CAR BATTERY TESTER

Checking the status of your car battery (accumulator) should be easier with this circuit which measures the internal resistance of the battery. Pulses generated by the 555 are used to drive a dummy load and the AC voltage which develops across the battery gives an indication of its internal resistance: the lower the voltage the healthier the battery. The AC voltage is read out by means of a digital meter connected at the output. Separate leads are used for the dummy load and for the metering circuit. They should be connected to their respective battery lugs but they should not touch each other. This avoids erroneous readings due to less than perfect contacts of the dummy load. The internal resistance depends on the battery temperature as well; this is the reason for the switch: hot means a battery (not ambient) temperature between 35 and 52 degrees Centigrade, normal is for a temperature between 16 and 34 degrees and cold is good for a temperature from -4 to 15. Beyond these ranges the reading is unreliable. The internal resistance depends also on the rated capacity of the battery. The 100 ohm potentiometer sets the battery capacity: it is rotated totally to positive for a 100Ah battery and totally to negative for a 32Ah battery. A dial with uniform markings from 32 to 100 was used in the prototype. This means we can measure internal resistance of batteries rated from 32 to 100Ah. As there are a number of smaller 12V batteries around, specially for alarm systems, a switch was introduced that, in the X1 position, will change the capacity range to 3.2 - 10Ah. The unit has six leads going out of the box: two for the dummy load, two for the metering section and two going to the digital meter. Operation is simple: set the range, temperature and battery rating, then connect the dummy load and the metering leads to the battery lugs and read the ac voltage: you should be safe if it reads below 10-12mV otherwise it is better to give the battery a good recharge and if it is still beyond 10-12mV then probably you need a new battery. A bright orange LED shows that the unit is connected and in operation.

From :: http://digilander.libero.it/bubblegate/eindex.html