A coulomb counter can measure charge that flows into or out of a battery, and small, dedicated ICs can directly interface with low-to-medium battery voltages of up to about 20 V. By using a high-voltage amplifier as a level shifter, you can extend the input operating range to much higher voltages of the measurement circuit. The LT6375 voltage-difference amplifier includes some features that make this circuit work accurately up to surprisingly wide voltages...
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