This circuit is used to measure the voltage on a 12V (nominal) lead acid rechargeable battery system. It was specifically designed for use in solar powered systems, but is general enough that it can be used for automotive or other 12V systems. Lead acid batteries normally spend their working lifetime in the voltage range of 11-15 Volts. This meter circuit was designed to show the voltage range of 10-15V on an analog meter movement, it can be used to show the battery charge state from empty to full.
The input voltage is filtered from transient voltage spikes via the 10 ohm resistor and the 0.1uF capacitor on the input of the 78L08 regulator. The 1 Amp series fuse and Transzorb protect the circuitry from short circuits and over-voltage conditions.
The 78L08 voltage regulator provides a constant 8V DC supply to the meter circuitry through the 10-15V input voltage range. The dual op-amp is supplied with 0V and 8V for the power rail inputs. The lower op-amp is set up as a buffer stage to provide a 4V virtual ground reference voltage. The virtual ground is used as a current sink for the meter, and as a reference point for the other op-amp.
A 78L08 regulator may be a difficult part to find, a good replacement would be an LM317L adjustable regulator circuit set to 8V. Other dual op-amps should work well in this circuit, the 1458 is a good substitution, an ultra-low power dual op-amp can be used if power drain is a consideration.
The upper op-amp is configured as a voltage summing circuit with adjustable gain. Gain is controlled by the 100K feedback resistor. The gain control adjusts the meter sensitivity across a voltage range. The measured voltage comes into the upper op-amp’s + input via a 100K/47K voltage divider. The offset control creates a second + input voltage that is summed with the measured voltage through the 68K resistor. Adjustment of this control moves the meter position up and down, it is used to set the meter movement at 0 when the minimum measured voltage (10V) is applied to the circuit. The meter shown has a sensitive 50uA full scale reading, more common 1ma meters can be used by reducing the value of the 47K meter series resistor to 2.5K.
Connect the circuit across a 12V battery and observe the voltages. The meter consumes a small enough amount of current that it can be left connected all of the time for all but the smallest of batteries.
source : www.solorb.com