The trick is switching the current being measured and the sense point separately. It's a 15A DPDT switch that bypasses the additional shunt in one position and leaves it in circuit in the other. The other side of the switch changes whether the sense amplifier senses the original shunt or the added shunt. That prevents it from sensing the voltage drop across the switch and added wiring, which can easily be more than the value of the original shunt.
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To my surprise, shortly after Naomi Wu gave me a bit of fame for making good use of solar power, Allie Moore got really jealous of her...
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