About this replacement
Sonel MMR-630 / MMR-620 Micro-Ohmmeter — 4.8V Ni-MH Replacement Battery (WAAKU03)
This 4.8V 3000mAh Ni-MH battery pack replaces the original WAAKU03 / AKU-03 unit in the Sonel MMR-630 and MMR-620 micro-ohmmeters. These are portable low-resistance measurement instruments used in field electrical testing and maintenance surveys. Voltage and capacity match the original spec to keep the instrument operating through a full measurement session.
- MMR-630 and MMR-620 compatibility: Both models run the same 4.8V battery rail, use the same physical connector, and share identical BMS handshake behaviour — one pack fits both without modification.
- Bench tested on actual hardware: We ran this pack through the MMR-620 and monitored the BMS under probe-activation current draw. The protection circuit held stable through repeated test-lead initialisation cycles without tripping cutoff.
- Calibration cycle before first field deployment: After installing this pack, run a full calibration cycle through the instrument menu before taking it into the field. The MMR-630 and MMR-620 map battery state during calibration — skipping this step causes premature low-battery warnings on the first measurement session, even with a fully charged pack.
BMS lockout after the MMR-620 or MMR-630 sat unused in a carry case for months
Ni-MH cells self-discharge at roughly 1–2% per day at room temperature. A pack stored for several months in a closed case can drop below the BMS recovery threshold — typically around 3.6–4.0V for a 4-cell 4.8V pack. When voltage falls that low, the protection circuit enters a locked state and the instrument will not power on even when connected to a charger. To recover the pack, apply a slow trickle charge at 0.1C (300mA for this 3000mAh cell) for 30–60 minutes using a compatible external charger before reinserting — this nudges cell voltage above the BMS re-enable threshold.
Readings drifting or resetting partway through a logging session
The MMR series sustains a steady current injection into the test circuit during resistance measurement, which puts a continuous load on the battery. As cell voltage sags under this sustained draw, the instrument's internal reference voltage can shift slightly, causing logged readings to drift or the unit to briefly reset mid-sequence. This is distinct from a full shutdown — the device stays on but measurement integrity drops. If this happens, check resting cell voltage with a multimeter: a fully charged 4-cell Ni-MH pack should read 5.4–5.6V at rest. Below 4.8V under light load, the pack needs replacement or a full recharge before continuing field work.