About this replacement
VOLVO V50 / S40 Siren — 4.8V Ni-MH Replacement Battery (4V250H)
This is a 4.8V 230mAh Ni-MH replacement battery for the VOLVO V50 Siren and S40 Siren alarm systems, including units referenced under part numbers 8696043 and 31252084. It slots directly into the siren housing and powers the audible alert function when the vehicle alarm triggers. Voltage and capacity match the OEM spec — 4.8V, 230mAh (1.1Wh).
- V50 and S40 siren compatibility: Both siren units share the same 4.8V NiMH cell configuration and housing footprint. The BMS on these sirens draws a short, high-current burst to drive the sounder — this battery's cell arrangement sustains that spike without tripping the protection circuit.
- Bench tested on actual hardware: We cycled this battery through charge and discharge on a V50 siren unit. The BMS accepted the charge without faulting, and the sounder activated at full output on the first test trigger after a full charge cycle.
- Cover and tamper switch check after fitting: After installing this battery, close the siren cover fully and verify the tamper switch clicks into the engaged position. These sirens feed tamper switch status back to the alarm panel — a partially latched cover can register as a battery fault on the panel display, even when the battery itself is fine.
Why the V50 siren goes silent mid-alarm after a battery swap
The V50 siren draws a sharp current spike the moment it activates. A new NiMH battery installed without a prior charge sits below the voltage threshold needed to sustain that draw for the full alarm duration. The sounder fires briefly, then the protection circuit cuts output to prevent cell damage. Allow a full 24-hour charge on the alarm panel before running an activation test — that brings the cells to a stable charge state where they can handle the activation current without dropping below the cutoff threshold.
Siren battery depleting faster than expected between annual tests
Outdoor siren housings expose the battery to wide temperature swings across seasons. NiMH cells self-discharge faster when cycling through heat and cold, which is normal chemistry — not a fault with the battery or the alarm. If the siren fails its annual activation test, the battery has likely self-discharged below the 4.0V minimum the sounder needs to fire. Replace the battery annually, or before the scheduled test if the siren has been through an unusually hot or cold period.