Sony NTM-910 Baby Monitor Replacement Battery 2.4V 1500mAh

The parent unit shows full bars after charging but loses signal within a few metres of the camera — what's wrong?

Range on the NTM-910 is directly tied to the radio transmit voltage in the parent unit, and a partially degraded or not-yet-fully-charged Ni-MH cell sags under radio load even when the charge indicator reads full. Ni-MH cells also show a flat discharge curve that makes the gauge unreliable until the cell has completed several full charge cycles. Charge the parent unit for the full recommended charge period before testing range — a Ni-MH cell that reads "full" after only an hour on charge has not yet reached its working voltage of 2.4V under load. Run two or three full charge-discharge cycles to condition the cell before judging range performance.

The night vision on the camera unit stopped working after I swapped the battery in the parent unit — are they related?

Night vision is controlled by the camera base unit, not the parent receiver, so a battery swap in the parent unit does not directly affect the IR LEDs. What does happen is that the re-pairing process sometimes completes with the parent unit in a default state where IR mode is toggled off. Go into the parent unit menu and manually enable night vision or auto-IR mode — it is a setting that can reset to off after a fresh pairing. If the IR indicator light on the camera base is on but the parent screen still shows a washed-out image, charge the parent unit fully, as display brightness and video processing are the first things to degrade when the cell voltage is low.

The NTM-910 parent unit won't turn on at all after I installed the new battery — what do I check first?

A Ni-MH cell that has been stored for an extended period can arrive at a low enough voltage that the parent unit's protection circuit will not initialise — this is not a fault with the unit or the cell. Connect the parent unit to its mains charger for at least 15 minutes before attempting to power it on; the charger bypasses the BMS lock-out and pushes enough current into the cell to bring it above the minimum startup threshold. If the charge indicator LED does not light within two minutes of connecting the charger, reseat the battery to ensure the contacts are fully engaged. Once the LED confirms charging, leave it on charge for the full cycle before powering on.