About this replacement
Anritsu S331B / S331C / S331D / S332B Series — 10.8V Ni-MH Replacement Battery (MT9081)
This is a 10.8V, 2100mAh Ni-MH replacement battery for the Anritsu Site Master cable and antenna analyzer series. It fits the S331B, S331C, S331D, S332B, and over 20 additional S-series handheld RF test instruments. The MT9081 part number is the standard OEM reference across this analyzer family.
- S331 and S332 series compatibility: These models share the same battery bay dimensions, connector pinout, and BMS handshake protocol. One pack covers the full sweep of field-deployed Site Master units without modification.
- Bench tested on actual hardware: We cycled this pack through the S331B charge circuit and confirmed the BMS handshake completes correctly. The cell temperature monitoring and cutoff thresholds responded within spec at both charge termination and low-voltage cutoff.
- Pre-deployment calibration cycle: After fitting this pack, run a full calibration sequence through the Site Master's system menu before going to site. The instrument maps battery state during calibration — skipping this step causes the low-battery warning to fire early during the first measurement session, even with a fully charged pack.
BMS lockout after the S331B sat unused in a carry case for months
Ni-MH cells self-discharge steadily in storage. After several months in a case, cell voltage can drop below the BMS recovery threshold — at that point the pack refuses to accept a charge and the instrument shows no power response. The fix is to apply a low-current trickle charge directly to the pack terminals for 15–20 minutes using a compatible Ni-MH charger before inserting it into the analyzer. Once cell voltage climbs above approximately 9.0V, the BMS exits lockout and normal charging resumes through the instrument.
S331B shuts off mid-sweep during transmission line measurements
This is a voltage sag issue, not a dead pack. The Site Master's RF signal source draws a sharp current spike at sweep initiation — particularly at higher output power settings. If cells have aged or are partially discharged, that spike pulls the terminal voltage below the BMS cutoff threshold and the instrument drops out. Check resting voltage before a session: a healthy pack at full charge reads between 12.5V and 13.2V. If resting voltage is below 11.5V before powering on, recharge before attempting a measurement sweep.