LEXUS IS200 Siren Alarm Compatible Battery 7.2V 300mAh 28AAAM6BML

LEXUS IS200 / IS300 Siren — 7.2V Ni-MH Replacement Battery (28AAAM6BML)

This is a 7.2V, 300mAh Ni-MH rechargeable battery for the LEXUS IS200 and IS300 factory alarm siren unit. It fits OEM siren part numbers 89040-53011 and 89040-53012. The siren draws on this cell to sound the alarm when the vehicle's main battery is disconnected or interrupted.

• IS200 and IS300 siren compatibility: Both models use the same siren housing and OEM part reference 28AAAM6BML. The connector, cell count, and voltage rail are identical across these two variants, so one replacement covers both.
• Bench tested on actual hardware: We cycled this cell through charge and discharge on a matched siren unit. The BMS accepted the full charge cycle without cutoff errors, and the cell held voltage within spec across the test window.
• Siren cover and tamper switch check: After fitting the battery, close the siren cover fully and confirm the tamper switch is physically engaged. If the cover is not fully latched, many alarm panels will flag a tamper fault — and some interpret that as a battery fault, not a housing issue.

Why the IS200 siren stops sounding mid-alarm after a battery swap

A freshly installed Ni-MH cell may not hold the current draw required to sustain the siren's output for a full alarm cycle. Ni-MH cells ship at a partial state of charge and need at least one full charge before they can deliver consistent current. If the alarm triggers before the battery has charged, the cell voltage sags under load and the siren cuts out early. Allow the vehicle's alarm system to charge the siren battery for 24 hours before testing — the siren should then sustain output to at least 6.5V under load.

Siren battery voltage reading low after six months without a triggered alarm

Ni-MH cells self-discharge at a higher rate than Li-ion, typically losing 1–3% capacity per day at room temperature. Outdoor siren enclosures accelerate this — thermal cycling between cold nights and warm days increases the discharge rate further. A siren battery that sits untested through a full winter can drop below the minimum activation threshold without any fault ever showing on the panel. If the battery reads below 6.0V on a multimeter with the cell removed, refit and allow 48 hours of trickle charge before the next alarm test.