About this replacement
Testo 350 Series — 8.4V Ni-MH Replacement Battery (0515 0098)
This 8.4V 3500mAh Ni-MH battery replaces the original pack in the Testo 350, 350 Plus, 350-S, and 350-XL flue gas analysers. It fits the standard battery bay without modification and connects via the same multi-pin interface as the OEM unit. The 29.4Wh capacity keeps the analyser powered through extended combustion measurement sessions in the field.
- 350 series compatibility: The 350, 350 Plus, 350-S, and 350-XL share the same 8.4V battery rail, connector pinout, and BMS communication protocol. One pack covers the full platform.
- Bench tested on actual hardware: We cycled this pack through the Testo 350 charge and discharge sequence and confirmed the BMS handshake cleared correctly. The pack accepted a full charge and held stable voltage under simulated probe module load without triggering a cutoff event.
- Post-install calibration: After installing this pack, run a full calibration cycle through the instrument menu before taking it to site. The 350 maps battery state during calibration — skipping this step causes premature low-battery warnings on the first measurement session, even with a fully charged pack.
BMS lockout after the Testo 350 sat unused in a carry case for months
Ni-MH cells self-discharge slowly during storage. If the analyser sat unused long enough, the pack voltage can drop below the BMS recovery threshold — typically around 6V for an 8.4V Ni-MH pack. When that happens, the instrument may not power on at all, or it may appear to charge but show no progress. Connect the analyser to the charger and leave it for at least two hours before attempting to power on — the BMS needs time to trickle-charge the cells back above the recovery threshold before it will allow a normal charge cycle.
Testo 350 shutting down mid-measurement when the probe module initialises
When the 350's probe module powers up, it draws a brief current spike as sensors initialise. On a degraded or partially discharged pack, that spike can pull voltage low enough to trip the BMS undervoltage cutoff — shutting the instrument down before a single reading is logged. This is not a fault in the battery or the instrument; it is a protection response to a momentary voltage sag. Charge the pack to full before field deployment and confirm resting voltage is at or above 8.4V before connecting the probe module.