Testo 300 L Replacement Battery 2.4V 1800mAh Ni-MH

Testo 300 L / 300 M / 300 XL — 2.4V Ni-MH Replacement Battery (0563 0310)

This is a 2.4V, 1800mAh Ni-MH replacement battery for the Testo 300 L, 300 M, and 300 XL combustion analysers. These handheld flue gas instruments are used on-site for heating system efficiency surveys and emissions testing. The original OEM part numbers are 0563 0310, 0563 0345, and 0563 0300 — all covered by this cell.

• 300 L, 300 M, and 300 XL compatibility: All three models share the same battery bay geometry, connector pinout, and 2.4V supply rail. The instrument's BMS handshake is identical across the series, so one cell fits all without modification.
• Bench tested on actual hardware: We ran this pack through the Testo 300 instrument power-on sequence, probe initialisation surge, and sustained sensor load. The BMS held stable across all draw phases with no cutoff events or cell imbalance detected.
• First-use calibration cycle: After fitting this pack, run a full calibration cycle through the instrument menu before going to site. The 300 series maps battery state during calibration — skipping this step causes premature low-battery warnings on your first measurement session, even with a fully charged cell.

Pack will not charge after the Testo 300 sat unused in a carry case for months

Ni-MH cells self-discharge over storage, and if the pack drops below roughly 1.0V per cell, the instrument's charger circuit may refuse to engage. This is a voltage threshold check — not a faulty charger. Some units will recover if you apply a slow external trickle charge for 20–30 minutes to bring the pack above the detection floor, then return it to the instrument dock. If the cell has sat discharged for an extended period, chemical reversal inside the cell may be irreversible, and replacement is the correct fix.

Testo 300 shuts down the moment the O₂ probe initialises

Probe initialisation on the Testo 300 draws a brief current spike as the electrochemical sensor heats and stabilises. On a degraded or deeply discharged pack, this spike is enough to trip the BMS undervoltage cutoff — the instrument powers off within seconds of entering measurement mode. The symptom looks like a software fault but the cause is the battery failing to hold voltage under transient load. Fit a fresh pack and confirm resting voltage is above 2.3V before probe insertion.