I've now gone through six charge/discharge cycles of my KX2's internal battery using the new KXUSBC2 to charge it and I've used different sources and USB-C cables for each charge.

Regardless of the charging voltage used, the KX2 battery gets a complete charge, whether the charging source was 5V or 15V.

Contrast that with different source voltages when used with Elecraft's KXIBC2 internal charger:

*MC = motorcycle battery

Some notes from the KXUSBC2 designer, Manuel HB9DQM:

• The KXUSBC2 charges the individual cells within the battery in series like the KXIBC2, and it's important that the battery pack has a protection circuit / BMS that ensures that the maximum voltage per cell is not exceeded if they are unbalanced. This is the case for the pack supplied by Elecraft, and also for any decent aftermarket pack.
• The reason why the KXUSBC2 can fully charge the pack regardless of input voltage is that it contains a fully fledged buck-boost DC-DC converter that can both step up and step down voltage, whereas the KXIBC2 is a simple linear regulator that can never produce an output voltage higher than the input.
• For the KXUSBC2, input voltage is not directly related to charging current. If the input voltage is lower, the KXUSBC2 will attempt to draw more current to reach the same output current into the battery. Of course there is a limit to the input current (3 A by default, or in case of a PD supply, whatever the supply advertises), and the charger IC has some smarts to prevent overloading the supply. So the charging current can to a certain extent be controlled by lowering the input voltage, but that is a rather crude way, and I would strongly recommend against using a separate USB-C trigger cable to supply the KXUSBC2 via the DC jack, instead of connecting the USB-C supply directly to the KXUSBC2 and letting it negotiate the voltage as intended.
• Instead, if you would like to charge your battery at less than the default of 2 A, you can change this setting using the button/LED based config menu (0.5, 1, 2 and 3 A options available in the menu, or any desired mA value using a UPDI programmer). See the user guide for details.

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