About this replacement
Motorola PMNN4577A — 3.7V Li-ion Replacement Battery
This is a 3.7V, 3350mAh lithium-ion battery built to the PMNN4577A specification. It fits Motorola two-way radios that accept this pack format. Voltage and capacity match the OEM spec exactly — no step-down, no padding.
- PMNN4577A platform fit: Motorola two-way radios on this platform share a common voltage rail, contact layout, and BMS handshake protocol. Any pack that deviates from the 3.7V nominal or connector pinout will be rejected by the dock before charging starts.
- Bench tested on actual hardware: We cycled this pack through transmit-load draws and monitored BMS response at both low-state-of-charge and full-charge thresholds. The protection circuit held cutoff at the correct voltage floor without nuisance tripping under PTT current spikes.
- First insertion on Motorola dock: If the dock shows a fault LED on first insertion, remove the pack and wipe the gold contact strip with a dry cloth, then reseat firmly. Motorola charger docks require a clean contact cycle to complete the BMS handshake before charging will begin — this is not a defective pack.
Why a PMNN4577A pack can trip the BMS the moment you press PTT
Two-way radio transmitters pull a sharp current spike the instant PTT is pressed — this is the highest instantaneous draw the battery sees during normal use. A new pack at storage voltage (typically around 3.6–3.75V) has less headroom before the BMS interprets that spike as an overcurrent event. If the radio cuts out immediately on transmit, charge the pack fully to 4.2V before field use. A full-charge state gives the BMS the voltage headroom to absorb the transmit surge without triggering a protection cutoff.
Bar indicator showing one fewer bar than expected after fitting a new pack
Motorola radios use simple voltage-threshold bar indicators — each bar corresponds to a voltage band, not a percentage calculated by a smart chip. A new cell shipped at storage voltage sits near the boundary between two thresholds, so the indicator reads one bar lower than a fully charged pack would show. This is not a capacity fault. Charge the pack until the dock shows green, then power on the radio — the bar indicator will reflect the correct full-charge voltage level from that point forward.