About this replacement
Nightstick TAC-400 / TAC-500 Series — 3.7V Li-ion Replacement Battery (400-BATT)
This is a 3.7V, 2600mAh Li-ion replacement battery for the Nightstick TAC-400, TAC-450, TAC-500, and TAC-550 tactical flashlights. It uses OEM part number 400-BATT and fits directly into the battery tube on all four models. These flashlights share the same cell format — 18650 cylindrical, 68.50 × 18.60mm — so one part number covers the full range.
- TAC-400 / TAC-450 / TAC-500 / TAC-550 compatibility: All four lights use the same battery tube diameter, the same charging circuit, and the same driver voltage floor. The 400-BATT part number was never model-specific — Nightstick designed the series around a single cell to simplify field replacement.
- Bench tested on actual hardware: We cycled this battery through full charge and discharge runs on the TAC-400. The BMS held stable voltage through turbo mode draw and tripped at the correct low-voltage cutoff without any false triggers or premature shutdowns.
- Turbo mode heat management: The TAC-series driver pulls significantly higher current in turbo mode than in standard or low modes. If you store the light unused for several weeks and then run turbo immediately, the cell voltage may sag faster than the indicator updates — drop to high or standard mode first to let the cell stabilise.
Why the TAC-400 steps down output before the battery indicator warns you
The TAC-400 driver uses a brownout protection threshold that sits above the battery indicator's low-warning trigger. When cell voltage drops under sustained turbo draw, the driver steps output down to protect the LED and the cell — before the indicator flashes. This is intentional behaviour, not a fault with the battery. If the light dims mid-use, switch to standard mode and the output stabilises at that level. The cutoff voltage the driver targets is approximately 2.8V at the cell terminals.
Flashlight cycling through modes on its own near end of charge
If the light starts toggling between modes or flickering without input, the cell voltage has dropped below the driver's stable operating window. The driver loses enough voltage to hold its current mode and resets its state. This is not a driver fault — it means the cell needs replacing or recharging immediately. Put the light on charge and confirm the charger shows a full charge before the next use. A healthy cell at full charge should read 4.1–4.2V off the charger.