Molicel 1821 Replacement Battery 7.4V 2600mAh

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

This 7.4V 2600mAh Li-ion pack replaces the original battery in the Trimble 1821 and 1821E handheld GPS/GNSS receivers. It matches the OEM voltage rail, connector pinout, and BMS communication protocol the instrument expects. Capacity is 2600mAh (19.24Wh) — taken from product data, not extrapolated.

• 1821 and 1821E platform compatibility: Both models share the same battery bay geometry, 7.4V nominal rail, and BMS handshake protocol. The instrument queries the pack at power-on; this replacement answers that query correctly so the charge indicator and low-battery alarm function as they should.
• Bench tested on actual hardware: We cycled this pack through full charge and discharge on the bench, monitoring BMS cutoff thresholds and cell balance across both series cells. The protection circuit tripped at the correct over-discharge floor and recovered cleanly on reconnect to a charger.
• Post-install calibration on the 1821: After fitting this pack, run a full calibration cycle from the instrument's system menu before heading into the field. The 1821 maps battery state during calibration — skip this step and the low-battery warning will trigger early on your first survey session, even with a full charge.

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

Li-ion cells self-discharge at roughly 2–3% per month. Leave a pack unused long enough and cell voltage drops below the BMS's minimum recovery threshold — typically around 2.5V per cell for this chemistry. When that happens, the protection circuit locks out entirely and the instrument shows no battery at all, even after you connect a charger. Most quality chargers can recover the pack with a trickle pre-charge that brings cell voltage back above the 2.5V threshold before switching to normal CC/CV charging.

Voltage dropout causing the 1821 to reset or drop readings mid-logging session

A degraded or partially discharged cell struggles under the sustained current draw of active GNSS tracking, especially when the receiver is logging at high update rates. Voltage sags momentarily below the instrument's operating floor, triggering a brown-out reset that clears the active log buffer. This is not a firmware fault — it is a cell capacity issue. Swap to a fresh pack, verify resting voltage reads at least 7.9V before deploying, and confirm the charge indicator shows full before starting a new logging session.