in there.

**EDIT**
peeking in the discharger, it had at least two transistors, some resistors and some caps, and the LED.
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BiGReD

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using a resistor to discharge a NiCd batt.
« Reply #5 on: September 30, 2003, 06:13:43 PM »

Quote from: viridius
Using an LED would create the risk of either damaging the battery from shorting it or burning out the LED.  I think the 5.5 ohm resistor is your best bet.  You could get a lower ohm, higher watt resistor if you like.


BURNING OUT A LED ON 1.7 VOLTS!?!!? HAHAHAHAHAHAHA!

Sorry, I had to say it, cause I have never seen a led less than 1.7V.  It would work perfectly for a small cell like the one he is using, especially if he hooked up 5 of them.
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viridius

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using a resistor to discharge a NiCd batt.
« Reply #6 on: September 30, 2003, 06:58:48 PM »

First: We do not know at what voltage the discharging begins to take place.  If you have some insight as to the maximum possible voltage the battery will be at during a discharge cycle, please share it.  All we know is that it seems to hang around 1.2 volts for a while.
Second: I will reiterate my stance that LEDs should never, for any reason be used without a current limiting resistor.  They are diodes, which means that they act as a short (with effectively zero resistance) when they are hooked up in forward bias.
Third: LEDs will simply not conduct below a certain point, even in forward bias.  This will prevent the battery from fully discharging, even if only one LED is used.  Resistors, on the other hand, will ensure complete battery depletion.
Fourth: Have you ever seen a battery discharger that used LEDs?  Neither have I.  Comet photoflash systems use resistors.
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