I have an elderly friend with a Lexus RC300h and he is unable to drive it for six months due to having had a heart operation. He has covered the car up on his driveway. I was concerned that the 12v battery may discharge over time, due to alarms and immobilizers. I have a 'smart' battery charger and wanted to loan it to my friend, to periodically top up the 12v battery. He allowed me to look at his car, but I knew nothing about the RC300h at the time. My own car is a Volvo with a front mounted battery, and to trickle charge that, I connect the +ve charger clamp to the +ve battery terminal, and the -ve clamp to an earth point, and it charges fine. This is what I expected to do on the Lexus, but initially I could not find the 12v battery, and my friend's Lexus handbook was very little help!
I have since discovered from online sources the 12v battery is in the left side of the boot under a cover.
However I want to mention what my friend did to the car, on the advice of his dealer. My friend talked to his dealer about my concerns on the telephone. His dealer advised the following - start the car, put it in 'D', press the footbrake pedal, and at the same time press the accelerator pedal, rev the engine, and this will charge the 'battery'! My friend did this while I was present because I failed to find the 12v battery! I was horrified at first because any automatic car I've known before would destroy the transmission by such a procedure! However, a screen display inside the car indicated that the engine was indeed charging a battery, and my friend kept this practice going until all the bars on the battery display on the screen had turned green. He then thanked me for my efforts and told me he would repeat this process every few weeks for the duration of his driving suspension!
It later occurred to me both my friend and the dealer had mistaken my concerns about the 12v battery for the hybrid battery, and that my friend's practice of revving the engine in 'Drive' with the brakes on had recharged the hybrid battery and had not done anything for the 12v battery. I can only assume that the transmission was not destroyed because it is a robotized manual, and that keeping the brakes on must had disengaged the clutch. I have to say though, that I disliked the whole process.
Returning to the 12v battery, I assume that will still be in danger of discharging, especially if the car goes nowhere for six months and keeps getting crank started to recharge the hybrid battery, which I see as not important at this stage. I've found an online PDF manual and from that, found instructions for starting a car with a discharged 12v battery by using another 12v battery and jumper cables, connecting to a dedicated positive 'jumper terminal' under the fuse box lid under the bonnet, and using an earthing point for the negative jumper cable, which appears to be a metal shield at the side of the engine in the illustrations. I am wondering if these are safe connection points for a 'smart' trickle charger like I have, and should I contact my friend again, and try to convince him that he needs to keep his 12 v battery healthy, and not run the car in 'Drive' with the brakes on!