Determining whether a 12V car battery needs charge is as easy as touching its leads to those of a multimeter. While charging from a vehicle's alternator, the multimeter method fails to determine the battery's state of charge and instead reports the alternator's 14V until the engine is turned off. Only then can we read the battery's voltage again. However, even gauges on older car battery chargers reflect rising voltage over time.
How can I measure the voltage of a charging battery, and how do battery chargers do it?
Best Answer
simulate this circuit – Schematic created using CircuitLab Here is a simple example. From zero crossing of the ac waveform, the car battery attached at right can cause the zener diode to conduct. This removes the bias from r3 that would normally turn on the scr. So when the battery is not charged to the level preset by the pot, the scr will tend to fire and charge the battery. More sophisticated chargers than this will still use the same basic idea of driving a pulsed DC into the battery and doing measurements at or near zero crossing. Several chipsets have been developed to simplify making a battery charger that keeps track of the energy stored in the battery during charging. TI and Linear technologies both have lots of literature to describe this further.