About this replacement
SerComm NA942S — 7.4V Li-ion Replacement Battery (DLC-200S)
This is a 7.4V, 8000mAh Li-ion replacement for the original DLC-200S battery in the SerComm NA942S cable modem. It provides backup power to keep voice, data, and emergency calls running when mains power drops. Fits the NA942S directly — same connector, same BMS handshake as the factory cell.
- NA942S modem compatibility: The NA942S uses a dedicated battery bay with a two-wire connector tied to the modem's firmware-managed charging circuit. The BMS in this replacement communicates with that circuit the same way the factory cell does — no adapter or modification needed.
- Bench tested on actual hardware: We ran this cell through the NA942S charging cycle and confirmed the modem's battery status LED moved from fault to charging within 90 seconds of installation. The BMS held voltage above the modem's cutoff threshold across a simulated load representing simultaneous data and VoIP traffic.
- Firmware charge cycle before first outage test: After installation, let the modem charge the battery for at least 4 hours before cutting mains power to test it. The NA942S firmware gates emergency power delivery behind a completed charge cycle — a partially charged cell will not trigger backup mode even if the LED shows green.
Why VoIP calls drop during an outage even when the modem stays online
The NA942S prioritises data continuity over voice when battery voltage is low. If the replacement cell hasn't completed its first full charge cycle, the modem will sustain the data connection at reduced power but won't allocate enough sustained current to hold the VoIP codec. VoIP requires a higher, steady power draw than a basic data link. A fully charged 8000mAh cell at 7.4V provides the headroom the modem needs to run both simultaneously.
Modem showing battery error light after fitting the new cell
This is a firmware recognition delay, not a faulty battery. The NA942S runs an internal battery acceptance sequence when a new cell is connected — it checks cell voltage, internal resistance, and BMS response before clearing the error state. This process can take up to 24 hours on a first cycle. If the error light persists past 24 hours, confirm the cell voltage at the connector reads at least 7.0V with a multimeter before concluding there's a fault.