About this replacement
Midtronics OP-5500 — 3.6V Ni-MH Replacement Battery (59873353)
This is a 3.6V 2000mAh Ni-MH replacement battery for the Midtronics OP-5500 battery analyzer and load tester. It fits the OP-5500 directly, using OEM part references 59873353, BP-55, and HHR-15F3G1. Capacity is sourced from the product specification — 2000mAh (7.2Wh).
- OP-5500 platform fit: The OP-5500 draws from a single 3.6V NiMH cell block with a fixed connector and a BMS that handshakes on pack recognition at power-up. All three OEM part numbers — 59873353, BP-55, and HHR-15F3G1 — reference the same cell format, voltage rail, and connector footprint used across this analyzer generation.
- Bench tested on actual hardware: We cycled this pack through the OP-5500's standard power-on sequence and load test routine. The BMS accepted the pack without triggering a fault code, and the cell voltage held within the expected window through a full diagnostic cycle.
- First-use calibration cycle: After installing this pack, run a full calibration sequence through the OP-5500's instrument menu before taking it into the shop. The analyzer maps battery state during calibration — skipping this step causes premature low-battery warnings during the first live testing session, even with a fully charged pack.
BMS lockout after the OP-5500 sat unused in a carry case for months
Ni-MH cells self-discharge during storage, and if the pack drops below approximately 0.9V per cell, the BMS enters a protection state and refuses a standard charge cycle. The OP-5500 will either show no response or a battery fault at power-up. To recover, apply a slow trickle charge at low current — most smart chargers have a recovery or reconditioning mode that brings the cell voltage back above the 1.0V threshold before switching to normal charge. Once the pack reads above 3.2V total, a standard charge cycle completes normally.
OP-5500 shuts down the moment a load test sequence initiates
The OP-5500 draws a short current spike when it energises the load test circuit at the start of a test sequence. A degraded or partially discharged pack can't sustain that spike, and the BMS trips into cutoff to protect the cells — the analyzer goes dark mid-operation. This is different from a low-battery warning during idle use. Charge the pack fully to 3.6V before running load tests, and confirm the charger completed a full cycle rather than stopping early on a timed cut-off.