A solar panel is a nonlinear current source. Its I-V curve has a distinctive knee, and somewhere on that knee sits a single operating point where P = V × I is maximized — the Maximum Power Point (MPP). Connect the panel directly to a battery and you'll clamp it to the battery voltage, which is almost never at the MPP. A 100W panel might deliver only 60W. MPPT circuits dynamically adjust the load impedance the panel sees, keeping it parked at the MPP as sunlight, temperature, and shading change.

Architecturally, an MPPT controller is a DC-DC converter (usually buck, sometimes boost or buck-boost) with a smart control loop. Instead of regulating output voltage, the loop regulates the input voltage to whatever value extracts maximum power. The converter's duty cycle becomes the knob that transforms the panel's V_MPP into whatever the battery needs.

The most common tracking algorithm is Perturb and Observe (P&O):

• Measure V_panel and I_panel, compute P = V × I
• Perturb the duty cycle by a small step ΔD
• If P increased, keep stepping the same direction; if P decreased, reverse
• Repeat every few milliseconds

P&O oscillates around the MPP by ±ΔD — a fundamental tradeoff between tracking speed and steady-state efficiency. Incremental Conductance is smarter: it uses dI/dV = -I/V at the MPP, so it knows when to stop perturbing. Better for fast-changing irradiance (passing clouds).

Concrete example: A 36-cell panel rated 100W, V_MPP ≈ 18V, I_MPP ≈ 5.5A, charging a 12V lead-acid battery (float ~13.8V). Direct connection: panel pulled to 13.8V, output ≈ 13.8V × 5.7A ≈ 79W (and you've shifted off the knee — likely much worse). With a buck MPPT: panel stays at 18V × 5.5A = 99W input, converter at 96% efficiency delivers ~95W to the battery at 13.8V (≈6.9A charging current). You gained 20% — pure software cleverness on top of a switching converter you'd build anyway.

Rule of thumb for step size: ΔD ≈ 0.5–1% of full duty cycle per iteration, with a 1–10 ms update rate. Too small and you can't keep up with clouds; too large and steady-state ripple eats your efficiency gain. A panel's MPP voltage drifts about -0.4%/°C with cell temperature, so cold mornings shift V_MPP up noticeably — your tracker handles this automatically.

Watch for: local maxima under partial shading (bypass diodes create multiple peaks — P&O can get stuck on the wrong one), and input capacitor sizing (must hold V_panel stable between perturbations, typically 470µF–2200µF).