Rx400h intermittent flat battery

[Lebouef]

Hi I'm very new to this but reaching out to you because I am loathed to take to my main dealer!

Out 400h has been faultless for 8 years apart from the normal wear and tear but since lockdown I have had problems with the aux Battery! Now it's well documented that there's a problem with these and was just about to replace when it didn't start again. I took the other car for that journey but and when I came back the Lexus started just fine so it is totally intermittent? I have cleaned up the earth strap but that has made no difference?

Any ideas would be great. It may well be a faulty aux Battery but it seems strange to be flat one minute then charged 10 minutes later?

 

[ColinBarber]

If you have a volt meter measure the voltage across the Battery - that will give you a good indication of whether you need a new Battery or not. I highly suspect you do.

[Herbie]

What Colin says above.

This may help:

[Lebouef]

Thanks guys will see if I can get hold of a voltmeter and take a look. 

How can a Battery be what seems flat one minute and then charged the next though? Surely a Battery is either charged or not?

[Britprius]

4 hours ago, Lebouef said:

Thanks guys will see if I can get hold of a voltmeter and take a look. 

How can a battery be what seems flat one minute and then charged the next though? Surely a battery is either charged or not?

All batteries recover a little of there charge when used discharged. When you first go to get in your car as you approach the brake booster pump comes on to bring up the brake pressure. you then open the door the locks opening, and then put you foot on the brake bringing on the brake lights, and press the start button to boot the computers. All this uses the Battery. If the voltage is to low the car refuses to go to ready mode.
Leaving the car the Battery recovers a little, and returning a short while later the brakes are already pressurised, and the Battery has enough voltage to get to ready mode.

John.

[Lebouef]

So after buying a voltmeter the voltage across the Battery was down at 10.3V so not good at all! Started the car up and the voltage jumps to 14v which is a good sign that it's charging so I have bought another Bosch S4 Battery as it's the same that was in the car anyway and it has lasted quite well considering the reports on here about poor performance and so far so good so hopefully that's a fix!

Thanks everyone for all your help.

[Thackeray]

1 hour ago, Lebouef said:

the voltage across the battery was down at 10.3V so not good at all! Started the car up

Interesting that it sounds as though the car started ok with the Battery at 10.3 volts. Maybe that was after the brake pressure was already boosted and the Battery had had a moment to recover. Or did you have to jump start it?

I've certainly started my car without problems for several months at around 11.5 volts. And reading a Prius forum I came across a report of someone who said they had had no problem starting the car with the Battery at 8 volts. This person commented that the computers run on 5 volts, so if that's correct obviously 8v would be enough to start the computers. Though it seems unlikely to be enough to boost the brakes and obviously the voltages may be different on Lexuses. There was much discussion that the boost process happened on this generation of Prius when the driver's door was opened, so by opening a different door you could avoid it happening to save the Battery and have a better chance of getting the car into Ready mode. If Lexuses boost the brakes as you approach (as Britprius mentioned above), instead of when you open the door, I suppose you could avoid this happening by turning off the key, if that's an option on a particular model.

So did it actually get into the Ready state at 10.3 volts without the need to jump start it?

[ColinBarber]

5v isn't inherently provided by the vehicle so the ECUs have 5v and 3.3v regulators or DC-DC converters in them to step down the 12v to 5v and/or 3.3v needed for the computer electronics - those systems expect a certain voltage above 5v to work. As the vehicle is powered on there is often a surge in demand from other systems which pulls the 12v rails down well below 8v if you have a bad Battery which affects the reset circuits on the computers and sensor reading, which is why you often get spurious error codes seemingly unrelated to the actual problem.

The RX400h, being an older generation vehicle and hybrid system, seems to cope better with a lower Battery voltage than say an IS300h but it is unpredictable. As well as the brake booster, you have auto lights that could come on if it isn't bright outside when you turn the key, the auto wipers may move if there is condensation on the screen, the steering wheel will start moving as soon as the key touches the ignition barrel - all of this makes it difficult to predict if you will be able to get into 'Ready' mode when your aux Battery is at a certain standing voltage - and it depends on the Battery condition and ability to hold a charge, not just voltage.