steely dan

Thanks for all your replies. I relented and called the supplying dealer and spoke to a senior technician. The engine is started via one of two electric motors within the gearbox , these are powered from the hybrid batteries. The boot mounted 12V battery powers the ignition circuitry inc. all required ECU's. The alarm would have initiated when the car was unlocked with a low voltage available at the ignition battery. This battery can be only be trickle charged to full power hence my "smart" charger being confused..... The violent push from the brake pedal is the brake booster response to low battery condition. The car can be jump started as correctly described in the manual above either by attaching the jump leads directly to the battery or via the red plastic covered power point within the engine bay and a suitable earth point. The hybrid batteries should hold their charge for 12 months or more and if unfortunately they do lose their charge a special Lexus charger is required . This is so rare that even the supplying dealer hasn't got one and has to have it loaned from Lexus themselves ......... So , I'll connect a trickle charger to the ignition battery and leave it for several days and then try it again. Thanks

Oh and theres a little notice inside the boot (albeit on the older GS) that says do not jump start this vehicle!

2014 GS 450h F sport . Car inherited by my wife in her new post , it's been sitting unused for 6 months . I tried to start it but 12v ignition battery is flat .Connected smart charger to battery in boot after disconnecting the neg connection and it wasn't having it ,charger says the battery is fully charged. Followed the handbooks instructions to the letter to jump start it via second vehicle with it's engine running but again no joy . When doing this and holding the brake pedal down it pushed back quite violently when start button engaged. Last time the car had any charge in the boot mounted battery the alarm went off . My question is how long does the main ( hybrid ) batteries hold sufficient charge to "start " the engine with a fully charged 12V ignition system battery fitted ,and has the last alarm initiation immobilised the damned thing....... TIA.