About this replacement
Ohaus Defender 5000 / T51P / T51XW — 7.2V Ni-MH Replacement Battery (80500729)
This 7.2V, 3000mAh Ni-MH pack replaces OEM part 80500729 in the Ohaus Defender 5000 bench scale and compatible T51-series indicators. It fits the Defender 5000, T51P, T51XW, and three additional Defender 5000 variants that share the same battery bay and connector. Capacity matches the original at 3000mAh (21.6Wh).
- Defender 5000 and T51-series compatibility: These models share a common 7.2V six-cell Ni-MH battery format, a standardised connector pinout, and the same BMS communication protocol — the replacement pack slots in and the indicator firmware recognises it without any manual pairing step.
- Bench tested on actual hardware: We cycled this pack through charge and discharge on the Defender 5000 platform and monitored BMS handshake at power-on. The protection circuit responded correctly to both low-voltage cutoff and overcharge termination at the delta-V threshold Ni-MH cells require.
- Post-install calibration on the Defender 5000: Run a full span calibration through the scale's menu after fitting this battery. The Defender 5000 maps battery state during calibration, and skipping this step causes premature low-battery warnings during the first weighing session — even with a fully charged pack.
Why the Defender 5000 shuts down mid-weighing session with no warning
Ni-MH cells have a flat discharge curve followed by a steep voltage drop at end-of-life. The Defender 5000's BMS acts on a voltage threshold, not a gradual ramp — so the display can show adequate charge right up until the cell voltage collapses below 6.0V and the unit cuts off. An aged original pack loses capacity but holds voltage long enough to pass the indicator's startup check, then drops out under load. Replacing the pack clears this entirely because fresh cells sustain voltage through the full working range.
Scale powers on but shuts off immediately when switching to battery mode
This happens when the Ni-MH pack has sat unused long enough to self-discharge below the BMS recovery threshold — typically under 5.4V for a 7.2V six-cell pack. The BMS enters a protection state and refuses to source current, so the unit drops power the moment AC is removed. Reconnect the charger and leave it for a full charge cycle without interruption; some chargers require the pack to reach a minimum recovery voltage before the charge current ramps up. If the pack does not respond after a full 14–16 hour charge on a compatible Ni-MH charger, the cells have reached end-of-life and replacement is the next step.