Cobra 840 Cordless Phone Replacement Battery 3.7V 180mAh

Cobra 840 — 3.7V Li-Polymer Replacement Battery

This is a 3.7V, 180mAh lithium-polymer battery for the Cobra 840 cordless phone handset. It replaces the original cell when the handset stops holding a charge or dies during calls. Dimensions are 30.60 × 16.00 × 5.10mm — confirm these against your existing battery before ordering.

• Cobra 840 handset fit: The 840 uses a slim Li-Polymer cell to fit inside the compact handset housing. Voltage must match at 3.7V nominal — the handset's charging circuit and low-battery cutoff are calibrated to this rail. A mismatched cell trips the protection circuit before the handset powers on.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on a cordless phone test rig. The BMS held cutoff correctly at low voltage and accepted a full charge from a standard 5V base station input without thermal events.
• First charge on the Cobra 840: Li-Polymer cells ship at partial charge for transport safety. Place the handset in the base immediately after installation and leave it for a full charge cycle before use — the 840's charging circuit needs uninterrupted contact to initialise the new cell correctly.

Talk time shorter than expected after fitting this battery

A new Li-Polymer cell in a cordless handset often delivers less than rated capacity for the first two or three cycles. The cell's internal resistance is slightly elevated until it has been fully charged and discharged a few times. Run the handset down to low-battery warning, then return it to the base for a full charge — repeat this two to three times. Capacity stabilises once the cell has been through these conditioning cycles.

Handset loses pairing with base station after battery replacement

Removing the battery cuts all power to the handset, which clears the DECT pairing stored in volatile memory on some Cobra models. The handset comes back up with no registered base, so it shows no signal or fails to connect to calls. Re-register the handset to the base using the pairing button — on most Cobra 840 units, hold the page/find button on the base for five seconds until the handset prompts for registration. Once paired, verify the handset shows full signal bars at close range before testing calls.