MaxCom MM550BB TJB-1 Replacement Battery 3.7V 1000mAh

MaxCom MM550BB / MM500BB Series — 3.7V Li-ion Replacement Battery (TJB-1)

This is a 3.7V, 1000mAh Li-ion replacement battery for the MaxCom MM550BB, MM500BB, MM132, and MM133 mobile phones. It fits directly in place of the original TJB-1 cell. Capacity figure is 1000mAh (3.7Wh) as rated by the manufacturer.

• MM550BB / MM500BB / MM132 / MM133 compatibility: These four MaxCom models share the same battery bay dimensions (54.61 × 35.16 × 5.31mm), connector pinout, and 3.7V voltage rail. One cell revision covers the full group without modification.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on a controlled bench rig. The BMS held the charge cutoff voltage correctly and did not trip prematurely under the steady low-current load typical of these MaxCom handsets.
• Fuel gauge recalibration on first use: After fitting this cell, disable fast charging if available and run one full discharge-to-charge cycle before normal use. The fuel gauge IC in these phones is calibrated to the old cell's discharge curve — one full cycle lets it remap to the new cell before reporting accurate percentages.

Sudden shutdown at 20–30% on the MM550BB after a cell swap

This is a voltage cliff issue, not a capacity fault. The fuel gauge IC still holds the old cell's internal resistance model, so it reads remaining charge as higher than the actual cell voltage. When the modem fires up for a call or the screen brightness peaks, current draw spikes and the new cell's terminal voltage drops below the cutoff threshold faster than the gauge predicted. The phone shuts off even though the percentage counter showed charge remaining. One full discharge cycle — draining to auto-shutoff, then charging uninterrupted to 100% — forces the coulomb counter to rebuild its model against the actual cell chemistry.

Phone will not power on after the replacement cell sat in storage

Li-ion cells self-discharge in storage. If this battery dropped below approximately 2.5V per cell, the BMS enters lockout mode to prevent damage and the phone will show no response when the power button is pressed. Connect the phone to a wall charger — not a PC USB port — and leave it for 15 to 30 minutes without pressing anything. The charge IC trickle-charges the cell back above the BMS recovery threshold, typically around 2.9V, at which point the BMS re-initialises and the phone will boot normally.