A Summer Circuits edition on ‘all things outdoors’ — good, but what of all the battery- powered circuits that remains indoors? Once the fine weather starts, the family car tends to remain increasingly in the garage – which is as beneficial to the owner, his/her bank account and the air that we breathe as it helps — to an extent to — reduce CO2 emissions. However, when we come to want to use the trusty vehicle again, it often happens that the battery shows serious, deeply worrying signs of being flat, sometimes to the point of preventing the engine from starting altogether. Pushstarting is no longer recommended or even possible with modern cars, so a topped up battery is highly appreciated. The solution of leaving an off-the-shelf charger permanently connected is not generally satisfactory, unless you’re lucky enough to have an ‘electronic’ one. The majority of dirt cheap ordinary chargers don’t include any regulation circuitry and so will over-charge a vehicle battery if you’re unwise enough to leave it permanently connected. So our proposed project is to build a charger that can act as both a standard charger, and a float charger that you can leave permanently connected without the slightest risk to your battery or fear of over-charging. What’s more, it doesn’t use any ‘exotic’ components and is ridiculously cheap.
Let’s have a look at the circuit diagram. The voltage supplied by our charger’s transformer is rectified by diodes D1 and D2 but is not smoothed. Strange as it may seem, this is vital for it to work properly, because as a result the rectified voltage consists of a succession of sinewave half-cycles, and hence falls to zero 100 times per second. When thyristor THY2 is conducting, the battery is effectively charged, the charging current being limited only by resistor R6, which has to be calculated as shown below. This thyristor is triggered via resistor R4 for each half-cycle of the mains, except when thyristor THY1 is itself triggered. In that event, THY2 turns off the first time its supply voltage drops to zero, and no further current can reach the battery. The voltage at the battery terminals is sampled by R5 and smoothed by C1 before turning on THY1 or not via P1 and D3. As long as this voltage is lower than a certain threshold, determined by the setting of P1 and corresponding to a battery that is not yet fully charged, THY1 is not triggered and so leaves THY2 conducting for all the mains half-cycles.
When the voltage at the battery terminals is high enough, THY1 is triggered and thus prevents THY2 from being triggered. This phenomenon is not in fact quite as clear-cutas we have just described, but takes place very progressively, so as it approaches full charge, the battery’s average charging current gradually reduces automatically, eventually stopping completely once the fullycharged voltage has been reached. LED1 acts as a charging indicator, while LED2 lights more when THY1 is being triggered frequently, thereby acting as a fully charged indicator. Three components of the circuit proposed here need to be selected according to the characteristics you want your charger to have; these are R6, THY2, and TR1. R6 needs to be calculated according to the maximum charging current you want, from: R6 = 16 / I where I is the current expressed in amps. Warning! Given the value of the other elements in the circuit (D1, D2, TR1, and the fuse), do not exceed 5 A.
The power dissipated in R6 can be calculated from PR6 =36 / R6 with P expressed in watts and R6 in ohms, of course. Thyristor THY2 should be a 100-V type (or more) rated at 1½ to 2 times the desired maximum charging current. And lastly the transformer, which should have a power in VA given by: P = 18 × 1.2 × I where I is the maximum desired charging current, expressed in amps.
The only adjustment to be made concerns pot P1 and will require access to a well charged battery. Connect it to the charger output and replace the 5-A fuse with an ammeter – preferably an old analogue type, better able to respond to average currents than certain modern digital types. Then adjust potentiometer P1 to obtain a current of around 100 mA. Later on, when you have the opportunity to charge a very flat battery, you will be able to fine-tune this adjustment by tweaking P1 to obtain a charging current close to the maximum you have set by means of R6. You’ll need to find a compromise setting between the float charging current, which mustn’t exceed around 100 mA, and this maximum current. Whatever the accuracy of your adjustment, you can be reassured that your battery will be treated better by this project than by many of its non-electronic counterparts to be found in the shops.