VEX EXP Controller 3.7V 900mAh Replacement Battery Li-ion

VEX EXP Controller / IQ Controller — 3.7V Li-ion Replacement Battery

This 3.7V, 900mAh lithium-ion cell replaces the original battery inside the VEX EXP Controller and VEX IQ Controller. Both are wireless handheld controllers used to operate VEX Robotics platforms in competition and classroom settings. At 53.00 × 34.00 × 4.30mm, it matches the original form factor for direct installation.

• EXP and IQ controller fitment: Both controllers run a 3.7V single-cell architecture with the same physical footprint and connector orientation. Swapping between the two models is straightforward because the battery specification is identical across both platforms.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on the EXP Controller platform and confirmed the BMS accepted charge from the standard VEX charging dock without fault flags or cutoff errors at full termination voltage.
• Controller firmware recalibration after install: After fitting a new cell, run the controller through one full discharge down to automatic shutoff, then charge to 100% before returning to competition use. The onboard fuel gauge IC is calibrated to the old cell's discharge curve — one full cycle resets it against the new cell so the charge indicator reads accurately.

Why the EXP Controller shows incorrect battery percentage after a cell replacement

The EXP Controller tracks charge state using a fuel gauge IC that builds a discharge curve model over time. When a new cell goes in, that stored curve no longer matches the actual cell chemistry, so the percentage readout drifts. The IC reports based on historical data, not live cell state. One complete discharge-to-shutoff followed by a full charge to 4.2V resets the coulomb counter and brings the display back in line.

Controller powers off unexpectedly while the display still shows charge remaining

This is a voltage cliff failure. The cell voltage drops sharply under the load of active wireless transmission and motor signal processing, falling below the BMS cutoff threshold even when the gauge reads 20–30% remaining. It happens most often on cells that have aged past 70% of original capacity. Replacing the cell and completing one full recalibration cycle eliminates the mismatch between gauge state and actual deliverable voltage — confirm the cell rests at 4.1–4.2V after a full charge before use.