Kirisun PT4200 Replacement Battery 7.4V 1200mAh KL-KB58B

Kirisun PT4200 / PT5200 Series — 7.4V Li-Polymer Replacement Battery (KL-KB58B)

This 7.4V, 1200mAh Li-Polymer pack replaces the Kirisun KL-KB58B battery in the PT4200, PT5200, PT558, and PT668 portable two-way radios. It ships at storage voltage, so expect the bar indicator to read low on first insertion — that is normal cell chemistry, not a fault. Seat the pack, charge to full before radio use, and the BMS will settle.

• PT4200 / PT5200 / PT558 / PT668 platform fit: These models share the same battery bay dimensions, contact layout, and 7.4V nominal voltage rail. The KL-KB58B form factor is common across the range, and the BMS handshake is consistent — swap is direct without reconfiguring the radio.
• Bench tested on actual hardware: We cycled this pack under simulated PTT load on the PT4200 platform. The BMS handled repeated transmit-current spikes without tripping to protect mode, and charge termination triggered cleanly at full capacity.
• Contact strip care on first dock insertion: If the Kirisun charger shows a fault LED on first insertion, remove the pack and wipe the gold contact strip with a dry cloth. Reseat firmly. The charger requires a clean contact cycle to complete the BMS handshake before it will begin charging.

PT4200 cutting out mid-transmission on a new KL-KB58B pack

When you press PTT, the radio draws a sharp current spike to power the RF output stage — significantly higher than standby draw. If the replacement pack is still at storage voltage (typically around 3.7V per cell), that spike can push the BMS into overcurrent protection before the cell voltage recovers. The radio drops the transmission and may restart or show low battery. The fix is a full charge cycle first: bring the pack to 8.4V before transmitting. After the first full charge, the BMS calibrates to the load profile and the cutout stops.

Bar indicator showing one bar fewer than expected after a full charge

The PT4200 uses a simple voltage-threshold bar indicator — each bar corresponds to a voltage band, not a percentage from a fuel gauge chip. A new Li-Polymer cell at rest can read slightly below the top threshold immediately after leaving the charger due to surface charge settling. Wait ten minutes after removing from the charger and check again. If the pack measures 8.2V or above at the battery contacts with a multimeter, the cell is full — the indicator will step up once the radio draws load and the resting voltage stabilises.