Separating Energy Harvester Cold Start From Steady Operation

An energy harvesting circuit may run once its storage element is charged but fail to start from a depleted state. Cold start can use different internal circuitry and require different input conditions from steady operation. Verify the startup energy path before concluding that a low voltage harvester is compatible with the board.

Read both operating regimes

Record cold start voltage and power requirements, the transition to normal operation, and the permitted storage configuration. The TI BQ25570 data sheet distinguishes cold start behavior from normal harvesting and states that sufficient input power is needed to exit cold start.

A source meeting an open circuit voltage threshold may still collapse under the startup load. Characterize the source’s current voltage behavior under the actual illumination, temperature, or other energy condition relevant to the product.

Trace the initial load

Identify every capacitor, storage element, leakage path, and active circuit that is connected during startup. Determine whether the controller holds the system load off until enough energy is available. A processor repeatedly attempting to boot can consume the energy needed to complete charging.

Include protection and monitoring components in the leakage budget. Very small continuous currents can matter when harvested power is limited. A board that starts with a debugger attached may be receiving unintended energy through its interfaces.

Define a repeatable depleted state

Follow the device’s recommended connection sequence and establish a known initial storage voltage. Record the source condition and any residual charge. A quick restart after power removal is not equivalent to a fully depleted cold start. Label these two experiments separately in the stored test results.

Measure input voltage, storage voltage, system enable, and load activity through the startup interval. Use instruments whose loading is appropriate to the available power. A low resistance probe or meter can materially change the result.

Verify the energy balance after release

Once the load turns on, observe whether storage continues rising, remains stable, or falls back below a threshold. Check the intended duty cycle and peak activity, not only an idle processor. Hysteresis or a controlled operating schedule may be needed to prevent repeated partial startups, according to the supported design.

Save initial conditions, source characterization, capacitance, leakage estimate, threshold settings, and timing waveforms. Report cold start time separately from steady harvesting performance. A successful design demonstrates that it can reach and sustain useful operation from the required initial state; efficiency measured after an externally precharged startup does not establish that capability.