Resistor to discharge NiCd batteries
ONG ENG TENG
mentioned to discharge the batteries when not in use for long time to
reduce the NiCd memory problem.
Anybody knows what resistor to get to do this discharging (presuming
for a single battery)? Like maybe one of those fat 2watt "square bar"?
Bruce Ritchie
I wouldn't use a resistor. I use a lamp (of the voltage rating
of the bateries, of course...). That way you can See when the
batteries are discharged.
brucer
--
(We excersise the `Oz-ian Option')
"I can't give you brains, but I can give you a diploma."
---------------------------
Bruce T. Ritchie "My employer can speak for itself.
ptim...@kathy.jpl.nasa.gov
You will would certianly do fine with a two watt resistor gold anodized Dale units.
The more imprtant thing is the ohm value. The idea is to discharge all of the
cells as far as possible without forcing the lowest capcity units too far into
reversal. Cells do reverse if you short down a whole battery, but they
don't if you short down individual cells. Not practical for most commercial
units, but worth considering. In aeorspace industry we size the resistors
according to the capacity of the cells. Smaller cells get more ohms. For
example a 10 Ampere-Hour cell might get a 1 Ohm resistor, so a 1 AH cell
should get a 10 Ohm resistor. If you go higher than this, you current desities
will be fairly high, and the risk in cell reversals may be greater than the
benifit due to reconditioning. If your cells are only 1 AH and your using
my 10 Ohm recommendation, You wont need a 2 watt unit, only a 0.2 Watt unit.
Also, it is always safer to go to higher resistance, although it will take longer.
Paul Timmerman - Battery Systems Group: Jet Propulsion Laboratory, Pasadena, Ca
Volker Soffel
you might want to take a look into the rec.video newsgroup, there's a
lot of info on this subject floating around there right now. It would be
to much to repeat everything here. If you can't get to this newsgroup
email me. I'll send you a summary.
--
Volker Soffel National Semiconductor Corporation
vol...@nsc.nsc.com Embedded Control Division Applications
(408) 721-4360 Santa Clara, CA95052
Simon Monk
>for a single battery)? Like maybe one of those fat 2watt "square bar"?
I find a torch bulb most satisfactory for this, for a number of reasons.
1) You can monitor how the discharge is going by the brightness of the
light. When it goes out I leave it for another hour to make sure.
2) As the batteries discharge (voltage accross them drops) the bulb
gets dimmer (cooler) and so the resistance drops allowing the now smaller
voltage accross the battery to safely discharge faster through the bulb.
This is almost as good as constant current discharging.
As for choosing a bulb, anything with the same or higher voltage than the
batterires will work fine. The bigger tha Wattage, the faster the
discharge. The bulb I use is from a large flash light, I dont remember
the power, but it takes about half an hour to completely flatten batteries
that have become too weak for my flash-gun.
SImon Monk.
Gregory Lukins
The value and power rating for the resistor depends on the voltage rating of
your battery. Here's a simple formula for MINIMUM power rating for the
resistor:
square the voltage of the battery and divide it
by the value of the resistor you are using.
For example:
If you have a 1.5 volt AA battery and you are using a 10 ohm resistor
(I'm just picking a value for the resistor because I know that a 10 ohm
resistor is readily available from places like Radio Shack in different power
ratings).
The power rating (in watts) will be (1.5)^2 / 10 = 2.25/10
= 0.225 watts
You'll need a 10 ohm 1/2 watt resistor. One word of caution, NiCad batteries
are designed to produce tremendous amounts of current for very short periods
of time. (The NiCad batteries used in toy cars can produce 100 amps or more
for a fraction of a second). My point is that they will heat up drastically
or explode if they are asked to produce large amounts of current for long
periods of time. I recommend that you double the power rating that the
above formula tells you to use. This will keep you safe. If you do feel the
battery getting very hot (or the resistor) unhook it.
Greg Lukins
gjlu...@uhura.cc.rochester.edu
Karl Schimmel
>>mentioned to discharge the batteries when not in use for long time to
>>reduce the NiCd memory problem.
>>
>>Anybody knows what resistor to get to do this discharging (presuming
>>for a single battery)? Like maybe one of those fat 2watt "square bar"?
>
the battery to only 1v which will not harm it. If you use a resistor you
can take it down to low and really mess up the cell. Ihave had much
experience with nicad from my RC hobbies (racing rc cars) A popular
way (and cheeper than the diode) is hook up a flash light bulb
to the battery when the bulb gets down to just the filament glowing
red take it off.
Note that will all discharging let the battery sit for a few hours
before recharging (trickle (slow) charge for 24hrs is best)
also not the zenor diode need to be able to handle a couple of amps
depending on how many cells you are discharging at once.
Karl R. Schimmel
The Wichita State University
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