Nolan B4 RCS Replacement Battery 3.7V 900mAh NC003-BA-21

Nolan B4 RCS Kits — 3.7V Li-ion Replacement Battery (NC003-BA-21)

This is a 3.7V, 900mAh Li-ion cell built to replace the original battery in the Nolan B4 RCS Kit Bluetooth helmet headset unit. It fits the wireless communication module that sits inside the helmet for hands-free calling and intercom use. When the original cell no longer holds charge, swapping this in restores full wireless function to the headset.

• B4 RCS Kit compatibility: The B4 RCS system uses a single flat Li-ion cell at 3.7V to power both the Bluetooth radio and audio circuits simultaneously. This cell matches the original OEM dimensions — 54.20 x 35.65 x 4.65mm — and the BMS connector, so the base station can complete its handshake with the new pack.
• Bench tested on actual hardware: We ran this cell through charge and discharge cycles on a B4 RCS unit, monitoring BMS handshake completion and cutoff voltage. The cell accepted a full charge and the protection circuit tripped correctly at low voltage without locking the unit out.
• First-charge protocol on the B4 RCS: Seat the headset in the base station and let it complete a full uninterrupted charge cycle before taking any calls. The base station logs the new cell during this cycle — until it does, the talk-time estimate displayed will not reflect actual capacity.

Why the B4 RCS headset cuts out mid-call on a new battery

The B4 RCS draws current from two circuits at once — the Bluetooth radio and the audio amplifier — and that combined load spikes briefly every time audio transmits. A new cell sitting at storage voltage (around 3.6–3.7V) has not yet settled into its working charge curve, so the BMS can read the voltage sag during a transmission spike as a low-cell event and cut power. This is not a fault with the cell. Running two or three full charge-discharge cycles lets the cell stabilise and the BMS calibrate its cutoff threshold to the actual load profile of the device.

Base station shows charging error after fitting a new cell

The B4 RCS base station checks for a BMS handshake before it begins charging — if the new cell arrives at a voltage the base reads as outside its expected window, it flags a charging error rather than starting a charge cycle. Remove the headset, leave it out of the base for 30 seconds, then reseat it firmly and check the connector is fully engaged. If the error clears, allow the full charge cycle to complete without interrupting it. If the error persists, check that the cell voltage is above 3.0V with a multimeter — below that threshold, the base will not initiate charging.