Graetz TC850B Survey Instrument Replacement Battery 7.2V 2000mAh

Graetz TC850B / TC850C — 7.2V Ni-MH Replacement Battery (NA150D05C100)

This is a 7.2V 2000mAh Ni-MH replacement battery for the Graetz TC850B and TC850C surveying instruments. It replaces OEM part number NA150D05C100 and restores field operation when the original pack can no longer hold charge. Capacity is 2000mAh (14.4Wh) — matching the original specification.

• TC850B and TC850C compatibility: Both models share the same battery bay dimensions, connector pinout, and 7.2V supply rail. The instrument's power management circuit draws from the same voltage threshold on both, so one pack covers either unit.
• Bench tested on actual hardware: We ran this pack through charge and discharge cycles on a Ni-MH compatible analyser, monitoring cell balance and BMS response at probe initialisation. The pack held voltage through the current spike at sensor power-up without tripping cutoff.
• First-use calibration on the TC850B/C: After installing a new pack, run a full calibration cycle through the instrument menu before field deployment. The TC850B and TC850C map battery state during that calibration routine — skipping it causes premature low-battery warnings during the first measurement session in the field.

TC850B shutting down mid-measurement despite a charged battery

The TC850B draws a brief current spike each time it powers a probe or sensor module. On a degraded or cold Ni-MH pack, internal resistance is high enough that this spike pulls the terminal voltage below the instrument's cutoff threshold — even when the cell capacity looks fine at rest. The instrument interprets the voltage dip as a depleted pack and shuts down to protect the measurement circuit. A new pack with lower internal resistance passes the same spike without voltage sag, and the instrument stays on through the full measurement cycle.

Pack will not charge after the instrument sat unused for months

Ni-MH cells self-discharge during storage. If a pack has sat unused long enough, cell voltage can drop below the threshold the charger checks before beginning a charge cycle — the charger sees the low voltage as a fault and refuses to start. This is a BMS sleep state, not a failed cell. To recover, use a Ni-MH analyser or recovery charger to apply a low-current trickle charge until the pack reaches approximately 5.0–6.0V, then place it back on the standard charger.