power: supply: Explain maintenance charging

In order for everyone to understand clearly why we want to use
maintenance charging for batteries, expand the description with two
diagrams and some text.
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>

include/linux/power_supply.h

@@ -374,9 +374,37 @@ struct power_supply_vbat_ri_table {
  *   These timers should be chosen to align with the typical discharge curve
  *   for the battery.
  *
- * When the main CC/CV charging is complete the battery can optionally be
- * maintenance charged at the voltages from this table: a table of settings is
- * traversed using a slightly lower current and voltage than what is used for
+ * Ordinary CC/CV charging will stop charging when the charge current goes
+ * below charge_term_current_ua, and then restart it (if the device is still
+ * plugged into the charger) at charge_restart_voltage_uv. This happens in most
+ * consumer products because the power usage while connected to a charger is
+ * not zero, and devices are not manufactured to draw power directly from the
+ * charger: instead they will at all times dissipate the battery a little, like
+ * the power used in standby mode. This will over time give a charge graph
+ * such as this:
+ *
+ * Energy
+ *  ^      ...        ...      ...      ...      ...      ...      ...
+ *  |    .   .       .  .     .  .     .  .     .  .     .  .     .
+ *  |  ..     .   ..     .  ..    .  ..    .  ..    .  ..    .  ..
+ *  |.          ..        ..       ..       ..       ..       ..
+ *  +-------------------------------------------------------------------> t
+ *
+ * Practically this means that the Li-ions are wandering back and forth in the
+ * battery and this causes degeneration of the battery anode and cathode.
+ * To prolong the life of the battery, maintenance charging is applied after
+ * reaching charge_term_current_ua to hold up the charge in the battery while
+ * consuming power, thus lowering the wear on the battery:
+ *
+ * Energy
+ *  ^      .......................................
+ *  |    .                                        ......................
+ *  |  ..
+ *  |.
+ *  +-------------------------------------------------------------------> t
+ *
+ * Maintenance charging uses the voltages from this table: a table of settings
+ * is traversed using a slightly lower current and voltage than what is used for
  * CC/CV charging. The maintenance charging will for safety reasons not go on
  * indefinately: we lower the current and voltage with successive maintenance
  * settings, then disable charging completely after we reach the last one,
@@ -385,14 +413,22 @@ struct power_supply_vbat_ri_table {
  * ordinary CC/CV charging from there.
  *
  * As an example, a Samsung EB425161LA Lithium-Ion battery is CC/CV charged
- * at 900mA to 4340mV, then maintenance charged at 600mA and 4150mV for
- * 60 hours, then maintenance charged at 600mA and 4100mV for 200 hours.
+ * at 900mA to 4340mV, then maintenance charged at 600mA and 4150mV for up to
+ * 60 hours, then maintenance charged at 600mA and 4100mV for up to 200 hours.
  * After this the charge cycle is restarted waiting for
  * charge_restart_voltage_uv.
  *
  * For most mobile electronics this type of maintenance charging is enough for
  * the user to disconnect the device and make use of it before both maintenance
- * charging cycles are complete.
+ * charging cycles are complete, if the current and voltage has been chosen
+ * appropriately. These need to be determined from battery discharge curves
+ * and expected standby current.
+ *
+ * If the voltage anyway drops to charge_restart_voltage_uv during maintenance
+ * charging, ordinary CC/CV charging is restarted. This can happen if the
+ * device is e.g. actively used during charging, so more current is drawn than
+ * the expected stand-by current. Also overvoltage protection will be applied
+ * as usual.
  */
 struct power_supply_maintenance_charge_table {
 	int charge_current_max_ua;