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From article <C16Csu.3LG@news.rich.bnr.ca>, by martyg@bnr.ca (Martin Gallant):
> I have just become the owner of a (new) Futaba 4-channel FM transmitter
> and corresponding receiver.  This is my first R/C radio set.  I would
> like to cycle both battery packs a couple of times before their first
> flight, to get an estimate of their capacity.
> 
> I have also been told this is a good idea for new NiCd's anyway.
> Opinions?
> 
> I need an inexpensive method to discharge the packs, ie. simple resistor
> and miniature light bulb method.  Can anyone supply a schematic and a
> parts list?  Is there an archive for this sort of information?

I have made several different dischargers.  The easiest one, and least 
expensive, consists of a zener-diode, and a power-resistor.  I am of the 
opinion that when discharging a pack of cells connected in series, you 
should not let the average cell voltage drop below .9volts per cell.  For
a 4-cell receiver pack, this works out to 3.6volts. 

I like to discharge 550mA nicads at their rated current, or about 1/2Amp.
This prevents any damage that can occur at higher discharge rates.  The way
to calculate the size of your power resistor is as follows:
Resistor-value(in ohms) = Pack-voltage/Max.current(in Amperes).  So, if you
have a 4-cell pack, it's "resting" voltage is around 4.8-5.5 volts, depending
on its state of charge.  Plug in R=5/.5 (500mA=.5A), you get 10 ohms.  So, 
my discharger for the receiver pack uses a 10 Ohm power resistor.  You must
also make sure your power resistor can dissipate sufficient heat.  This is
calculated as: power-needed = Pack-voltage x discharge-current.  In this
case, 5volts x .5amps = 2.5 Watts.  Now, if you used a 2.5 watt power resistor,
(assuming there was such a thing), it would get REAL hot.  I use the equivelant
of a 60 watt, 10-ohm resistor, to keep the generated down.  This is accomplishedby using four 10-ohm resistors, wiring two sets of two in parallel, and then
those two parallel sets in series.  By wiring two 10-ohm resistors in
parallel, you get a 5-ohm resistor that can dissipate twice as much power.
Wire two of these in series and you get a 10-ohm resistor that can handle
four times the power of just one of these resistors.  I use 15-watt 10-ohm
resistors.

Circuit:


                                                     Resistors
|===========|                                    --|=======|--
|  Battery  |          b                         |           | 
|           |----------/zener/---------------------|=======|--
|           | Pos.                                            | 
|           |                                    --|=======|--
|           |                                |---|           |
|           |                                |   --|=======|--
|           | Neg.                           |    
|===========|--------------------------------|

where --|=====|-- is a resistor, with all four connected in a series-parallel
configuration to give a 10-ohm load.  The zener diode is reverse-biased
(the band goes towards the battery) to only allow current to flow when the 
battery voltage is greater than the value of the zener diode.  In this case,
(4 1.2v cells) use a 3.6 volt or something close (3.2-4.0).  Be sure that
the diode can handle about one amp of current with no problem, or else it
will blow!

You can do without the zener diode if you just watch the battery voltage
while it's discharging.  Just unhook the circuit when voltage reaches
3.6v (4-cells) or 7.2v(8-cells).

Good luck-
Jason Worley
 

In article <1993Jan22.025009.20035@a.cs.okstate.edu> worley@a.cs.okstate.edu (WORLEY LAWRENCE JA) writes:
>                                                             I am of the 
>opinion that when discharging a pack of cells connected in series, you 
>should not let the average cell voltage drop below .9volts per cell.

Right on.

>Resistor-value(in ohms) = Pack-voltage/Max.current(in Amperes).  So, if you
>have a 4-cell pack, it's "resting" voltage is around 4.8-5.5 volts, depending
>on its state of charge.  Plug in R=5/.5 (500mA=.5A), you get 10 ohms.  So, 
>my discharger for the receiver pack uses a 10 Ohm power resistor.  

You are forgetting the zener diode. In all circuits where there is a
predictable voltage drop (diodes, zener diodes, base-emitter junctions, etc)
you must subtract the diode voltage from the source voltage, so the correct
formula is:  R = (pack voltage - zener voltage) / current. 

In your example, this gives a resistor value closer to 2 ohms. Remember that
the zener diode is dissipating most of the power (dropping around 3.6 volts)
and the resistor drops the voltage by only another volt or so.

>also make sure your power resistor can dissipate sufficient heat.  This is
>calculated as: power-needed = Pack-voltage x discharge-current.  In this
>case, 5volts x .5amps = 2.5 Watts.  Now, if you used a 2.5 watt power resistor,
>(assuming there was such a thing), it would get REAL hot.  I use the equivelant
>of a 60 watt, 10-ohm resistor, to keep the generated down.  This is accomp-
>lished by using four 10-ohm resistors, wiring two sets of two in parallel, and
>then those two parallel sets in series.  By wiring two 10-ohm resistors in
>parallel, you get a 5-ohm resistor that can dissipate twice as much power.
>Wire two of these in series and you get a 10-ohm resistor that can handle
>four times the power of just one of these resistors.  I use 15-watt 10-ohm
>resistors.

