Charging Heat

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The Zero battery will emit heat above a charging level of 0.4C (4kW for a 4-brick pack), and will naturally cool below that threshold.

Expect several degrees Fahrenheit rise when performing a full charge near a 1C rate.
The cause is best characterized as wikipedia:Internal resistance of the battery, driven by current flow.
The power wasted from this characteristic will be roughly proportional to the square of the current flowing into the battery.
External Inputs
On the SDS Platform, the Onboard Charger is contacts the bottom face of the battery case and will conduct some heat into the battery as it operates.
Discharge Heat
Discharging a battery at a very high rate can also produce heat.
The internal resistance from charging seems to be higher for a given current than the internal resistance from driving/discharge.
High Temperature Guidance
See the hot weather usage recommendations for thorough guidance.
If the battery temperature is already high enough to cause concern about the next charging phase, ride at relatively modest speeds to allow ventilation to cool it faster than discharge heats it up.
Use a battery temperature indication to help understand this.