Electrical Fundamentals

Blocking Diode

A blocking diode is placed in the circuit between a solar panel (or panel string) and a battery bank to prevent reverse current flow — stopping the battery from discharging back through the panels at night or during low-light conditions when panel voltage drops below battery voltage.

Without a blocking diode, current flows from the higher-voltage source to the lower-voltage source. During the day, current flows from the panels (higher voltage) to the battery (lower voltage) — the desired direction. At night, the panels produce no voltage, and the battery would push current backward through the panels, slowly draining itself and potentially damaging the cells.

In modern solar installations, blocking diodes have been largely replaced by charge controllers, which electronically prevent reverse current flow with near-zero voltage drop. A Schottky blocking diode drops about 0.3-0.5V, causing a small but constant power loss during charging. Charge controllers (both PWM and MPPT) handle reverse-current protection internally with MOSFETs that have negligible voltage drop.

Blocking diodes are still used in some specific situations: small direct-panel-to-battery setups without a charge controller (like trickle chargers), parallel-connected panel strings where one string might shadow another, and some marine and RV installations where simplicity is valued over marginal efficiency.

If your system has a charge controller, you do not need a separate blocking diode — the controller handles this function.

Frequently Asked Questions

Do I need a blocking diode if I have a charge controller?

No. All modern charge controllers (both PWM and MPPT) have built-in reverse-current protection that replaces the need for a blocking diode. Adding a blocking diode to a charge-controller system would only add unnecessary voltage drop and reduce efficiency.

What is the difference between a bypass diode and a blocking diode?

A bypass diode is inside the panel and routes current around shaded cells. A blocking diode is in the circuit between the panel and battery, preventing the battery from discharging back through the panels. They serve completely different protective functions.