CheshireDriver

That wasn't my experience with the Yaris Hybrid. The dropping voltage on the aux 12 volt battery meant that the DC/DC converter was being powered by the engine runnng more frequently in an attempt to charge the failing aux battery. It was noticeably revving quite hard when the car came to a stop and would keep revving all the time, so I think it was wasting fuel. Whether it would apply to the Lexus 400h I don't know. It does seem that when stationary the NiMH traction battery will not charge the 12v aux battery unless the engine is running (via the DC/DC converter). I think there is also a difference in the N and P settings on the transmission. I believe that the aux battery is draining when stationary when set to N, and it is better to use P if stopped for any period of time in traffic. Especially if the AC is on as it uses an electric compressor rather than belt driven. The steering is also electric on both Lexus and Yaris hybrid. I think with both hybrid and electric cars, the absence of an alternator means that the 12 aux battery depends on the dc/dc converter to be sending current from the high voltage traction battery. A seriously failing 12 volt aux battery will tend to throw up error codes for this reason especially in electric cars. And there is a fair drain on the Aux 12V battery from the audio, power steering, AC compressor and lights in both types of car.

With respect I think a failing 12volt battery could affect the engine as the dropping voltage will mean that instead of running on the electric motors around town, the engine will be instructed to keep topping up the battery. This seemed to happen not on my Lexus but my wife's Toyota Yaris Hybrid which has a very similar drive train albeit with only one electric motor. When we first got this car it seemed to be revving all the time, when stationary, and the fuel economy was down a little. Once I spotted the battery voltage was down and I had to jump start it once, I installed a new 12 volt battery (Yuasa AGM £130ouch!) and the car suddenly worked better. Better economy and more running on the electric motor. Very economical in fact - now always above 60mpg and usually around 64-66. Not sure if this would apply to a Lexus Hybrid but the principle is similar. The correct 12v battery to put in my 400h is tricky as the space is not large and the terminals as fitted are the smaller Japanese type. I would really like to put in a 60ah battery if I could fit it in to give a better reserve of 12V energy. I have got on OK with the Bosch S4022 which is 45aH but the resting voltage is only 12.1volts so I think its days are numbered.