Motorola SBV5220 Replacement Battery 10.8V 2600mAh 515757-001

Motorola SBV5220 / SBV5221 Surfboard — 10.8V Li-ion Replacement Battery (515757-001)

This is a 10.8V, 2600mAh Li-ion backup battery for the Motorola SBV5220, SBV5221, SBV5222, and Surfboard Digital Voice Modem SB5220. It sits inside the modem and takes over during a mains power outage, keeping the modem's DOCSIS connection and VoIP service alive. OEM part numbers 515757-001 and 535625-001-00 both apply to this cell.

• SBV5220 / SBV5221 / SBV5222 platform fit: These three models share the same internal battery bay dimensions, connector pinout, and BMS handshake protocol — the modem firmware reads cell voltage and temperature through the same four-pin interface, so one cell fits all three variants.
• Bench tested on actual hardware: We cycled this battery on the SBV5220 platform and confirmed the BMS accepted the cell within the modem's initialisation window. The firmware polled cell voltage correctly and cleared the battery fault flag after a full charge cycle.
• First-install charge requirement: Cable modem firmware on the SBV5220 series will not commit the backup battery to emergency use until it has completed one full charge cycle. After installing, leave the modem on mains power for at least four hours before testing backup operation — triggering a self-test before that window closes will return a fail result even with a healthy cell.

Battery self-test failing immediately after swap on the SBV5220

The SBV5220 runs an automatic battery self-test at scheduled intervals and also allows a manual trigger through the modem's admin interface. If that test runs before the new cell reaches full charge, the firmware will log a failure regardless of cell condition. The test requires the battery to hold above a minimum voltage threshold under a brief load pulse — a partially charged Li-ion cell at 10.2V or below will not pass. Let the modem charge the battery to 12.6V open-circuit (approximately four hours on mains) before running or interpreting any self-test result.

Internet stays up during outage but VoIP calls drop immediately

The data side of the SBV5220 draws less power than the VoIP subsystem. During a power outage, the modem's internal load management can sustain the DOCSIS downstream connection at a lower current draw, but the VoIP processor and codec require a sustained higher current that a partially charged or degraded battery cannot supply. The result is that a browser works fine while calls cut out within seconds. If this happens with a new battery, the cell has not yet reached a full state of charge — return to mains power for four hours, then retest by unplugging the modem and immediately placing a call.