NiCad Battery Cycler
for Rx & Tx

[Cycler for 4.8 Volt Receiver]
[Cycler for 9 Volt Transmitter]

Part Description Radio Shack Digikey Notes
IC1  TL431ILP, Motorola
 n/a  296-1292-ND  Prec. Voltage Ref.
D1, LED  Light Emitting diode    P374-ND  5mm, red
D2  1N4001, diode    1N4001DICT-ND  Gen. Purp. Rec.
R1  470 ohm, resistor  271-1317    5%, 1/4W, carbon
R2/R3  10 ohm, resistor  271-132    5%, 25W, WW
R4  1.5K, resistor      5%, 1/4W, carbon
P1   10K, trimmer-pot      10-turn, Bourns
K1  5V-1A, relay  275-240 (5V)    3V type is best
S1  SPST, switch  275-8077    momentary 'on'

Part Description Radio Shack DigiKey Notes
IC1  TL431ILP, Motorola  n/a  296-1292-ND  Prec. Voltage Ref.
D1, LED  Light Emitting diode    P374-ND  5mm, red
D2  1N4001, diode    1N4001DICT-ND  Gen. Purp. Rec.
R1   1K, resistor  271-1321    5%, 1/4W, carbon
R2/R3  10 ohm, resistor  271-132    5%, 5W, WW
R4/R5  10 ohm, resistor  271-132    5%, 5W, WW
R6   10K, resistor  271-1335    5%, 1/4W, carbon
P1   10K, trimmer-pot      10-turn, Bourns
K1  7V-2A, relay  275-005 (7V)    6V preferred
S1  SPST, switch  275-8077    momentary 'on'

A KIT for this project is available here: [NiCad Cycler]

Technical Notes: This NiCad cycler is based on the Motorola Solid-State precision voltage reference device, TL431(ILP). The trip point (adjustable with P1 for both units) can be set to 4.4volt for the receiver pack and 8.8volt for the transmitter pack or whatever else you prefer as trip-point for your battery packs. Radio Shack can order this part in I was told.TL431 pins TL431ILP senses the preset voltage reference and trips the relay when that control voltage point is reached, adjusted with the 10-turn trimmer pots, which in turn activates the charger. The resistors used in this circuit provide an approximate discharge rate of 250mA. Since the remainder of the circuits power is also provided by the battery being discharged, an additional 50mA or so is discharged from the NiCad battery packs. The relay is configured as latch so that once the unit trips from discharge to charge, the unit cannot be recycled until the start switch is pressed. The component values, setting up the discharge values and trip points can be adjusted to handle any size or battery voltage up to the 30 volt maximum rating of the TL431. Remember, the relay coil voltage must also be taken into consideration when changing the operating voltage of the circuit. All components listed in the circuit can be easily obtained from your local electronics store or Tandy/Radio Shack, although the TL431 may have to be ordered in. If you find a significant drop in discharge time you have a clue that something is going bad with your pack and close examination or a new purchase may be needed.

Description and Calibration: When you have completed building the cycler, go back and make sure that all your connections are soldered solidly and that all connections are correct. If you're not sure, try to get help from someone with electronics experience. Although highly unlikely, it is possible to destroy the TL431 by reversing the positive/negative connections so try to make sure this particular device is hooked up correctly. Take your time checking your wiring and connections; the last thing you want is damage to your charger.
To calibrate this unit you need 6 regular (1.5v) dry cells, it does not matter what size they are, AA, C or D cells are all good. Just make sure they are new. NiCads will not work for this step in the process. [I used a variable Power Supply instead so I can simulate depletion of a battery pack, by adjusting the voltage, to the point I prefer the relays to trip.]
Your goal in calibrating the cycler is to adjust the trimmer pots in such a way that the unit will change from 'discharge' to 'charge' when the cells reach 1.1 volts per cell. On the receiver battery pack this will be 4.4 volts. On the transmitter pack this is 8.8 volt.
Preset the trimmer controls all the way to one end. If you followed the the parts list above you will have about 10-turns to go from one end to the other, and believe me, with the regular trimpot adjusting the cycler is almost impossible. A 10-turn trimmer is a necessity!
Okay, on with it. Connect 3 of the dry cells in series, giving you 4.5 volts total. This is just above the voltage you want the receiver pack to change over from 'cycle' to 'charge'. Connect the dry cell combination across the receiver battery leads of the cycler. Press and release the start button. If the LED lights and stays lit, turn the control all the way to the other end, and repeat the step above. Not turn the trimmer potentiometer back 1/8 turn, in the opposite direction you turned it to get the LED to go off. Press and release the start button one more time. IF the LED stays on, the receiver battery adjustment portion of the cycler is complete. If the LED still goes out, turn the trimmer an additional 1/8 turn back. Now the LED should stay lit when the start button is pressed and released. If it does not, or the relay seems to 'rattle' when you press the start button recheck your wiring; something is not connected right.

For the transmitter section of the cycler use the same calibration method as described above but now use all 6 cells. At 1.5volts per cell this will add to 9 volts and again is just above the 8.8 volt trip level to change over from cycle to charge.
CAUTION: Do NOT plug your wall charger in during the calibration procedure. It should only be plugged in when the NiCads are connected to the cycler!
Once you completed the adjustments, connect your fully charged NiCad packs to the cycler. Plug in the wall charger. The LED's on your wall charger should be on indicating the packs are being charged. If not, check your wiring again. Now press and release the start button. The Led's on the cycler should go on and the led's on the wall charger go out. This state indicates your battery packs being 'discharged'. When the led's on the wall charger are on, the batteries are being charged. About 16-hours after the cycler has switched from discharge to charge, your batteries are ready for use.
Remember, you must allow the batteries to fully charge before flying. I don't need to remind you to check the battery voltage with your expanded scale voltmeter before each flight.
Also note that the adjustments can be bit tricky, more so for the Rx part and especially if you're using a 5 volt relay for the receiver part, a 3 volt relay is much easier to work with. So just take your time and be patient.

Personal note: If you have problems with the relay actuation (Rx only), try a different relay, like the prefered 3.0 volt type instead. It is usually a reed type relay. The TL431 drawing is shown front-view. My personal settings for the threshold cut-off is for Rx @ 3.8volt and Tx @ 7.6volt, which is 0.95volt-per-cell. I like to 'deep-cycle' the battery packs. Why? For the simple reason that as soon as the the threshold is reached the charge cycle starts so there is absolutely no harm done. I have used this cycler for 8 years and combined the rx/tx in a 3"x4"x1.5" plastic case. I cycle once per month no matter if I fly or not.
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Copyright © 1995 - Tony van Roon (VA3AVR). ALL RIGHTS RESERVED
Last Updated: May 24, 2009