About this replacement
MetalSub FX1211 / PR1209 — 6V Ni-MH Replacement Battery (105020816)
This is a 6V, 5000mAh Ni-MH replacement battery for the MetalSub FX1211 and PR1209 flashlights. It replaces OEM part number 105020816 when the original cell pack no longer holds a charge. At 30Wh, it restores full output to both models without modification.
- FX1211 and PR1209 compatibility: Both models share the same 6V cell pack format and connector pinout. The driver circuit in each light is tuned for this voltage rail, so swapping in a correctly rated Ni-MH pack keeps the driver operating within its intended range.
- Bench tested on actual hardware: We ran this pack through charge and discharge cycles on the FX1211 driver. The BMS held cutoff cleanly at the low-voltage threshold and the pack accepted a full charge without thermal event across multiple cycles.
- Multi-cell replacement rule: This pack contains multiple Ni-MH cells wired in series. Never mix this new pack with individual cells of unknown cycle count — mismatched cells cause the weakest cell to invert under load, shortening total pack life and generating excess heat at high draw.
FX1211 output stepping down before the battery indicator reads low
The FX1211 driver uses a brownout protection threshold that kicks in before the fuel gauge registers empty. As the Ni-MH pack voltage sags under load, the driver steps output down to protect itself — this happens faster at high-current modes. The pack voltage can read 5.8V on a meter while the light has already dropped a mode. This is normal driver behaviour, not a faulty battery.
Driver flickering or mode-cycling near end of charge
If the FX1211 starts cycling between modes or flickering without input near the end of a charge cycle, the driver is brownout-tripping on voltage dips from the ageing cell pack. A fresh pack eliminates the dip that triggers the cycle. After fitting this replacement, verify the pack reads at least 6.0V off the charger before first use — anything below that suggests the charger did not complete a full cycle.