Battery Rating vs Capacity Monitoring
Some things: Shipping batteries in the US requires the Watt-Hour rating to be marked on the battery, and the rule is to use the "nominal" voltage. So LFP uses their 12.8v figure for that 100AH x ...
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The shunt itself is just a precise, very low-value resistor. Its function is to convert amps to millivolts for the monitor to sense (because, at least traditionally, it's a whole lot easier to measure small voltages directly than to measure DC current more directly). The voltage across the shunt is a side-effect of how the shunt works, and is not dependent on the battery voltage.
The battery monitor is where all the interesting stuff is happening. It's constantly sampling both the battery voltage and the current. To determine instantaneous wattage (power), the little computer in it multiplies the current and voltage. To determine watt-hours, it integrates that wattage over time, which is a bit more mathematically precise way of saying it accumulates the individual samples. To measure charge, it integrates just the current over time. (Integrating just voltage over time doesn't lead to any particularly useful quantity; a volt-hour is not something you're likely to run across.)
The battery monitor is where all the interesting stuff is happening. It's constantly sampling both the battery voltage and the current. To determine instantaneous wattage (power), the little computer in it multiplies the current and voltage. To determine watt-hours, it integrates that wattage over time, which is a bit more mathematically precise way of saying it accumulates the individual samples. To measure charge, it integrates just the current over time. (Integrating just voltage over time doesn't lead to any particularly useful quantity; a volt-hour is not something you're likely to run across.)
