Motorola SBV5220 Replacement Battery 10.8V 3400mAh

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

This 10.8V, 3400mAh lithium-ion battery replaces the internal backup cell in the Motorola SBV5220, SBV5221, SURFboard SBV5222, and Surfboard Digital Voice Modem SB5220. It holds the modem online during a power outage, keeping both data and VoIP active until utility power returns. OEM part numbers 515757-001 and 535625-001-00 both cross to this cell.

• SBV5220 / SBV5221 / SBV5222 platform fit: These three modems share the same 10.8V three-cell pack format, connector pinout, and BMS handshake protocol. The SBV5222 adds one additional port but draws from the same battery bay — voltage rail and BMS communication remain identical across the family.
• Bench tested on actual hardware: We ran this cell in an SBV5220 and monitored the BMS charge acceptance sequence. The modem firmware began balancing the pack within 20 minutes of installation, and the BMS reported a healthy cell after a full charge cycle with no error flags thrown.
• First-charge protocol for cable modem backup cells: After installing, leave the modem plugged into mains power for a full four hours before testing backup function. The SBV5220 firmware will not commit the battery to emergency use until it has completed one full supervised charge cycle — pulling the plug before that window ends triggers a false battery error.

Why the SBV5220 drops VoIP during an outage but keeps the data light on

The SBV5220 prioritises its DOCSIS data channel when running on battery backup. VoIP requires sustained higher power than a basic data connection — the modem's power management firmware can shed the voice subsystem first if the backup cell cannot deliver adequate current. A degraded original cell often passes the modem's basic battery-present check but fails under the load spike the voice processor creates at call setup. Replacing with a full-capacity 3400mAh cell and completing the four-hour initial charge resolves this in most cases.

Modem showing battery error light after replacement

A steady or flashing battery error indicator after swapping the cell does not mean the new battery is faulty. The SBV5220 firmware runs an internal battery-check sequence that can take up to 24 hours after a new cell is installed before it clears the error flag. The BMS needs to complete at least one full charge-discharge handshake with the modem controller. If the error light persists beyond 24 hours on mains power, check that the battery connector is fully seated — the three-pin connector requires firm pressure until it clicks flush.