This fine Summer’s day, you’ve decided to go out for a breath of fresh air – but without wanting to give up your ‘hi-tech toys’ — whether it’s your son’s radio-controlled car, your daughter’s MP3 player (all borrowed of course after due negotations), or your own favourite portable DVD player. All these appliances share the feature of usually operating on rechargeable batteries – which is of course no problem when the mains is at hand, as they all come with their own chargers. But the problem gets a bit more complicated out in the country, and as Murphy’s Law has it, that’s always just the moment you find your batteries are flat or nearly so. If your car is parked nearby, we can suggest a solution in the form of this very simple project – and what’s more, it will cost you practically nothing, since it uses mostly components that any good electronics enthusiast is likely to already have in a drawer or the junk box. Even if you did have to buy everything, the whole thing shouldn’t cost more than about £ 10. As Figure 1 shows, it’s a project that smacks of good old-fashioned – we almost said granddad’s – electronics, as it doesn’t use a microcontroller, nor even the slightest specialized integrated circuit (IC).
In spite of this, it will look after your batteries, especially if you are reasonable about the charging time. Whether they are old nicads (NiCd) – becoming extinct these days because of their many shortcomings and toxicity – or the omnipresent nickel-metal-hydrides (NiMH), these types of battery have to be charged at constant current. This charging current should be 10% of their rated capacity (printed on the label) for a normal or slow charge, or a maximum of 100% of their capacity, if you want a fast charge. So, to recharge the NiMH or NiCd batteries in our various portable devices from a car battery – for that’s what it’s all about – all you have to do is build a constant current generator.
To achieve this takes just two common or garden transistors, T2 and T3. The latter is turned on to a greater or lesser extent by way of R3 and T2. By virtue of the very principle of transistors, there cannot be more than about 0.6 V between the base and emitter of T2. If this voltage tends to drop, T2 will tend to turn off, which will then increase the conduction of T3 – and vice-versa. In other words, the base-emitter voltage of T2 will virtually always remain at 0.6 V. Now this voltage is produced by the current passing through one of the resistors R4–R7, and hence also the battery to be charged. So it’s easy to see that this current is quite simply given by Ich = 0.6 / R where Ich is the desired charging current and R is one of the resistors R4–R7.
As T2 turns on (and hence the battery is charging), transistor T1 increasingly saturates. If this current drops too much, or falls to zero in the event of a poor contact or faulty battery, the LED goes out to indicate a problem. Diode D1 protects the circuit from possible reversed polarity of the battery being charged. We have designed a small PCB for this project with provision for direct mounting of a rotary switch to be mounted, thereby reducing the wiring needed to nothing. This switch is Lorlin part no. PT6422/BMH and is available, for example, from Farnell under product ref. 1123675. However any other equivalent may be used if it can be adapted to the circuit board. In most case, that means installing some extra wires between the PCB and the switch pole and contacts. Transistor T2 may be required to dissipate quite a lot of heat for low-voltage batteries being charged at high currents, so space has been provided to fit it with a U-shaped heatsink.
The various designed charging currents are 400, 130, 60, and 10 mA for positions 1–4 of the switch. The unavoidable voltage drop across T2 means the maximum voltage of the battery to be recharged cannot exceed 9.6 V. If you want different charging currents from those designed, all you need do is simply replace one or other of R4–R7 by a resistor whose value has been calculated as above (R = 0.6 / Ich) and whose power is given by P = 0.36 / R. As a constant-current generator, the circuit is naturally protected against short-circuits, but do take care all the same if you increase the charging current too much not to exceed the maximum power dissipation in T3 (65 W) and more importantly, the power allowed by the small heatsink provided for on the PCB. A current of 500 mA seems to us a reasonable maximum value to not exceed. The value should cater for most NIMH and NiCd batteries if a few hours are allowed to charge them. But then it was a sunny day so that shouldn’t be a serious concern.