Wow, this is a long running thread I noticed. Well, sisters, in our RC world, we had to learn a lot about Nicad and NIMH charging. Parejo sila actually, magaan lang and MH for the same capacity. Pero mas malakas and current delivery rate ng Nicad. Anyway, NOT ALL can be fast charged altho it is abused in most cases. The safest charging ratge is called the C10, 10 for 10% of capacity. So, for a 500 mah nicad, 50 ma ang C10 for 15 hours assuming galing sa 1.1V and nicad. The standard charger "for dummies" from Futaba is a 45-50 ma wall charger. The 4 cell pack is charged in series as well as the 8 cell pack in the transmitter.We have duplicated this charger many times when someone needed a charger.
While constant current is suggested in many mags, the tapering current worked well for us for many years which means no voltage regulator is used. Instead it is a simple traffo cum diode charger. If you will put an ammeter in series, you will note that the current tapers as the battery gets charged. That is because as the battery gets charged, its internal resistance drops and it therefore draws less current. The rule of thumb is to put 50% more charge on an exhausted battery. At 10%, it would take 10 hours to put back 100 % of the spent energy. But the rule is to let it charge 50% more, thus 15 hours. Actually, this is not technically accurate since the charging rate actually tapers and is not constant. so the current put back is not really 150%.
For the Futaba charger, it is interesting that the traffo is a plain 200ma traffo with 19V on its secondary with a center tap. Let's call the terminals 1,2, and 3. 1 and 2 is used to charge the 4 cells with a diode, and LED, and resistor in series. A resistor is paralled to the LED so the LED does not burn up. Forget the voltage, calibrate the charger as to the current running to the battery. Adjust the resistor to get the 50 ma. Choose the parallel resistor to an LED to get the right brightness. Do the same for the 8 cells.
If you want to build a charger for higher capacity batteries, get a bigger traffo. The current should remain at C10. But you have to adjust the parallel resistor across the LED. It should drop a voltage of 1.5 only, the usual LED voltage.
THIS CHARGER IS "FOR DUMMIES" CUZ YOU CAN LEAVE IT HOOKED UP FOR WEEKS WITHOUT HARMING YOUR CELLS.
HOW TO DISCHARGE THE BATTERIES
If you had gone to a race track in the old days, you would see battery packs being discharged with auto light bulbs. The RC car packs had 6 cells so the the max V was 1.4v x 6 or 8.4V. 12V auto bulbs can be used. to increase the discharge rate, just parallel more bulbs. BUT THE VOLTAGE WAS BEING WATCHED. AS SOON AS IT HITS 1.1 V PER CELL OR 6.6V , THE CHARGER IS REMOVED. At 1.1V, the voltage drop in nicads accelerates.
There must be a way to stop the discharge by itself without having to plunge P2k to P3k on a TLP charger(I have one). I have seen one style used in AA nicads. A diode of sufficient current rating shorts out the cell. Usually it is done on spent battery so the current is now weak. The diode will not let the V go below .7V. so, the cell is not fully zapped. Then it is charged. Well, how do we do this on a 4 cell pack. Threshold V is 4.4V, so we divide that by .7 and get 6 diodes in series. 6 x .7 is 4.2 volts. So the battery will be discharged to 4.2V before charging. if you want to discharge high current, used big diodes but better to series an auto bulb to limit the discharge rate. When it is out, the discharge is complete.