Given this is the same technology as used by computers I might be able to contribute this.
Modern laptops charge very quickly to 80% and then slowly for the last 20% as this is where it places stress on the battery wears them faster if you continue to blast them.
The charging electronics are monitored to boost the rate up to 80% (along with other factors such as temperature and age) and then more gradually. Some of the new systems are over-provisioned allowing a few blocks at a time to be relaxed or cycled for a period to benefit long term reliability.
As we have seen with the recent laptops and tablet/phones the efficiency of these devices is improving as much from the control and charging capabilities as the actual storage.
