20 Minutes of Battery Operation in 0.30 Seconds

That's the output of a pytest test running against a simulated 10 kWh battery — 20 minutes of physics, 0.30 seconds of wall clock time. In Part 1 I explained why I built DER Twin — a Modbus simulator for energy devices. The short version: I spent a year building EMS software without a proper test environment and it was painful. Testing required physical hardware, every cycle took too long, and we were essentially testing on production. This article is about what becomes possible once you have a simulator. Specifically: how to write automated tests for EMS control logic that run in seconds, reproduce any scenario deterministically, and require no hardware at all. The Setup To make this concrete I put together a small demo project alongside the simulator. It has a simple EMS — SimpleEMS — that connects to a BESS over Modbus TCP and cycles it between 40% and 60% SOC. There's a thin Modbus client that reads and writes registers by name, a main.py to run it against a live simulator, and the