About this replacement
Bigblue CB6500P — 14.8V Li-ion Replacement Battery (BATCELL21700X4)
This is a 14.8V 5000mAh (74Wh) lithium-ion battery for the Bigblue CB6500P, CB 6500P, and CB6500PB flashlights. It slots into the same battery compartment as the original pack and uses the same voltage rail. Replace it when your light drops brightness sooner than it used to or no longer holds a full charge.
- CB6500P / CB6500PB compatibility: Both variants run the same 14.8V four-cell architecture and share the BATCELL21700X4 part number. The driver circuit expects this voltage to regulate output across turbo and standard modes correctly.
- Bench tested on actual hardware: We ran this pack through full charge and discharge cycles on a CB6500P unit. The BMS handled cell balancing without fault, and the protection circuit tripped correctly at low-voltage cutoff before any cell dipped below safe threshold.
- Multi-cell replacement rule: This pack contains four 21700 cells. If your light takes separate cells rather than a prebuilt pack, replace all four from the same batch at the same time. Mixing aged and new cells causes the weakest cell to discharge first under the high current turbo mode draws, accelerating imbalance and shortening overall pack life.
Why the CB6500P steps down brightness before the battery indicator reads low
The CB6500P's driver uses brownout protection to step output down when pack voltage sags under load — even if the indicator still shows partial charge. This is not a fault. As the pack ages, internal resistance rises, causing voltage to sag more sharply during high-current draw. A new battery with lower internal resistance holds voltage steadier, so the driver stays at full output longer before stepping down.
Turbo mode draining the pack significantly faster than standard mode
Turbo mode on the CB6500P draws five to ten times more current than standard mode. That load is handled across all four cells in series, so a pack with even minor cell imbalance will hit low-voltage cutoff on the weakest cell well before the others are depleted. If turbo runtime has dropped sharply, check whether the pack voltage reads close to 14.8V fully charged — a reading below 14.0V at rest after a full charge indicates a degraded cell. Replace the pack and verify the charger is bringing voltage up to 16.8V at charge completion.