The information below is taken from MDK v.0.2
The Ara power architecture is designed to accommodate the unique and flexible nature of the platform. Users of an Ara phone will be able to power their device with one or multiple batteries; they will be able to swap a depleted battery with a fresh one, without powering off their phone; they will be able to charge one or more batteries in their phone from one or multiple charging devices. The design of the power architecture enables all of these use cases and provides a flexible power platform for new applications.
Figure below shows a high-level view of the Ara power architecture. A common power bus is central to the architecture. The power bus voltage is specified to be between 3.3 and 4.8 V. The power bus is distributed through the Endo frame to each interface block. A power manager in the Endo optimizes power consumption, reduces current leakages, and provides protection against overcurrents and overvoltages. The Endo also has a small battery or capacitor bank, and associated charge controller. The power stored in the Endo provides power to the phone during brief periods when the battery and/or charger modules are being hot-swapped or reconfigured and no other power source is available to keep the device on.
Ara modules fall into three categories in relation to the power architecture: power consumers, charging modules, and power storage modules (batteries, fuel cells, etc.). Modules can change categories throughout their usage life. For instance, a battery module may have a built-in charging port. Such a module would serve as a power consumer when being charged from the bus (e.g., if another module’s charging port is active), a charging module when powered through it’s charging port, and a power storage device when functioning as a regular battery. Figure below also illustrates a simplified view of these three module types. The following subsections describe each of the types.
Power Consumer Module
At the top of figure above is a power consumer module, which is the simplest in its operation. The power consumer module draws power directly from the power bus, or more commonly, voltage regulator(s) steps down and regulates the voltage from the bus for various circuits within the module.
Charging modules can supply externally derived power to the power bus. Charging modules may draw a small amount of current from the power bus during module initialization, but current Qows nominally in one direction, from the module to the bus. Figure above shows a traditional DC charger module.
Power Storage Module
Power storage modules primarily include battery modules, but other power storage devices are envisioned. Power storage modules can supply power to the rest of the system or draw power from the bus for recharging.
Battery modules supply power to the power bus by default. Figure below shows a configuration in which there are multiple battery modules connected to the power bus. Ideal diode circuits are required to prevent current Qow into a battery in default discharge mode. The power bus voltage is equal to the highest voltage presented by any of the batteries connected to the power bus; the battery with the highest voltage bears the full load. The load is shared when the voltage falls to level of the next highest battery. This trend continues until the voltage across all batteries is equal and the load is shared evenly among them.
Figure below shows when a charger module is connected to the system and ready to deliver charging current to the batteries, it signals the supervisory controller in the Endo (power manager) via UniPro messaging. The supervisory controller then signals battery modules to cut off power delivery to the power bus and activate their charging circuits to draw power from the bus. The supervisory controller periodically polls battery modules to report self diagnostics such as state of charge and uses this information in its power management routines. Power storage modules must declare their total capacity and report their remaining capacity to the Endo to ensure the power manager can accurately determine the overall power budget of the device.