While most devices have an optimum power requirement in order to fast-charge, they will continue to charge gradually slower as the available power diminishes. Additionally, many devices require more power when they are fully drained compared to when they are partially charged.
For example, an iPad would like 2.1 amps for optimum fast-charge but will continue to charge almost just as fast at 2 amps and then a little slower at 1.5 amps, etc. Typically, the lower threshold where devices will stop charging would be a 0.5amp or 1amp.
The following scenarios explain what happens when drained (having little to no battery life left) iPads were plugged in:
- One iPad will get 2.1 amps
- Two iPads will each get 2 amps
- Three iPads will each get 1.3 amps
- Four iPads will each get 1 amp