About this replacement
Motorola MTP8500 / MTP8550 Series — 7.4V Li-ion Replacement Battery (NNTN8570)
This is a 7.4V, 1900mAh lithium-ion battery for the Motorola MTP8500, MTP8550, MTP8550Ex, and MTP8500Ex TETRA portable radios. It replaces OEM part numbers NNTN8570, NNTN8570A, and NNTN8570B. The pack fits the same bay and connects with the same contact strip as the original.
- MTP8500 and MTP8550 platform fit: All four models in this series share the same 7.4V battery bay, contact layout, and BMS handshake protocol. Swapping between Ex and non-Ex variants does not require a different pack — the BMS communicates identically across the platform.
- Bench tested on actual hardware: We cycled this pack on an MTP8550 dock and monitored the BMS through charge acceptance, PTT transmit spikes, and cutoff thresholds. The protection circuit held within the expected voltage window under sustained RF output.
- First insertion on a Motorola dock: If the charger shows a fault LED on first insertion, remove the battery, wipe the gold contact strip with a dry cloth, and reseat firmly. The Motorola dock requires a clean contact cycle to complete the initial BMS handshake before charging will begin.
Why the MTP8500 cuts out mid-transmission on a freshly inserted battery
New lithium-ion cells ship at storage voltage — typically 3.6–3.7V per cell, not full charge. When a radio draws the PTT transmit current spike from a cell at storage voltage, the BMS can read a momentary undervoltage condition and trip the overcurrent protection. The radio cuts mid-transmission not because the pack is faulty, but because the cell voltage is too low to absorb a high-current draw without a brief sag. Charge the pack fully before putting it into service — once both cells reach 4.2V, the BMS headroom is sufficient to handle transmit current without tripping.
Bar indicator showing one fewer bar than expected on a new pack
The MTP8500 uses voltage-threshold bar indicators, not a fuel gauge chip. Each bar corresponds to a voltage bracket, and a new pack at storage voltage typically sits in the second-lowest bracket. This is not a capacity fault — the cell simply hasn't been charged yet. Put the pack on the dock for a full charge cycle and the indicator will step up to the correct level. At full charge, each cell should read approximately 4.2V, placing the display at the highest bar threshold.