Leica GEB121 Replacement Battery 6V 3600mAh Ni-MH

Leica GS50 GPS / TCR802 Power Series — 6V Ni-MH Replacement Battery (GEB121 / GEB122)

This 6V Ni-MH battery replaces the GEB121 and GEB122 packs used in the Leica GS50 GPS receiver, TCR802 Power, TCR803 Power, and 400 series instruments. Capacity is 3600mAh (21.6Wh), matching the original specification. It fits the standard battery bay without modification.

• GS50, TCR802/TCR803 Power, and 400 series compatibility: These instruments share the same 6V supply rail, GEB121/GEB122 form factor, and battery-bay connector pinout. The BMS handshake is voltage-threshold based — no proprietary authentication chip to negotiate.
• Bench tested on actual hardware: We cycled this pack through charge and discharge on a GS50-class load profile. The BMS held cutoff at the correct low-voltage threshold and accepted a full charge without triggering fault flags.
• First-use calibration on Leica instruments: After fitting a new pack, run a full instrument calibration cycle through the Leica menu before field deployment. The GS50 and TCR series map battery state during calibration — skipping this step causes the low-battery warning to fire well before actual depletion on the first session.

GS50 shutting down mid-GNSS session with a battery that charged fully

The GS50 draws a sustained load during satellite lock and active logging that is higher than during standby. If a Ni-MH pack has developed cell imbalance from shallow cycling, terminal voltage drops sharply under this sustained draw even though the pack reads full at rest. The instrument's under-voltage protection triggers a clean shutdown rather than corrupting a log file. Cycling the pack three times — full charge to the instrument's cutoff threshold — lets the cells re-equalize and restores stable voltage under load.

New pack fitted but the GS50 shows no charge or won't power on

Ni-MH packs that have sat in storage below roughly 5.4V can put the internal protection circuit into a sleep state where the instrument sees no valid voltage at the contacts. The GS50 interprets this as no battery fitted and refuses to boot. Connect the pack to the Leica charger rather than the instrument first — the charger applies a low-current recovery charge that brings cell voltage back above the wake threshold. Once the charger confirms a charge cycle has started, the pack will accept a normal charge and the instrument will recognise it at next insertion.