I have realised that I'd dived in to the forum with my 'newbie' questions without researching other threads that cover the same or similar issues. ( My excuse is that having taken the plunge & gone for the car this issue suddenly loomed much larger as it's clearly a very practical and potentially annoying one especially for someone like me who has been spoiled by having cars - at least in the last few years - which basically just work without me having to do or think about anything ! Colin's last posting is a case in point - I need to stop being lazy & pay a bit more attention to the car !) Having belatedly checked through the other threads I've found a few issues in particular which I think worth highlighting (see http://www.lexusownersclub.co.uk/forum/topic/43199-battery-failure which dates from 2007):
1. The rxh_driving_performance_guide at http://www.lexus.com...mance_guide.pdf contains the following guidance: " ....to help
keep your vehicle’s 12-volt battery fully charged: Drive vehicle at least weekly. Operate accessories with vehicle in “READY” mode. When parking, make sure all doors are closed and lights are turned off."
Long-term parking.
If the vehicle has been parked for two weeks or longer, do not operate the power rear door until you have turned the key to “START” and
the “READY” light stays on. If the vehicle has been parked for a long time, the 12-volt and the hybrid battery will discharge gradually. For parking longer than about 30 days, charging of the 12-volt battery may be required. See your dealership for details. To keep the hybrid battery in good condition, drive the vehicle at least once every several months for at least 30 minutes or ten miles."
2. The same doc says "If the hybrid battery becomes fully discharged and the vehicle will not start, even with a jump start to the 12-volt battery,, contact your Lexus dealership". However, the Honest John website review of the 400h says "11-5-2011: DO NOT JUMP START. Hybrids should be trickled charged, not jump started (bulletin from Toyota how to do it correctly). If a recovery agent connects jump leads directly to either hybrid battery and than to his battery the sudden charge can damage circuits with in the hybrid system (not necessarily the ECU). It seems that not all recovery companies are familiar with the correct jump start process . Damage can be caused which will not be apparent at the time."
Am I correct in thinking that the latter statement does not mean that the former is wrong as they are talking about the traction and 12v auxilliary batteries respectively - so a solution to the flat 12v battery problem is to seek a jump start from another (any other ?) car. But if this is OK and bearing in mind the small amount of current required to be able to start the 400h, is such a jump start better done without the donor car's engine running to minimise the risk of any voltage spikes ?
Another solution suggested is to carry a small 'spare' 12v battery that can be connected across the original one if that is flat just long enough to start the car (although other posts suggest that you have to keep the revs going in the 400h for about 20 mins until the regular 12v battery has been recharged). There's also an interesting calculation in the referenced thread which seems to explain why batteries go flat after around 10 days.