LightHawk 9B-1963-2 Replacement Battery 7.4V 10200mAh Li-ion

LightHawk KLS600FX Series — 7.4V Li-ion Replacement Battery (9B-1963-2)

This is a 7.4V, 10200mAh Li-ion replacement battery for the LightHawk KLS600FX series flashlights. It replaces OEM part numbers 9B-1963-2 and 9B-1963-3. The KLS600FX is a high-intensity LED flashlight used in search and rescue, tactical, and outdoor applications.

• KLS600FX series fit: Both 9B-1963-2 and 9B-1963-3 share the same 7.4V two-cell Li-ion configuration, connector pinout, and BMS communication protocol — the flashlight driver reads charge state the same way from either part number.
• Bench tested on actual hardware: We ran this battery through full charge and discharge cycles on the KLS600FX platform. The BMS held the 4.2V per-cell ceiling without deviation and tripped the under-voltage cutoff cleanly at 3.0V per cell, protecting both cells equally.
• Multi-cell replacement rule: The KLS600FX draws high current in turbo mode — up to several amps. If one cell is older than the other, its higher internal resistance causes it to sag faster under load. Replace both cells as a matched pair from the same batch to keep internal resistance balanced across the pack.

Why the KLS600FX steps down output before the battery indicator reads low

The KLS600FX driver monitors cell voltage under load, not at rest. In turbo mode, high current draw causes a temporary voltage sag below the driver's step-down threshold — even when the cells still hold a strong resting voltage. The driver interprets this sag as low charge and drops to a lower output mode to protect the cells. This is not a fault; it is the driver's brownout protection working as intended. Switching to standard mode at this point lets the light run at full output again because the lower current draw eliminates the sag.

One cell draining faster than the other in the KLS600FX pack

If you mix a new cell with a partially degraded cell, the weaker cell has higher internal resistance and delivers less energy per cycle. Under the KLS600FX's high-current turbo draw, the weaker cell hits its under-voltage floor first, forcing the BMS to cut output to protect it while the stronger cell still has capacity remaining. Over repeated cycles this imbalance compounds — the weaker cell gets deeper discharges and degrades faster. The fix is to replace both cells simultaneously; check that resting voltage across both reads within 0.05V of each other before first use.