The Qi-Hardware Ben's NanoNote operates with a rechargeable lithium battery (could be of -ion or -polymer). This feature determines that the board requires a power supply circuit to carry out the tasks of recharge battery and provides the right voltage to the different IC regulators circuits.
SE9016 is a Seaward Electronics inc. Standalone Linear Lithium Battery Charger Integrated Circuit.
It is a complete constant-current and constant voltage linear charger for Lithium-Polymer and single cell lithium-ion batteries. the SE9016 suits ideally in portable applications due to its SOT-23 package and low external component count. In addition, the SE9016 is specifically designed to work within USB power specification.
the MOSFET architecture allows non external sense resistor and no blocking diode. Thermal feedback regulates the charge current to limit the die temperature during high power operation or high ambient temperature. The charge voltage is fixed at 4.2V, and the charge current can be programmed externally with a resistor. The SE9016 automatically terminates the charge cycle when the charge current drops to the tenth part of the programmed value after the final float voltage is reached.
When the input supply that could be a wall adapter or a USB supply is removed, the SE9016 automatically enters into a low current stage, dropping the battery drain current to less than 2uA. The SE9016 can be put into shutdown mode, reducing the supply current to 20uA. SE9016 is intentionally designed to have slightly negative Tempco. This provides extra protection to Lithium battery during charging.
this IC include features like charge current monitor, undervoltage lockout, automatic recharge and status pin to indicate charge termination and presence of any input voltage.
- Programmable Charge Current Up to 800mA.
- No MOSFET, Sense Resistor or Blocking Diode Required.
- Constant-Current/Constant-Voltage Operation with Thermal Protection to Maximize Charge Rate without Risk of Overheating.*
- Charges Single Cell Li-Ion Batteries Directly from USB Port.
- Preset 4.2V Charge Voltage with ±1% Accuracy.
- 20uA Supply Current in Shutdown.
- 2.9V Trickle Charge Threshold.
- Soft-Start Limits Inrush Current.
- Available in 5-Lead SOT-23 Package.
- RoHS Compliant and 100% Lead (Pb)-Free.
 Pin Configuration & Functions
VCC: Positive input supply voltage. Provides power to the charger. the value of VCC covers a range from 4.25V to 6.5V and should be bypassed with at least a 1uF capacitor.
BAT: Charge current output. provides charge current to the battery and regulates the final float voltage to 4.2V.
CHRG: Open-Drain Charge Status Output. When the battery is charging, the CHRG pin is pulled low by an internal N-channel MOSFET. When the charge cycle is completed, a weak pull down of aproximately 20uA is connected to the CHRG pin, indicating an "AC present" condition.
PROG: Charge Current Program, Charge Current Monitor and Shutdown pin.
 Flow Chart
 SE9016 datasheet, Seaward Electronics inc. http://pdf1.alldatasheet.com/datasheet-pdf/view/291191/SEAWARD/SE9016.html