Design a Supercharger backend: billing, stall telemetry and finding a free charger

Design the backend for a network of EV fast-charging sites, each with several stalls. Three requirements:

1. Billing: measure the energy delivered in every charging session and store it so the customer can be billed correctly.
2. Telemetry: collect status and measurements from every stall.
3. Nearest available charger: let a driver find the closest site with a free stall.

Cover the data stalls send and how often, ingestion, storage, making billing correct when connections drop, and keeping availability fresh enough for drivers.