It's easier to think of this in terms of current. Power dissipated is
proportional to current squared, so you want to divide the current
with a current divider so that each half of the divider is only
dissipating one quarter of the original power. For a 10 ohm final value,
all you need is two 20 ohm values in parallel, where the 20 ohms is
made up of two 10 ohm resistors. This is essentially what you said. 
However since we know we only need a 2 ohm resistor, the total power it
needs to dissipate is only .5 watts - which is easy to find, i.e. only
one resistor is needed.

>                                                             Be sure that
>the diode can handle about one amp of current with no problem, or else it
>will blow!

Yes. You want at least a .5x3.6 = 1.8, let's say 2 watt zener. It is dissipating
most of the total 2.5 watts.

>Good luck-

Max
--
Max Feil  mkfeil@bnr.ca | What do I know? I'm just a Nerd on the Big Ranch.
Bell-Northern Research  |------------------------------------------------------
CAD Software Development| "I have a funny feeling I know who he is: Mr. X"
Ottawa, Ontario, Canada.|                                            - Ultravox

In article <1993Jan22.025009.20035@a.cs.okstate.edu>, worley@a.cs.okstate.edu (WORLEY LAWRENCE JA) writes:

|> I like to discharge 550mA nicads at their rated current, or about 1/2Amp.
|> This prevents any damage that can occur at higher discharge rates.  The way
|> to calculate the size of your power resistor is as follows:
|> Resistor-value(in ohms) = Pack-voltage/Max.current(in Amperes).  So, if you
|> have a 4-cell pack, it's "resting" voltage is around 4.8-5.5 volts, depending
|> on its state of charge.  Plug in R=5/.5 (500mA=.5A), you get 10 ohms.  So, 
|> my discharger for the receiver pack uses a 10 Ohm power resistor.  You must
|> also make sure your power resistor can dissipate sufficient heat.  This is
|> calculated as: power-needed = Pack-voltage x discharge-current.  In this
|> case, 5volts x .5amps = 2.5 Watts.  Now, if you used a 2.5 watt power resistor,
|> (assuming there was such a thing), it would get REAL hot.  I use the equivelant
|> of a 60 watt, 10-ohm resistor, to keep the generated down.  This is accomplishedby using four 10-ohm resistors, wiring two sets of two in parallel, and then
|> those two parallel sets in series.  By wiring two 10-ohm resistors in
|> parallel, you get a 5-ohm resistor that can dissipate twice as much power.
|> Wire two of these in series and you get a 10-ohm resistor that can handle
|> four times the power of just one of these resistors.  I use 15-watt 10-ohm
|> resistors.

One point that was overlooked here is that the Zener diode will dissipate most
of the power, not the resistors.  Also you have to subtract the Zener voltage from the
bettery voltage to properly calculate the resistor value.  As a result you really
want about a 4 ohm resistor.  The resistor will dissipate only one watt, no big deal.
But you will need a two-watt zener diode, which might be a little bigger that
what Radio Shack, etc. carries.   You could use a couple of smaller zener diodes in series.

-- 

"There is no human problem which could not be solved if
 people would simply do as I advise." -- Gore Vidal


 Doug Braun                         Intel Design Technology
                                    408 765-4279

 dbraun@scdt.intel.com

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                 | hplabs |
 or maybe:      -| oliveb |- !intelca!mipos3!cadev6!dbraun
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In article <C16Csu.3LG@news.rich.bnr.ca>, martyg@bnr.ca (Martin Gallant) writes:
> I have just become the owner of a (new) Futaba 4-channel FM transmitter
> and corresponding receiver.  This is my first R/C radio set.  I would
> like to cycle both battery packs a couple of times before their first
> flight, to get an estimate of their capacity.
> 
> I have also been told this is a good idea for new NiCd's anyway.
> Opinions?
> 
> I need an inexpensive method to discharge the packs, ie. simple resistor
> and miniature light bulb method.  Can anyone supply a schematic and a
> parts list?  Is there an archive for this sort of information?
> 

Your transmitter may have a diode in the charging circuit to 
prevent cycling thru the charge jack.  In this case, you have to 
remove the battery pack.  Hook up an ESV, if no voltage is 
measured, you have a diode.  A real cheap cycling method is to 
charge up your system for 14 hrs using the standard wall charger, 
and then turn your transmitter and receiver on, and operate 
the sticks while you sit and watch TV.  You should do this for 
about 90 minutes for the standard discharge.  This also burns 
in your servos, so you find out if there is a problem before 
you fly the plane.  Repeat the procedure a few times and you 
should be confident about your battery capacity and radio 
/ servo reliability.