Product Specifications

Overview:
BA21d is a name that combines both the model and the base type, referring to a classic single-filament, single-contact, bayonet-style halogen bulb. It is primarily used as a turn signal light on motorcycles and can also serve as a rear position light or brake light on certain models. Its design is simple, reliable, and cost-effective.

Core Features and Specifications:

1. Model & Specifications

   • BA21d: This name contains complete information. It represents both the bulb's overall model and explicitly indicates its base type.

   • Rated Voltage & Wattage:

     • Operating Voltage: 12V.

     • Standard Wattage: This is the common wattage for motorcycle turn signals and brake lights.

   • BA21d Base Details: It is a single-contact, symmetrical double-pin bayonet base.

     • "BA" stands for Bayonet.

     • "21" relates to its standard wattage (21 watts) and size code.

     • "d" here is commonly interpreted as double pin, referring to the two symmetrical locating pins on the base. Electrically, it is a single contact (one center positive tip, with the shell as negative).

2. Working Principle & Application

   • Working Principle: As a standard single-filament bulb, it lights up when powered.

   • Primary Applications:

     • Motorcycle/Scooter Turn Signal Lights: This is its primary and most common use. Typically used with an amber (yellow) glass bulb or lens.

     • Brake Light: On vehicles with a single-function bulb design (where tail and brake lights are separate bulbs), it can be used as a brake light (behind a red lens).

     • Rear Position Light: On some older models, it may also function as a constantly-on rear position light.

3. Key Distinctions & Compatibility

   • Vs. BA20d Series: BA21d and the previously mentioned BA20d are different sized and specified bases and are NOT interchangeable. The BA20d base is smaller and typically used for headlights like H4/1HD or S2 tail lights. The BA21d is a larger, more common base specifically designed for 21W signal lights.

   • Relation to P21W: The BA21d bulb is functionally and in wattage completely equivalent to the P21W bulb. The difference lies in the base: P21W usually refers to a wedge-base bulb of the same 21W rating, while BA21d specifically denotes the bayonet base. They serve the same function but have different physical interfaces and are not interchangeable.

   • Vs. Dual-Power Bulbs: It is a single-power  bulb and does not have the dual-power tail/brake light function of bulbs like the S2.