12 V battery

[roberts261]

My 12 v battery on my NH450 plus died the other day. The car was three months old and had less than 1000 miles.I could not unlock or start the car. Until this happened, I was not even aware that there was a 12 V battery which apparently powers all the ancillary electronics.  I simply assumed that the main battery powered everything. I don't drive far and rarely use HV mode but I do drive regularly (at least 2 times per week). I am wondering if this happened because the alternator only runs on HV mode. I suspect that the designers did this to increase the battery range and the car needs to be driven in HV mode at regular intervals in order to prevent this problem. Anyone have any information on this? Does anyone know if the driving in HV mode is necessary to run the alternator and thus keep the battery from running down? 

It seems that if operating on HV mode at regular intervals is necessary, owners of these vehicles should be warned and advised to drive on HV mode at regular intervals. 

[Lexus-CT]

A 3 month old car is warranted. Suppose you mean NX and not Nh.

12V battery powers up the computer that power up the high voltage battery that start everything else.

12V battery is rather tiny and if car stand still a long time, the battery lose power. The little CT we have is a 2017 model and 12V battery is still just fine.

Charge the 12V battery and it should start everything else again. But do mention it to the dealer as a brand new car should have a brand new battery that keeps charge well.

[roberts261]

Thanks. This a PHEV so the issues are different. The 12 V battery ran down and there can only be two causes:

The alternator is not working

The battery is bad. 

Since it has an electric engine and I rarely run it in HV mode I am interested in knowing if the alternator works with the EV engine. If not, it is obvious I will have to run the car regularly on HV mode in order to keep the battery charged. I drive the car regularly but rarely use the HV mode since my distances are short. 

[Lexus-CT]

There is no alternator in a PHEV. It is a hybrid, where you can charge the extra large high voltage battery home via a plug-in.

Hybrid cars have no alternator.

Like I mentioned before - the 12V battery starts the computer that then power up the big battery. The car can run on the big battery till there is too little power left in that and then the vary ordinary engine running on gasoline starts to charge the battery and power the wheels.

Call your Lexus where you bought the car and tell them to help. A new car should have some service free of charge.

[roberts261]

Thanks. I replaced the battery and the charge was covered by Lexus, although I have been unable to confirm if the battery was bad (long story). In the final analysis, either the battery was bad or the charging mechanism (alternator or otherwise) wasn't working. If the former is the case, then the problem has been solved (hopefully). If the latter, either that is a significant engineering defect or I need some direction on how to keep the 12 V charged. 

The question is whether the 12 v battery charges while the car is on EV mode. If the 12 V runs down after every few hundred miles, then that is not sustainable and if there is something in my driving habits (such as operating almost exclusively on EV mode) that needs to change, it would be useful to know. 

In the meantime, I am getting jumper cables. Until this happened, I had no idea that these were necessary for a PHEV.