South BP-5S X11 Data Controller Replacement Battery 7.4V

South X11 Data Controller — 7.4V Li-ion Replacement Battery (BP-5S)

This is a 7.4V, 3400mAh Li-ion replacement battery for the South X11 Data Controller. It fits the X11 directly, using the same connector and BMS handshake as the original BP-5S pack. Capacity figures come from the product data — 25.16Wh at the rated 3400mAh.

• X11 platform fit: The X11 Data Controller runs a 7.4V two-cell Li-ion architecture. The BP-5S form factor matches the bay dimensions — 70.70 × 39.40 × 21.70mm — and the BMS communicates over the same data line the X11 uses to report state of charge to the instrument firmware.
• Bench tested on actual hardware: We cycled this pack under a sustained sensor-load profile, simulating continuous GNSS logging and probe polling. The BMS held voltage regulation within tolerance and did not trip under the initialisation current spike that occurs when the X11 powers up its sensor modules.
• Pre-deployment calibration step: After fitting this pack, run a full calibration cycle through the X11 instrument menu before heading into the field. The controller maps battery state during that calibration pass — skipping it causes the low-battery warning to fire early during your first measurement session, even with a full charge.

BMS lockout after the X11 sat unused in a carry case for months

Li-ion cells in the BP-5S self-discharge slowly during storage. If the pack drops below approximately 2.5V per cell, the BMS enters a protective sleep state and refuses to accept a charge through the X11's standard charge port. The instrument will show no charge indicator at all — not a slow charge, nothing. To recover the pack, apply a low-current pre-charge (around 0.1C) directly to the pack terminals using a compatible bench charger until cell voltage climbs above 3.0V per cell, then return it to the X11 charger normally.

X11 display showing erratic battery percentage after swapping packs

The X11 tracks battery state using a voltage-threshold model calibrated to the previous pack's discharge curve. When a new cell set with slightly different internal resistance is installed, the instrument's percentage readout can jump or drop unexpectedly across the first few charge-discharge cycles. This is not a fault in the battery — it is the controller recalibrating its threshold map. Run two full charge-discharge cycles through normal field use and the percentage display will stabilise to within a few percent of actual remaining capacity.