Pentax 29518 Replacement Battery 7.4V 3400mAh Li-ion

Pentax 29518 / 38403 Series — 7.4V Li-ion Replacement Battery

This 7.4V, 3400mAh Li-ion battery replaces the internal pack in Pentax surveying and measurement instruments including the 29518, 38403, 46607, and 52030 models. It delivers 25.16Wh of capacity across the full voltage range these instruments draw from during sensor initialisation and active logging. Built to the same physical footprint as the original — 70.35 × 38.50 × 20.65mm — so it seats correctly in the battery bay without modification.

• 29518, 38403, 46607, 52030 fit group: These models share a common 7.4V power rail, the same connector orientation, and a BMS handshake protocol that expects cell voltage within a fixed window on power-up. A pack outside that window causes the instrument to reject the battery or report a fault before a measurement session starts.
• Bench tested on actual hardware: We cycled this pack through simulated probe initialisation events — the brief high-draw spike when the instrument powers a sensor module. The BMS held without tripping on startup, and cell voltage recovered cleanly after each initialisation pulse.
• Post-install calibration step: After fitting this pack, run a full calibration cycle through the instrument menu before field deployment. The instrument maps battery state during calibration — skipping this step causes premature low-battery warnings during your first measurement session, even with a fully charged pack.

BMS cutoff when the sensor module initialises at power-up

Pentax survey instruments draw a short but sharp current spike the moment they power the probe or sensor module at startup. If the battery's BMS has a low trip threshold — common in aged or counterfeit packs — this spike triggers a protection cutoff before the instrument completes its boot sequence. The instrument either restarts in a loop or shows a battery fault code. This pack's BMS is rated to handle the startup surge without latching off, and the cell chemistry sustains voltage through that transient rather than sagging below the cutoff point.

Pack will not charge after the instrument sat unused for several months

Li-ion cells left in storage self-discharge over time. If the voltage drops below roughly 2.5V per cell — 5.0V across a 7.4V pack — most chargers and instrument charging circuits will refuse to engage, reading the pack as faulty. The BMS may also enter a deep-sleep lockout state. To recover, connect the pack to a compatible external Li-ion charger with a recovery or "wake" mode and apply a trickle charge at 0.1C until cell voltage rises above 3.0V per cell, then resume normal charging through the instrument.