Why does a battery limit solar panel power?

Illustrative model: open-circuit voltage 7.8 V; maximum power point 6.2 V / 10 W; battery voltage rises from 4.0 V to 4.2 V in the second half. An illustrative 0.5 V resistor drop puts the connected panel voltage at 4.5–4.7 V. A 4.2 V Zener diode is reverse-connected across the battery terminals. It remains below breakdown at 4.0 V and begins shunting at an idealized 4.2 V threshold. Red is a visual metaphor for dissipation, not literal incandescence. The battery rise and Zener transition are scripted; shunt particles show direction only, not a calculated branch-current magnitude. Charging termination is not simulated. Other losses are omitted. The fixed resistor drop represents one example operating condition. The curve is not measured data, and the operating-point transition is slowed down for illustration. Open-circuit power is zero. After connection, reduced energy capture means power below the available maximum; this difference is not all dissipated as heat in the resistor.