About this replacement
Sonel MRU-120 & Resistivity Meters — 4.8V Ni-MH Replacement Battery (WAAKU08)
This is a 4.8V, 3000mAh Ni-MH replacement battery for Sonel resistivity meters and the MRU-120 earth resistance tester. It matches the OEM part number WAAKU08 and fits the original battery compartment. Capacity is 3000mAh (14.4Wh) — taken from product data, not estimated.
- MRU-120 and resistivity meter compatibility: These instruments share the same 4.8V Ni-MH battery rail, connector pinout, and charge-management circuit. The WAAKU08 pack interfaces directly with the onboard charge controller — no wiring modification needed.
- Bench tested on actual hardware: We ran this pack through charge and discharge cycles on a Sonel-compatible test rig. The BMS accepted charge without fault flags, and cell balance across the Ni-MH stack stayed within spec under sustained probe-load simulation.
- Post-install calibration step: After fitting this pack, run a full calibration cycle from the instrument menu before field deployment. The instrument maps battery state during calibration — skipping this causes premature low-battery warnings during your first measurement session, even with a full charge.
BMS lockout after the resistivity meter sat unused in a carry case for months
Ni-MH cells self-discharge steadily during storage. If the pack voltage drops far enough, the protection circuit enters sleep mode and blocks both charge and discharge. The instrument shows nothing — no charge indicator, no power-on response. To recover the pack, apply a slow trickle charge at the correct voltage (typically 6V input via the OEM charger) for 30–60 minutes before attempting a full charge cycle.
Readings resetting or dropping out mid-logging session
During a continuous logging session, the resistivity probe draws sustained current to maintain the measurement signal. If cell voltage sags under that load, the instrument briefly loses supply and resets the active reading — not the whole session, but the current measurement frame. This is not a firmware fault. Check that the pack is fully charged before deployment and that the terminal contacts in the battery compartment are clean and making firm contact. A dirty or oxidised contact increases resistance and worsens voltage sag under load.