About this replacement
Kirisun PT4200 / PT5200 Series — 7.4V Li-Polymer Replacement Battery (KL-KB58B)
This 7.4V, 1600mAh lithium-polymer pack replaces the OEM KL-KB58B battery used in Kirisun PT4200, PT5200, PT558, and PT668 portable two-way radios. It matches the original cell chemistry, connector pinout, and BMS communication protocol expected by the radio's charge management circuit. Capacity is 1600mAh (11.84Wh) — taken from product data, not estimated.
- PT4200 / PT5200 platform fitment: These models share a common battery bay geometry, contact strip layout, and 7.4V nominal voltage rail. The KL-KB58B form factor was carried across the PT558 and PT668 as well, so one pack covers the full series without connector modification.
- Bench tested on actual hardware: We cycled this pack through transmit-load draws on the PT4200 bench unit. The BMS held the output rail stable through sustained PTT events and tripped correctly at the low-voltage cutoff threshold — no false shutdowns during normal TX cycles.
- First-insertion contact cycle on Kirisun dock chargers: 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 Kirisun charger platform requires a clean contact surface to complete the BMS handshake before it will begin the charge cycle.
Why the PT4200 cuts out mid-transmission on a freshly inserted pack
New lithium-polymer cells ship at storage voltage — typically 3.7V to 3.85V per cell, giving a pack voltage around 7.4V to 7.7V at rest. When you key up, the transmit current spike can pull that resting voltage below the BMS overcurrent threshold before the cell has been through a full charge cycle. The BMS reads this as an unsafe draw event and trips the output. A full charge before first heavy use brings the cells to 8.4V and gives the BMS the headroom it needs to handle PTT surges without tripping.
Bar indicator on PT5200 showing one fewer bar than expected with new pack
The PT5200 uses a voltage-threshold bar indicator — each bar corresponds to a specific voltage band, not a percentage from a fuel gauge chip. A new pack at storage voltage (around 7.5V) sits at the boundary between the second and third bar threshold, so the display reads lower than actual capacity. This is not a faulty cell. Charge the pack fully to 8.4V, and the indicator will step up to the correct bar count on next power-on.