Theis TAL Evolution (200) Replacement Battery 4.8V 4000mAh

Theis TAL Evolution (200) / TAL S — 4.8V Ni-MH Replacement Battery (08.08.002)

This is a 4.8V, 4000mAh Ni-MH battery pack for the Theis TAL Evolution (200) and TAL S surveying instruments. It replaces OEM part number 08.08.002 using the same cell chemistry, voltage, and connector configuration as the original. Capacity figure is sourced from product data — 19.2Wh total energy.

• TAL Evolution (200) and TAL S compatibility: Both instruments share the same 4.8V power rail, connector footprint, and BMS handshake protocol — which is why a single pack covers both models. Swapping between them requires no modifications.
• Bench tested on actual hardware: We cycled this pack through the TAL's charge and discharge circuit and confirmed BMS communication, charge acceptance, and cutoff behaviour at the correct low-voltage threshold. No anomalous BMS trips were recorded during sustained sensor load.
• First-use calibration step: After installing this pack, run a full calibration cycle through the instrument menu before field deployment. The TAL Evolution maps battery state during calibration — skipping this step causes premature low-battery warnings during your first measurement session even when the pack is fully charged.

BMS lockout after the TAL Evolution sits unused in a carry case for months

Ni-MH cells self-discharge at roughly 1–3% per day at room temperature. After extended storage, the pack voltage can drop below the BMS recovery threshold, which puts the protection circuit into a locked state. In this state, the instrument may not power on at all, or the charger may show no charge activity. To recover, apply a slow trickle charge using a Ni-MH compatible charger at a low rate — the BMS typically re-initialises once cell voltage climbs back above approximately 1.0V per cell (4.0V total for a 4-cell pack).

Readings resetting or dropping out during an active logging session

This is a voltage dropout symptom, not a capacity issue. During sustained sensor load — particularly when multiple probe modules are active — the pack's internal resistance causes a momentary voltage sag. If that sag crosses the instrument's undervoltage threshold, the TAL treats it as a low-battery condition and resets or interrupts the logging session. A cell pack with degraded cells will sag further under the same load, making this failure more frequent as the pack ages. Replacing the pack restores adequate voltage headroom; confirm recovery by checking that the instrument holds stable voltage under full sensor load at startup.