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Voltage Calibration
It is common for chargers
to not accurately measure the battery’s voltage, and to see voltage variations in
battery cables between the battery and balance of system. With most other battery chemistries a
discrepancy of a couple tenths of a volt is not an issue; however this can be an issue with your Lithium
Iron battery. Voltage discrepancies between the battery and charger(s) can result in the battery not being
properly charged or potentially overcharged resulting in a FAULT.
Iron Edison recommends measuring the battery’s voltage at the battery’s terminals using a high quality
voltmeter, and comparing this measurement with your charge
r’s battery voltage measurement. Using a
high quality voltmeter with fresh batteries is critical for this process.
Any discrepancy found between the battery’s true voltage and your charger’s voltage measurement
needs to be factored into your
charger’s programming. Some chargers have a voltage calibration feature
to address this issue; otherwise you will need to adjust the charge voltage set points to compensate for
any discrepancy found.
This calibration should be done during initial commissioning while the battery is at rest, and again while
charging at your system
’s highest charge rate to fine-tune the calibration.
Example:
If your charger measures the battery voltage 0.5 VDC higher than the battery’s true voltage,
the battery will never reach its required bulk / absorb voltage. Therefore the bulk / absorb voltage setting
on your charger should be increased 0.5 VDC to compensate for this discrepancy, so the battery will
reach the required bulk / absorb voltage. If the charger measures the battery voltage 0.5 VDC lower than
the battery’s true voltage, the battery could be overcharged and possibly cause the battery to shutdown
with a FAULT. Decreasing your charger’s bulk / absorb setting by 0.5 VDC in this case will prevent this.
Voltage vs. State of Charge
With most battery chemistries the battery’s state of charge can be determined by the battery’s resting
voltage. Because lithium batteries have such a narrow voltage window and a relatively flat voltage / SoC
curve, determining the state of charge by resting voltage can be imprecise. This combined with voltage
variations while the battery is under load or chargi
ng can make determining the battery’s state of charge
very difficult.
If your system has triggers based on the battery’s state of charge, it is recommended to use a current
sensing battery monitor that calculates
the battery’s state of charge based on current flow rather than
battery voltage. When configured correctly, this will provide a more accurate representation of the
battery’s state of charge and better overall performance of your system.
Below are values needed by current sensing battery monitors to accurately calculate state of charge:
Charge Efficiency
99%
Peukert Coefficient
1.02
