One thing is the Phantom Lipo and their Charger is not like a typical Lipo that we use in our RC Cars/Planes/Trucks and so on. As there is software and safety's put in place on the DJI so it is much harder to hurt them. The 57w (1c Charger) and 100w (almost a 2c charger) are both smart chargers that also balance the cells as charging. You can not over discharge the battery while flying as there is software to protect it against that. It also monitors the internal temp of the battery while in use and while charging.
Best advice I could give based on my experience with RC lipos. Run the battery as low as the phantom will allow you or at least to 10% of that before charging. Which it should not discharge below 3.5v per cell. Never store a Lipo for periods longer than a few weeks with 100% power or very low on power. The voltage that you want the lipo at is 3.85v per cell for long term storage (2 weeks or more). Be cautious if there is a large difference in voltage between cells. If you see a difference of .2 or more I would be very concerned about the condition of that battery. Honestly I would be concerned at .05 difference. Never Charge a Hot battery.
Then take DJI Adivce on this,
- Charge and discharge the battery completely once every 20 charge/discharge cycles. Discharge the battery until there is less than 8% power or until it can no longer be turned on, then recharge it to the maximum capacity. This power cycling procedure will optimize the battery life.