How can I determine LI-ion battery charge while varying the load?
Every battery is built to drop the voltage when you draw a larger current, this is why automotive batteries have a nominal voltage of 12V under high load current and normal voltage of 14.4V.

In order to check the battery voltage you have to stop the load and charging circuits, wait until the battery voltage settles and then measure it.

Measuring by current is not good, your load can drain different amounts of current, which will give you false results. You need to measure the charging current. The battery will drain a different charging current, depending on its state of discharge, but you have to disconnect the load first.