VEX IQ Controller 3.7V Replacement Battery VEX-228-2779 900mAh

VEX IQ Controller — 3.7V Li-ion Replacement Battery (VEX-228-2779)

This is a 3.7V, 900mAh lithium-ion cell that replaces the internal battery in the VEX IQ Controller. The IQ Controller is the central processing unit of VEX IQ robot kits — it coordinates motor outputs and sensor inputs during competitions and build sessions. Voltage and capacity match the OEM specification exactly.

• VEX IQ Controller fit: The IQ Controller runs a single lithium-ion cell at 3.7V nominal. The BMS in this unit monitors cell voltage directly — any deviation from the expected voltage window triggers a low-battery warning or shutdown before a match cycle completes. This replacement cell sits within that window.
• Bench tested on actual hardware: We cycled this cell under load conditions that replicate motor-drive draw across the IQ Controller's output channels. The BMS accepted the cell without error flags, and charge termination triggered correctly at full capacity.
• Fuel gauge recalibration after swap: After installing this cell, run one complete discharge down to the controller's low-battery cutoff, then charge fully before use. The IQ Controller's fuel gauge IC reads state-of-charge against a stored cell profile — a full cycle forces it to remap against the new cell's actual discharge curve.

Why the IQ Controller shuts down mid-match even with battery showing charge

The IQ Controller cuts power when cell voltage drops below approximately 3.0V under load — even if the on-screen indicator still shows remaining charge. During a match, motor-drive current spikes cause a momentary voltage sag on a degraded or deeply discharged cell. The BMS reads that sag as a brownout condition and triggers an emergency cutoff to protect the cell. A new cell at full state-of-charge maintains voltage above the cutoff threshold even during peak motor demand.

IQ Controller battery percentage stuck or jumping after cell replacement

The fuel gauge IC in the IQ Controller stores a discharge curve calibrated to the previous cell's internal resistance and capacity profile. When a new cell is installed, that stored curve no longer matches the actual cell, so the reported percentage can freeze, skip, or drop suddenly. This is not a fault with the replacement cell — it is the gauge IC working from stale data. Run one full discharge-to-cutoff followed by a full charge cycle, and the IC will recalibrate to the new cell's actual curve.