Regulating output current
Transistors Q3 and Q4 with resistors R5 and R6 form a regulated constant current source which limits the maximum DC current that can be delivered to battery pack to: Vbe / R5 = 100 mA. If larger current tries to flow to batteries through Q3 emitter and R5, Q4 will start conducting more strongly which will drag a portion of Q3 base current from it. Even if charger output contats get shortcircuited, no current higher than 0,1A will flow. This is a reliable current regulating mechanism well worth remembering.
Additional components on the left to Q3/Q4 are used to stop the charging current completely if the pack is connected with the wrong contact polarity to the charger. In that case, battery pack voltage lights up an LED inside the 4N25 optocupler enabling its internal transistor to turn on which completely takes on Q3 base polarising current stopping it dead in tracks. Therefore, no charging current flows if battery pack is connected to the charger in reverse. This saves not only the pack but also Q3 as it will otherwise be easily fried up by the internal power dissipation which can be approximately calculated as: (33V + 20V) * 0.1A = 5W. Along with optocoupler internal IR LED, a visible red "Error" LED lights up which gives the user a clear indication of the irregular circuit operation. Diodes D6, D7 and D8 protect the circuit if battery pack is connected to the charger currently not being connected to the power supply.
Protective mechanisms described above ensure that nothing unexpected can happen to either the battery pack or the charger. But if everything is right and 0,1A charging current flows through the pack, voltage drop on R7 polarises Q5 which keeps the blue "Charging" LED turned on.
Stopping the charger
There are several possible methods for regulating the amount of charge delivered to NiCd and NiMH batteries. The simplest one is to keep the charging current below 10% of a particular battery rated cappacity as these batteries are designed so that they can tollerate unlimited overcharging of that sort. This is routinely done in production by intentional making the positive battery electrode some 10% larger in surface area than the negative one which enables regenerative chemical reactions to kick in and trap the produced gasses that would otherwise blow up the battery. Charging current of 0.1A that the charger described here deliveres is well below the safe limit for modern day cordless tools battery packs so no real danger of overcharging them exists.
Unfortunately, batteries can be fataly damaged by prolonged overcharging even if they do not vent or explode catastrophically. The problem is caused by the formation of sharp crystals inside the electrolyte which grow under overcharge conditions. These are very conductive and will short the battery out by penetrating the electrolyte completely if charging current is not limited to at most 1/100 of the battery rated cappacity after a reasonable amount of time. This is the task for the rest of the electronics in this circuit.
The method of choice for sensing the state of charge in non-critical applications like this is monitoring the battery pack voltage. In fast chargers it would not give good enough results, but it is absolutely ok to be used here. The most important component for monitoring battery voltage is inexpensive but very precise voltage regulator TL431. It continually measures the pack voltage via R18 so that if the pack voltage exceeds approximately 21V, it starts drawing a certain amount of current from the positive power supply through the "K" electrode. This in turn activates Q6 which turns on the green "Stop" LED and both Q7 and Q2. Transistor Q7 is used only to stop Q5 from conducting so that "Charging" LED stops glowing. Simultaneously, Q2 stops the DC-DC converter by blocking Q1 base current which cuts off the battery charging current. A benefit of voltage monitoring charging method is that "Stop" LED will be lit if the pack is not connected to the charger as in that case voltage on charger output contacts surely rises above the precalculated threshold.
Voltage regulator TL431 compares voltage at its "Reg" input pin to a highly stable internal voltage reference of 2.495V which makes calculating R18 easy - voltage divider R14/R18 should provide this voltage at "Reg" pin when battery pack voltage reaches the optimal value at full charge. R18 value marked on the circuit diagram was calculated so that battery pack of 19.2V nominal voltage gets a near full charge before automatics cuts off the charging current. A charger intended for conditioning 14.4V packs should use 36 kΩ resistor for R18 instead.
A certain amount of hystheresis in TL431 comparator operation is provided by adding R13 and D9 into the mix. This leads to sharp cutting off of the charging current and sharp and clear turning on and off of "Charging" and "Stop" LEDs. Some time after cutting off the charging current, battery voltage starts dropping down which temporarily turns TL431 off and consequently turns the DC-DC converter on. LEDs follow this too by inverting their operation. Soon after, battery voltage rises again turning the TL431 on and DC-DC converter off. LEDs follow. The period of this oscillatory process varies from a fraction of a second after the initial charging cutoff to about ten seconds after an hour or so. The LED light show that results makes the state of charge indication rather unusual and amusing.