Some applications require a reduction in the charging voltage, one reason can be to increase the cycle life of the battery.
Reducing the charging voltage will often cause some discrepancies with the State of Health (SoH) calculation and completing the learning cycle.
To successfully complete a learning cycle there must be a 90% change in Depth of Discharge (DoD), this is based on the range of the chemistry ID. The learning cycle must always be completed with the full range of the battery specification, the same range that is used in the GPCCHEM chemistry ID matching process.
After the learning cycle is completed the discharge termination and charge termination voltage can be adjusted. After the adjustment another full charge and discharge cycle should be performed. The reason for the second cycle is to update the DoD at End of Charge (EoC) value. The DoD at EoC is what the gauge uses to calculate the starting point for State of Charge (SoC) calculations during discharge, since it would not be correct to have the gauge report 80% SoC at “full charge” with a de-rated charging voltage.
The SoH is calculated using the design capacity and an ideally calculated Full Charge Capacity (FCC) value that is calculated using a present temperature and load. The equation is SoH = SoH_FCC/Design_Capacity. Because the gauge uses a preset design capacity, this can cause some initial errors in the SOH calculation because each cell does not have the same full charge value when first deployed. The new BQ40Z50-R4 implements a new SoH feature that learns the max FCC for the specific cell, giving more accurate SoH results for the lifetime of each pack.
If an older firmware version is being used it is common for the host to adjust the reported initial SoH to 100%, since when the charge voltage is de-rated the SoH_FCC will also reduce, which will cause the SoH reported initially to sometimes be below 100%.