Curtis Mathes F690 Compatible Battery 6V 4200mAh Ni-MH

Curtis Mathes F690 / FV600 Series — 6V Ni-MH 4200mAh Replacement Battery

This is a 6V Ni-MH replacement battery rated at 4200mAh (25.2Wh) for the Curtis Mathes F690, F820, FV600, FV900, and six additional compatible models. It uses the same voltage rail and cell format as the original pack. Fits the camera body without modification to the contact plate or door latch.

• F690 / FV-series platform fit: The F690, F820, FV600, and FV900 share the same 6V battery cavity and contact pinout. One cell format covers the full range — no adapters, no rewiring. The BMS in each body reads the same charge termination signal from this Ni-MH pack.
• Bench tested on actual hardware: We cycled this pack through charge and discharge on a camera body load simulator. The BMS accepted the cell on the first charge cycle and terminated correctly at peak delta-V — no overcharge events, no premature cutoff under flash-draw conditions.
• First-cycle conditioning for the F690: Run one full charge inside the camera body or OEM charger before heavy shooting. The F690's battery-remaining indicator maps to the discharge curve on that first cycle — skipping it causes the indicator to read incorrectly for the life of the pack.

Flash capacitor recharge sag at the end of a Ni-MH cell's cycle

Ni-MH cells have a gradual voltage taper in the final 15–20% of discharge. The flash capacitor recharge circuit pulls a burst of current to top up between shots. When cell voltage is already low, that burst causes a momentary sag that the camera body reads as pack failure. The body either locks the shutter or disables the flash entirely before the cell is truly empty. This is not a fault in the replacement cell — it is the body responding to a real voltage event. Keep shots per charge within the body's rated count and the sag stays within tolerance.

Battery percentage jumping erratically on the F690 display after fitting a new cell

The F690 maps its battery indicator to voltage thresholds calibrated during the first charge cycle. A new Ni-MH cell has a flatter discharge curve than a partially aged pack, so the body's lookup table misfires — percentage can jump from 80% to 20% with no warning. The fix is one full charge-to-discharge cycle completed inside the camera body. After that cycle, the indicator tracks correctly because the body has sampled the actual discharge curve of the new cell. Do not pull the battery mid-cycle — let it run to the body's auto-shutoff at approximately 5.0V.