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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?

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    \$\begingroup\$ By stopping for a moment. \$\endgroup\$ – Ignacio Vazquez-Abrams Dec 20 '14 at 16:14
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    \$\begingroup\$ By measuring the current also, to see if it's still drawing current \$\endgroup\$ – PkP Dec 20 '14 at 17:11
  • \$\begingroup\$ Smart battery chargers keep track of time and the current flowing into the battery. Some dumb battery chargers simply charge to voltage set point and don't care about anything else. These usually are not the fastest to recharge a battery, but they are easy to design, and if it is not important to recharge the battery quickly, they work fine. \$\endgroup\$ – mkeith Dec 20 '14 at 17:37
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schematic

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.

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For cars, an option is to use a Hall sensor around the lead going into the battery. The magnetic field depends on the current flowing: Magnetic Field of Current

However it may not be easy or accurate: on AllegroMicro I found linear Hall sensors with up to 10 mV/G. A current of 1 A at 10 mm distance from axis of the wire (reasonable for car battery leads) gives you 0.2 G, resulting in 1 mV signal.

Basically the most sensitive Hall sensor from Allegro Micro gives you 1 A resolution.

If you use specifically designed current sensors you will get about 20 mV/A, resulting in about 0.05 A minimum sensitivity (I assumed ADC with 10 bit and 1 V reference voltage).

The initial spike during engine startup may still be difficult to measure with good time resolution.

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