Hi...how can I test the battery....I have one which charged after about 2.5 hours...2nd battery is still cell balancing after 6 hours..(red/green) flashing.. using the genuine yuneec charger
I recommend a meter that connects to the balancing lead that comes with your charger. You can read the voltage of each cell. You can also determine the internal resistance of the cells which is the best indicator of cell health.
This is one such meter:
When I have a battery doing a very long balance charge I remove it, unplug the charger, wait several minutes and then resume charging. If it continues the balance charge for more than another hour, the battery may be defective.