About this replacement
Yokogawa AAQ7250 — 10.8V Ni-MH Replacement Battery (9HR-4/3FAU)
This is a 10.8V 3500mAh Ni-MH replacement battery for the Yokogawa AAQ7250 portable surveying instrument. It replaces OEM part number 9HR-4/3FAU. The battery restores field operation for survey and measurement sessions where the original pack has degraded or failed.
- AAQ7250 platform fit: The AAQ7250 uses a 10.8V Ni-MH pack with a specific connector pinout and BMS handshake. This replacement matches that voltage rail and connector configuration so the instrument's power management system accepts the pack without fault codes.
- Bench tested on actual hardware: We cycled this pack through charge and load sequences that replicate probe initialisation and sustained sensor logging. The BMS handled current spikes at sensor power-up without tripping into protection mode, and voltage held stable under continuous measurement load.
- Pre-deployment calibration cycle: After installing a new pack, run a full calibration cycle through the AAQ7250's instrument menu before heading into the field. The instrument maps battery state during that process — skipping it causes premature low-battery warnings to appear during the first measurement session even when the pack has a full charge.
BMS lockout after the AAQ7250 sat unused in a carry case for months
Ni-MH cells self-discharge during storage. If the pack sat unused long enough, cell voltage can drop below the BMS's recovery threshold, putting the pack into lockout. In that state, the instrument either shows no battery indication or refuses to power on entirely. To recover, connect the pack to the charger for a minimum of 30 minutes before attempting to power the instrument — most BMS circuits re-initialise once they detect a valid charge voltage above 10.0V.
Readings resetting or dropping mid-logging session with no low-battery warning
This failure mode is distinct from a flat battery. It happens when sustained sensor load causes a brief voltage dropout that the instrument interprets as a fault rather than a dead pack. Ni-MH cells can sag under continuous draw, especially if the pack has shallow-cycled repeatedly and hasn't been fully discharged. Run a full discharge-to-cutoff cycle followed by a complete charge before the next deployment. After conditioning, check that open-circuit voltage reads at least 12.5V before inserting the pack into the instrument.