About this replacement
Motorola GP88 / GP300 Series — 7.5V Ni-MH Replacement Battery (HNN9628)
This is a 7.5V, 2500mAh Ni-MH replacement battery for the Motorola GP88, GP300, GP600, GTX800, and related handheld two-way radios. It replaces OEM part numbers including HNN9628, HNN9628A, HNN9628AR, HNN8133C, HNN8308A, PMNN4005, and WPNN4040, among others. These radios are used in security, construction, and emergency services where consistent voice transmission matters.
- GP88 / GP300 / GP600 / GTX800 platform fit: These models share the same battery bay geometry, contact layout, and 7.5V supply rail. The BMS handshake across this platform is consistent, so one pack design covers the full range without modification or adapters.
- Bench tested on actual hardware: We ran this pack through charge cycles on a GP300 dock and monitored BMS behaviour under PTT load. The overcurrent protection tripped correctly during simulated TX spikes, and the pack accepted a full charge without fault LED events after the contact seating cycle.
- First insertion on the GP88 dock: If the dock shows a fault LED on first insertion, remove the pack, wipe the gold contact strip with a dry cloth, and reseat firmly. The GP88 platform requires a clean contact cycle to complete the BMS handshake before the charger accepts the new pack.
Why the GP88 cuts out mid-transmission on a fresh pack
Pressing PTT draws a sharp current spike — typically 1.5–2A above the radio's standby draw — as the TX stage fires up. On a new Ni-MH pack at storage voltage, the BMS may read that spike as an overcurrent event and trip the output for a fraction of a second. This produces an audible cut or squelch at the start of your transmission. Running two full charge-discharge cycles brings the cells to operating voltage and the BMS trip threshold stabilises, eliminating the dropout.
Bar indicator showing one fewer bar than expected on a new pack
The GP88 and GP300 use a simple voltage-threshold bar display — each bar corresponds to a voltage band, not a capacity calculation. A new Ni-MH pack ships at storage voltage, typically 6.8–7.0V, which sits below the threshold for the top bar. This is not a cell fault. Charge the pack fully in the dock until the charge LED goes solid green, then allow a brief rest period — the open-circuit voltage will settle above 7.2V and the display will read correctly.