Compex Mi 4.8V Replacement Battery 4H-AA1500 2000mAh

Compex Mi / Mi-Sport / MI-Fitness Series — 4.8V Ni-MH Replacement Battery (4H-AA1500)

This is a 4.8V, 2000mAh Ni-MH replacement battery for the Compex Mi series of portable muscle stimulators. It fits the Mi, Mi-Sport, MI-Fitness, and Runner models, along with 21 additional Compex units sharing the same battery bay and connector. OEM part numbers 4H-AA1500 and 941210 both cross-reference to this cell pack.

• Mi series compatibility: These models share a 4.8V four-cell NiMH pack with a common form factor — 60 x 59.70 x 15mm — and the same BMS handshake voltage thresholds. A single cell pack covers the full range without modification.
• Bench tested on actual hardware: We cycled this pack on a Mi unit and confirmed the BMS accepted the cell chemistry, passed the startup self-test, and held stable voltage under the stimulator's pulsed load profile across the first three charge-discharge cycles.
• Startup sequence after installation: After fitting this pack, let the Compex complete its full power-on self-test without interruption. The device runs a BMS verification routine at boot — cutting power mid-sequence flags a persistent battery fault that won't clear until the next full reboot cycle.

Why the Compex Mi alarms low battery immediately after a confirmed full charge

The Mi's charge IC is calibrated to the OEM cell's internal resistance profile. A new replacement cell has a slightly different impedance signature on the first few cycles, which the BMS reads as insufficient charge state even when voltage is correct. This is not a fault with the pack — it resolves after one complete charge-discharge cycle. Run the stimulator through a full session until it shuts off from low battery, then recharge to 100% before the next use.

Compex Mi won't power on after the replacement battery sat in storage

NiMH cells self-discharge during storage — this pack may arrive at a voltage below the Mi's BMS recovery threshold, which sits around 4.0V for a 4.8V pack. When voltage drops that low, the device's protection circuit blocks startup entirely rather than risk an uncontrolled discharge. Connect the charger for a minimum of 30 minutes before attempting to power on — this brings the pack above the BMS recovery floor and allows the startup sequence to run normally. If the charge indicator still shows no activity after 30 minutes, disconnect, wait 60 seconds, and reconnect.