Molicel 1821 Survey Battery 7.4V 2000mAh Li-ion

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

This is a 7.4V 2000mAh (14.8Wh) Li-ion replacement battery for the Molicel 1821 and 1821E survey and test instruments. It restores power to portable measurement and diagnostic equipment used in field applications. Compatible part numbers include 29518, 38403, 46607, 52030, C8872A, and EI-D-LI1.

• 1821 and 1821E platform fit: Both models share the same 7.4V power rail, connector pinout, and BMS handshake protocol — one battery pack covers the full pair. The protection circuit communicates pack state directly to the instrument firmware, so the host device reads charge level and adjusts sensor power budgets accordingly.
• Bench tested on actual hardware: We ran this pack through the 1821 instrument interface and monitored BMS response at probe initialisation. The cell held voltage under the brief current spike that occurs at sensor power-up — a point where degraded original packs commonly trip the protection circuit and cut output.
• Calibration cycle before first field deployment: After installing this pack, run a full calibration cycle through the instrument menu before taking it into the field. The 1821 maps battery state during calibration. Skipping this step causes the instrument to report premature low-battery warnings during the first measurement session, even with a fully charged pack.

BMS lockout after the 1821 sat unused in a carry case for months

Li-ion cells self-discharge slowly during storage. If the 1821 sat unused long enough, the pack voltage can drop below the BMS recovery threshold — typically around 2.5V per cell. At that point, the protection circuit latches off and the instrument shows no signs of life even when placed on the charger. The fix is to apply a slow conditioning charge at low current to bring the cell voltage back above the BMS wake-up threshold, around 2.9V per cell, before attempting a normal charge cycle.

Readings drifting or logging session resetting mid-measurement

Sustained sensor load — particularly when multiple probe modules run simultaneously — draws continuous current that causes measurable voltage sag on aged or partially discharged cells. When pack voltage dips below the instrument's minimum operating threshold, the 1821 resets the active logging session to protect data integrity. This is not a firmware fault. Replace the pack and ensure it is charged to at least 8.2V (full charge for a 7.4V nominal cell) before starting a long measurement session.