How the IV sweep works

Each of the six panels connects to the T-Relay boards: three relay boards, two panels each. The T-Relay's only job is to switch relays on command and report back when it has. All the electrical measurement happens through a single shared energy meter, read over RS485/Modbus by the ECM50 controller. A per-panel select relay decides which one panel, out of six, is patched through to that meter.

The T-Relay sits close to the panels to keep relay wiring short; the ECM50 sits with the meter and the rest of the sensor bus. The two talk over ESP-NOW, a direct wireless link that needs no WiFi router or internet connection (§5).

Each held phase lasts about 1.5 s and the break about 0.1 s, so one panel takes roughly 4.6 s and a full six-panel sweep about 27.6 s.

Relay contacts don't switch instantly. Between the Voc phase ending and the Isc phase beginning there is a real electromechanical transition, measured on the bench at roughly 120–150 ms. During that window the panel's terminal voltage and current pass through every intermediate value between Voc and Isc, as a slow sweep would.

The firmware samples the meter as fast as the RS485 bus allows during that window, instead of at its normal background rate. Those samples, typically 10–15 per panel, are the IV curve. The relay's own switching transient does the sweeping; the firmware only has to listen fast enough to catch it.

The same window yields V_mp/I_mp/P_max: the firmware scans every transition sample for the highest V×I product. It is a genuine point on the measured curve, not an estimate.

A plateau isn't accepted as-is: the firmware finds the longest contiguous stretch where the reading holds within a tight tolerance, discarding the settling edges. A plateau that never settles (too short, too noisy) is rejected rather than averaged into a misleading number.

ECM50 and T-Relay run on independent clocks with no shared wiring beyond the wireless link. ECM50 only knows which meter samples belong to which panel and phase because T-Relay tells it, in real time, every time a relay state changes.

MSG_START_QUICK_LOAD is the fifth message: a lighter variant (§7) that reuses the same ack / marker / done exchange.

Automatic sweeps fire three ways: on a 15-minute clock-aligned schedule, when irradiance crosses a bright-sun threshold, or manually from the console (SWEEPNOW). Automatic triggers are suppressed after dark, since V and I read ~0 with no light and every sweep wears the relays for no usable data.

A quick load poll skips straight to the Load relay state on each panel in turn (the phase 3 circuit, set directly), takes one reading, and moves on. It reuses the same messages and the same ECM50 state machine as a full sweep, so the two can never run at once or collide on the shared bus.