What caused my Lexus CT200H's battery to discharge in three weeks?

I am the owner of a Lexus CT200h, which is just over one year old and has 11,000 miles on the clock. Over Christmas I left my car parked locked and secure in my garage for three weeks. On my return I found that all batteries were completely flat and a jump-start was necessary. Lexus subsequently checked the car reporting no faults, although from the paperwork it would appear that they only checked the batteries for recharge status. I have tried to speak to a Lexus mechanic but get no further than the admin staff and I therefore wonder if you have any idea as to the problem? The manual states that the car should be driven once every few months to keep the batteries in good condition. Many thanks for your time.

Asked on 6 April 2013 by BB, Fordham, Cambs

Answered by Honest John
It's most likely to be a faulty interior light dimmer which, after slowly switching the light off, remains open circuit. Try switching the interior light off completely whenever you leave the car for some time. Also switch off the radio at the radio and switch off the heater fan at the heater fan switch. Do not relay on the ignition or ‘start’ button to isolate them.
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