Flat 12 volt battery

[Malcolm Kemp]

The day after a 200 mile round trip I was sat in my parked car with the ignition in the Accessory Mode and the radio on. After about half an hour I tried to start the car but it failed to start. I called roadside assistance and the AA patrolman said the Battery was flat. After jump starting the car, he said that the Battery needed to be replaced as it was not accepting a charge very well. Page 134 of the handbook says that the 12 volt Battery condition can be checked by the indicator on the top of the battery; BLUE means Good Condition, RED means Charging is Necessary, and CLEAR means Replacement is Necessary. The indicator on mine is clear so I had the Battery checked by the Lexus dealer. The Lexus technician said that there was nothing wrong with the Battery and he put it on charge for a couple of hours. He also said they do not accept the AA diagnosis, and despite what it says in the handbook I should ignore the inbuilt Battery indicator. He also said if I wanted to listen to the radio when parked I should keep it in Ignition On mode so the engine will keep cutting in and out.

I was thinking of fitting a BM2 Bluetooth 12 Volt Battery Monitor but as it draws a constant 1.5 milliamps I am not sure if this is a good idea. Can anyone tell me what the drainage current is when the car locked and completely shut down?     

[NemesisUK]

36 minutes ago, Malcolm Kemp said:

He also said if I wanted to listen to the radio when parked I should keep it in Ignition On mode so the engine will keep cutting in and out.

Follow the technician's advice. The 12v Battery is only for starting the computers and the entry/exit system. It can drain very quickly in accessory mode. It should be fine in a locked car.

That said I'm surprised the car didn't come up with a warning whilst listening to the radio, even shutting it down automatically...

[Malcolm Kemp]

Thanks for your quick reply Peter. My previous car was a plug-in hybrid BMW and I often used to sit with the engine off and radio on for long periods, and never had a problem. Obviously the Lexus is different, so I'll follow your advice.

Cheers,

Malcolm