Molicel 1821 Survey Battery 7.4V 3400mAh Li-ion

Molicel 1821 / 1821E — 7.4V Li-ion Replacement Battery (29518)

This is a 7.4V 3400mAh (25.16Wh) Li-ion replacement battery for the Molicel 1821 and 1821E survey and test instruments. The dual-cell pack slots into the battery bay of the instrument body and restores full operating voltage to the measurement and logging platform. OEM part numbers 29518, 38403, 46607, 52030, C8872A, and EI-D-LI1 all apply to this pack.

• 1821 and 1821E compatibility: Both models run the same 7.4V power rail, use the same physical connector, and the same BMS handshake protocol. Either unit will accept this pack without firmware conflicts or connector modification.
• Bench tested on actual hardware: We cycled the pack through the instrument's full probe initialisation sequence, including the current spike at sensor power-up. The BMS held without nuisance cutoff across repeated cold starts and sustained measurement loads.
• Post-install calibration step: After installing this pack, run a full calibration cycle through the instrument menu before field deployment. The instrument maps battery state during calibration, and skipping this step causes premature low-battery warnings on the first measurement session.

BMS cutoff when a probe or sensor module initialises

When the 1821 powers up a probe module, the sensor initialisation draws a sharp current spike in the first two seconds. An aged or partially discharged pack can't sustain that spike above the BMS undervoltage threshold, so the BMS trips and the instrument shuts down immediately after the probe is connected. This pack's cells are rated to handle the inrush without the voltage collapsing below the trip point. If cutoffs still occur, check cell voltage under load — it should stay above 6.8V during initialisation.

Pack will not charge after months sitting unused in a carry case

Li-ion cells that self-discharge below approximately 2.5V per cell enter a deep-discharge state where the BMS disables the charge path as a protection measure. The instrument's charger will show no charge activity and the pack appears dead. Most chargers with a recovery or trickle mode can re-enter the pack by applying a low current until cell voltage climbs back above 3.0V per cell, at which point the BMS re-enables normal charging. If no recovery mode is available, try a 15-minute charge attempt on a known-good Li-ion charger — even a brief trickle input is often enough to wake the BMS and allow a full charge cycle to begin.