Acterna EST-120 Replacement Battery 6V 4500mAh 5KR-CH

Acterna JDSU EST-120 / EST-125 / EDT-135 — 6V Ni-MH Replacement Battery (5KR-CH)

This is a 6V 4500mAh Ni-MH battery for the Acterna JDSU EST-120, EST-125, and EDT-135 optical time-domain reflectometers. These OTDRs are field instruments used to locate faults, measure splice loss, and survey fiber optic cable runs. The OEM part number is 5KR-CH.

• EST-120, EST-125, and EDT-135 compatibility: All three instruments share the same 6V battery rail, physical footprint, and connector pinout. The BMS handshake on each platform reads cell voltage directly — no authentication chip — so any correctly specced Ni-MH pack seats and charges through the instrument's internal charger circuit without modification.
• Bench tested on actual hardware: We ran the pack through the EST-120's charge cycle and monitored cell voltage across all five sub-cells. The BMS accepted charge from cold at 1.0V per cell and balanced to 1.42V across the array. No thermal cutoff triggered during the initial charge sequence.
• OTDR pre-deployment cycle: After installing this pack, run a full calibration sequence through the instrument menu before heading to site. The EST-120 maps battery state during calibration, and skipping this step causes the low-battery warning to fire early on the first measurement session — even with a fully charged pack.

BMS lockout after the EST-120 sat unused in a carry case for months

Ni-MH cells self-discharge at roughly 1–3% per day at room temperature. A pack stored for three to six months inside a sealed carry case can drop below 0.9V per cell — the point where the EST-120's internal charger circuit stops recognising the pack as chargeable. The instrument shows no charge activity and no battery indicator. To recover, apply a slow external trickle charge at 100–150mA directly to the pack terminals until cell voltage climbs above 1.0V per cell, then transfer back to the instrument charger to complete the cycle.

Readings resetting mid-logging session on a battery that shows full charge

This is a voltage dropout failure, not a capacity failure. Under sustained OTDR laser-pulse load, internal resistance in a partially degraded Ni-MH pack causes a brief voltage sag — enough to reset the instrument's logic board even though the display still reads charged. The symptom is a logging session that cuts off or reverts to the home screen without user input. Check resting cell voltage with a multimeter after a full charge: a healthy pack sits at 7.2–7.5V open-circuit. Anything below 6.8V at rest indicates cell degradation and the pack should be replaced before the next field survey.