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Q1) my charge current varies by 20% over the complete charge and 25% from unit to unit. Is this an issue?
A1) The constant current charge is controlled by the SNS signal which is filtered quite a bit and is a function of input and output voltage along with the inductor and capacitor values and parasitics. This is very difficult to calculate and it basically is fine tuned imperically by making adjustments with the sense filter of current sense resistor. This seems very uncertain but it is a battery charger. The key point to remember is that charging higher than C/3 (one third the battery capacity) will allow a good detection signal. Yes the DCA amplitude does change as the battery increases in voltage, but it is slow compared to the change in battery voltage so this would never cause a -dV change at the battery. Once the pack nears full capacity the battery voltage flattens out and then the output constant current does not change in this steady state condition and one gets proper termination. The charge current and time may vary by 20 to 30% but most people don't know how discharged the pack is each time so this should not be a big deal.
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