My application features a Li-Ion battery (NCR18650B) which I'm charging with the standard CC/CV method. Various constraints force me to use a generic buck converter (with current limiter) for the charging, instead of a more specialized IC. I have temperature monitoring and MCU control over the charging process.
I'm wondering what's the recommended approach to calculate the feedback resistors that set the buck converter's output voltage. This voltage should canonicaly be 4.20V ±0.05V, but I'm using 1% resistors, and another 1% variance comes from the feedback reference; in total, about 3% of uncertainity. For example, I can select resistors that give me 4.20V typically, but span a range 4.08..4.32 with all tolerances considered ("the simplistic approach"). I can instead design conservatively and select a divider that sets the voltage to 4.10V typically, with [3.98..4.22] variance, which is well within spec ("the conservative approach").
In the current PCB revision I've designed with the conservative approach and in the lab my devices consistently charge to about 4.05V OCV (I believe because the Vref is actually not uniformly distributed in the specified range, but skewed towards lower values). The bigger problem is in the actual usage scenarios, where the energy source is not a reliable wall-wart, but an intermittent source whose availability pattern is unpredictable. As it is "free to use", I want to optimize time-to-charge. Charging with voltage (typically) limited to 4.05 means the charging gets into the CV mode too soon, and the battery could rarely ever exceed 70% SoC.
If the cells feature protection circuitry, which is the recommended, industry-standard way to select the CV charging voltage? The simplistic one (target 4.20 volts, don't care about tolerances) or the conservative (consider worst-possible tolerances, so your charging voltage could never exceed 4.20V)?