The voltage of your 18650 battery dropped by 0.02V (from 4.3V to 4.28V) about two days after it was fully charged. This voltage drop is normal, because even a healthy battery will have a slight voltage drop.
However, based on this information alone, we can't determine the health of the battery. The following are some factors that may affect battery health:
1. Battery frequency: If the battery is frequently discharged and charged with large current, the battery life may be shortened.
2. Storage temperature: If the battery is exposed to high temperature for a long time, its life will also be affected.
3. Battery charging method: If the battery is often undercharged or overcharged, its life will also be shortened.
4. Battery manufacturing batch and brand: Different manufacturing batches and brands may affect the performance and quality of batteries.
If you want to assess the health of the battery, please consider the following suggestions:
1. Observe the appearance of the battery: check whether the battery is swollen, deformed or damaged.
2. Check the capacity of the battery: compare the new battery with known capacity and test whether the capacity of the old and new batteries is the same.
3. Charge according to the correct charging method: avoid overcharging and undercharging, and avoid using inferior chargers and battery protection plates.
4. If the performance and quality of the battery are found to be poor, we can consider replacing it with a new battery to ensure the safe and stable operation of the equipment.
Generally speaking, if your 18650 battery still has obvious voltage drop or other problems under the correct use and storage conditions, it is best to replace it with a new battery in time.