As you progress in R/C you begin to run into more and more situations that require a new rechargeable battery. The wetcell Start Battery, a Ni Cad for glow plugs, a larger capacity airborne pack, more than one airborne pack, etc. When these additions occur, most of the time they do not include a charger. So do we go down and buy a $40.00 supercharger each time we run across another battery that needs charging? Why not whip up a simple charger that will do the job and can be added to when another charging need arises?

I'll try to do this without causing you to stall out on technical descriptions. That may be the hardest part. This won't be a highly sophisticated machine, just a reliable charger that will work time after time, and doesn't break the bank to make. I'll include a Radio Shack number for each of the components. There's nothing critical in the design, just beef it up if you increase the current demand. The 3 amp transformer should handle 3 channels.

As we may want to add more than one charging channel in order to charge more than one battery pack at the same time, the transformer should be capable of supplying enough current, i.e. 3.0 amps or more. The output of the transformer will be 13 or 14 volts a.c. This alternating current must be changed to direct current and we use a diode bridge to do this. Use 4 ea. 1N4001 diodes, or a component bridge (RS 276-1101, 276-1146).

A 7805 voltage regular chip controls the output voltage. It normally regulates to 5 volts DC. But using the adjustable pot we change it into a variable supply that varies from 5 volts to about 14 volts. This way we can charge different voltages and currents as our batteries require. To increase the current regulation and provide an indication that charging current is flowing, we place a lamp in series with the load current (276-1770).

An incandescent bulb's filament has a property that, as the heat increases and decreases, the resistance changes also, and tries to keep the current constant. If we should set up a charge current of 50 milliamps the lamp will keep it constant as the battery voltage charges up over the charge time (14 to 16 hours). In selecting a bulb it is best to choose a bulb that has a low voltage (2V) and a current rating as high as the greatest current you wish to charge. I have chosen a #222 rated at 2V and 250 ma. When it is burning brightly that indicates that about 250 ma is flowing to the batteries. More on caliberation later.

As shown, hook up the positive lead (+) to the red lead on your battery pack. (If you are not sure which lead is positive, read it with a meter, or get help to insure which lead is which.) Then hook up the negative lead to the black battery lead (-). When it is turned on the bulb should glow dim to bright depending on the pot setting. I recommend that any R/Cer that deals with batteries should have a good digital readout multimeter. Radio Shack has a good one for about $49.95 (22-189). This will read your battery voltage to within 1/100th of a volt, and you need this accuracy. Any of the "Expanded Scale Meters" advertized in the R/C magazines are also great.

If, for example, you are charging a 4-cell, 4.8V battery pack with a capacity of 500 ma, the charging current would be CAPACITY/10 or 50 ma of charging current. By hooking up your current meter in series with the lamp circuit, adjust the pot for a reading of 50 ma. Now notice how much of a glow is on the lamp so that you have a reference. The charger should now charge that pack at 50 ma, until you make some change in the pot setting. The same for other batteries, just adjust the pot for the correct setting for that particular pack and observe the lamp glow. If the next time you plug it in and you get the same glow you know it's still set up right. The best thing is that a quick glance, even in a dark room, will assure you that the batteries are indeed charging.

Higher voltages can be obtained with a higher voltage transformer, and possibly using a 7812 (12V) regular chip. If you plan to charge at more than 1 amp of current you will have to provide diodes or bridges that will handle the higher current. I'd be happy to advise anyone that has any particularly difficult charging problem. Call me if you have any questions. I'm including below a sketch showing how to hook it up. I like to use perf boards and perf pins from R/S to build the circuit on. It makes it much neater and last longer (270-1392, 276-1394).

If you wish to add other channels, boost the diode capacity to 3 or 4 amps (276-1141 or 276-1180) then just top off the bridge and reproduce the circuitry for the additional channels. Each would have its own regulator, pot circuit and lamp. This would work very well with a redundant (two pack) 500 ma airborne battery system.

   Transformer, 12.6 vac c.t., 3 amp ............ 273-1511
   Diodes (4), 1N4001 ........................... 273-1101 
   or Diode Bridge (1) ................(276-1151) 276-1146
   Voltage Regulator IC 7805, 5 Volt .............276-1770
   Resistor, 4.7k ohm, 1/2 watt (Yel-Viol-Red)....271-030
   Potentiometer (Pot) 5k ohms..........(271-217) 271-1714
   Lamp, 2v, 250ma, #222..........................272-1124
   Lamp Holder....................................276-1394
   Perfboard 4 1/2 X 6 ...........................276-1394
   Perfpins.......................................270-1392