Collection: BA9s bulbs

The BA9s Base: What Does This Designation Mean?

BA9s means: B for Bayonet (quarter-turn lock), A for standard shape, 9 for the base diameter in mm, s for Single contact (1 centered pin). It is the compact format of the bayonet family — the smaller brother of the BA15S (15 mm).

It mainly accommodates the T4W (4 W) — hence the parallel use of the two designations BA9s and T4W for the same bulb.

BA9s = T4W: Two Designations for One Bulb

Common confusion: BA9s and T4W refer to the same bulb. BA9s describes the base, T4W describes the complete bulb (T for tubular, 4W for wattage). On the packaging, you will find one or the other designation depending on the manufacturer — it is the same physical product.

Uses of BA9s

The BA9s base is used in the circular dome lights of many vehicles:

• Main round dome light (vs elongated C5W festoon lights)
• Vanity lights on sun visors (illuminated mirrors)
• Map lights integrated into the dome light
• Some door sill and door handle lights
• Indicator lights on older vehicles

Distinguishing BA9s from a Festoon Type Dome Light

If your dome light is circular, it is most likely a BA9s (or T4W).

If your dome light is elongated (rectangular), it is most likely a C5W festoon bulb (or C10W) — a festoon base with contacts at both ends, without a bayonet.

Simple identification procedure: remove the faulty bulb. BA9s = short bulb with metal bayonet base; festoon = elongated bulb with metal bases at both ends.

Switching to LED BA9s

Switching to LED provides modern white light (vs yellow halogen) and a lifespan multiplied by 10. Canbus compatible without resistors on most vehicles. Available interior colors: cool white, warm white, blue, red, green — choose according to your aesthetic preferences.