Honestly, I think only an EE could answer the question and they'd need knowledge of battery chemistry and design, charger design, solar array output voltage, regulation circuitry, etc.

IIUC a properly designed system can tolerate the inherent variability of a solar array, and generally speaking Li-ion cells don't have memory issues that plague some other battery formulas (causing diminishing capacity due to partial charge/discharge cycles) but ideally the Li-ion cells should also be protected and should be connected to smart circuitry that regulates voltage and prevents overcharging.

Cheap solar chargers won't have any of that.

Cheers,