Measuring the charge of a battery while charging
simulate this circuit &ndash; 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.