Bigblue TL4000P Replacement Battery 14.8V 3400mAh

Bigblue TL4000P Series — 14.8V Li-ion Replacement Battery (BATCELL18650x4)

This is a 14.8V 3400mAh lithium-ion battery pack for the Bigblue TL4000P tactical diving flashlight and its close variants. It uses four 18650 cells wired in series to match the original pack's voltage rail and physical form factor. Compatible models include the TL4500P, TL4800P, and VTL5500P, among others.

• TL4000P series platform fit: These lights share the same four-cell 14.8V architecture, connector pinout, and driver input range — so one pack covers the full lineup without modification.
• Bench tested on actual hardware: We ran this pack through full charge and discharge cycles on the TL4000P driver. The BMS handled high-current draw at full turbo output without tripping into protective cutoff, and cell balancing stayed within acceptable variance across all four cells.
• Multi-cell replacement rule: Replace this pack as a complete unit — never mix cells from different batches or cycle counts. In a four-cell series pack under high turbo draw, the weakest cell drains first, gets reverse-charged by the stronger three, and can fail permanently within a handful of cycles.

Battery draining faster in turbo mode than the indicator suggests

Turbo mode on the TL4000P draws significantly more current than standard or medium modes — often five to ten times the amperage. Voltage under that load drops sharply across all four cells, which can make the pack read as depleted before the resting voltage confirms it. This is normal discharge behaviour under high current, not a faulty battery. Drop to standard mode when you need to extend a dive or work session — the pack will recover its indicated charge level as current draw falls.

One cell reading low after a full charge cycle

If one cell in the pack shows noticeably lower voltage than the other three after a full charge, cell imbalance has set in — most often from mixing an older cell with newer ones during a previous repair. In a series pack, the weakest cell caps the entire pack's capacity and takes the heaviest stress on every discharge. Check individual cell voltage with a multimeter across each pair of contacts; a healthy 18650 after full charge should read between 4.15V and 4.20V. Replace the full pack rather than a single cell to prevent the cycle repeating.