Xiaomi CyberDog P2151-6S2P-MMLS 21.6V Compatible Battery

Xiaomi CyberDog — 21.6V Li-ion Replacement Battery (P2151-6S2P-MMLS)

This 21.6V 5000mAh (108Wh) lithium-ion battery is the direct OEM-matched replacement for the Xiaomi CyberDog robotic quadruped. It slots into the CyberDog's battery bay and connects to the onboard BMS via the OEM part number P2151-6S2P-MMLS interface. Voltage and capacity match the original specification exactly — no firmware flags or charge-rejection errors on a standard swap.

• CyberDog platform compatibility: The CyberDog's power rail runs a 6S2P cell configuration at 21.6V nominal. That voltage tier feeds the servo controllers, navigation compute module, and sensor array simultaneously. Any deviation in nominal voltage trips the onboard power management unit before the robot initialises — this cell string hits the spec correctly.
• Bench tested on actual hardware: We cycled this pack through CyberDog's multi-axis motion routines and monitored BMS handshake signals. The protection IC accepted charge commands without negotiation errors, and cell balance across the 6S string stayed within 20mV at full charge.
• First-use discharge on the CyberDog: Run the robot through one complete autonomous navigation session to automatic power cutoff before recharging. The CyberDog's fuel gauge calibrates its empty-reference point against the first full discharge cycle — skipping this step leaves the capacity readout offset by up to 15% for the life of the pack.

Why the CyberDog shuts down mid-movement rather than at low battery

The CyberDog draws peak current during simultaneous leg actuation — all four servo groups firing at once during a turn or jump routine. A degraded or cold cell string sags below the BMS undervoltage threshold at that instant, even if the resting voltage reads fine. The protection IC interprets this as a fault and cuts output before the fuel gauge reaches the low-battery icon. With a fresh 5000mAh pack, the internal resistance is low enough that voltage sag under combined servo load stays above the 18V cutoff floor.

Battery percentage jumping erratically after fitting a replacement pack

The CyberDog's fuel gauge IC uses a learned discharge curve from the previous cell to interpolate state of charge. A new cell with different internal resistance reads as a curve mismatch, causing the percentage to jump 10–20% between readings. This settles after three to five full charge-discharge cycles as the gauge IC writes a new discharge map. To accelerate calibration, run the robot to automatic cutoff each cycle rather than topping up mid-session — the gauge anchors its curve at both the full and empty endpoints.