Using seat heaters in aux mode

[Ext3h]

MY 2024, Sport edition.

So I tried to use the heat seaters and ventilation on low while sitting on a parking lot, waiting. Put the car in aux mode, as waiting with ignition on isn't permitted.

All fine - until after about half an hour all warning lights light up, the central unit is locking up and the car turns generally unresponsive. Central pane of the HUD reports a critical failure in the brakes. Had to turn it off (taking several presses on the keyless ignition button), wait a minute and everything was back to normal.

Tried again - and half an hour later exactly the same.

Meanwhile, the high voltage battery still claims a 70% charge, and didn't flinch the least.

All fine if ignition is on - high voltage battery is showing a slow drain and the engine is running for 1-2 minutes every 20 minutes or so.

But in aux mode, something about battery management or even just monitoring appears to be failing.

Is the seat heater by chance running of the 12V net, and the 12V net is only recharged from the high voltage net when the ignition is on?

[richardfrost]

Is the seat heater by chance running of the 12V net, and the 12V net is only recharged from the high voltage net when the ignition is on?

I would say YES. Just start the car ready to drive away. Where does it say you can't do that?

Context: I have a 2016 Toyota Rav4 as my main car. So long as the car is displaying 'Ready', this is equivalent to ignition on, and I can sit there as long as I like. From time to time the engine kicks in to top up the 12v battery. The one time I forgot to do this, the car wouldn't start. AA came out and 'jump started' me from their battery and instructed me to put it in 'Ready' mode for about an hour. I couldn't drive it to recharge as I was parked in a field for the weekend at a 24 hour running festival and my car was my 'tent'. I fully expect the Honda system works the same way as Toyota, in this respect. I know the Honda drive system is different but the two battery setup is going to be the same.

[Kremmen]

I would never use 'aux' mode for anything with our tiny 12v battery

Leave it in 'on' and let the engine kick in when needed ....... which is what I did when first setting up the car

[shufty]

I would never use 'aux' mode for anything with our tiny 12v battery

Leave it in 'on' and let the engine kick in when needed ....... which is what I did when first setting up the car

...Indeedy, not worth risking anything else.
I made the mistake of playing with the settings when in aux mode. Had to wait for the HV battery to charge the 12v enough for me to just lock the car or put the windows up.

[Lord Voltermore]

Can you reasonably expect a relatively small 12v battery, or even the larger HV  battery ,to power high  demand things such as heated seats for very long,(half an hour or more)  whilst parked, , unless the car is able to  periodically restart the ICE and recharge the battery/batteries  as required? 
   The various systems that 'fail' with strange manifestations   are just the first  to malfunction due to a flat battery . No battery management system could extend a fundamentally   excessive battery  drain.   
 
Possibly a fully charged HV battery could power heated seats for longer than 30 minutes  if all its power were used to keep the 12v battery charged  but this would risk flattening and damaging the HV battery.

[Lincolnshire Rambler]

Agree with LV . My last two Nissan Notes only let you use the radio for 20 mins with the ignition switched off before it turned the aux circuits off. I think only the hazard warning lights ending up been the only noticeable item to still work after 20 mins. The jazz has a very small 12 v battery -it’s half the size and capacity my wifes’Toyota 1.0 l aygo with its stop start tech

[Ext3h]

All right, just going to straight out avoid aux mode from then on.

The part about not turning ignition on is due to some strict local laws were you aren't even allowed to use your smartphone or alike while the ignition is running. Gruntled police may take that as an excuse for a ticket.

Still worrying that apparently the whole 12V net will happily go into brown out, without even as much as a warning ahead of time. I would had expected it to shed consumers first before everything goes down at once, including safety critical ECUs.

[Lord Voltermore]

Even with conventional cars its always been the case that if you leave a car for too long on aux with things drawing power, even just the radio ,its possible to flatten the battery.The first warning back then was usually finding the car wont start.  And you couldnt expect heating without the engine running.     I have seen police officers having to push start a police car that I assume had been left too long at the scene of a crash with its blue lights running.  (or its radio) .

I agree that some sort of  early shutdown  of power consuming circuits  just  prior to everything else  failing would be a good idea.  From what others are saying its possible if things do go 'weird' that waiting a few minutes for the 12v battery to recover a bit may be enough to restore sufficent power to power up the car so the HV battery can start the ICE.,restoring safety critical circuits at the same time.    But the 12v battery would still need a long time/several miles driving   to charge enough  to allow another long session on Aux mode.   
'
A low mileage user who maybe  only do a short weekly trip to the supermarket then waits in the car while the 'expert' shops might need to bear this in mind.(  At least they wont be joining the other 'old codgers' who always seem to wait with their  trolly blocking the busiest and narrowest aisles .      )

If you frequently sit  for long periods on Aux  I would recommend carrying a  small rechargeable  'jump starter' powerbank in the car.  These are only about the size of a paperback book. If necessarily they could easily be connected to the 12v battery to get you out of trouble.   

[Nicksey]

Aux is always going to drain the battery, either radio or the more energy sapping electric seats.

Renaults only let you play a radio for about 10 minutes before it shuts it down. If memory serves, there was a message on the display that said something like 'radio shutting down to conserve battery power'.

[embee]

I think if I was wanting to use significant power while the car was "off " on any sort of regular basis I'd definitely invest in a powerbank as a backup.  There have been a couple of threads on these recently.

[Jazzik]

If I was wanting to use significant power while the car was "off " on any sort of regular basis I'd definitely give up on that idea...

[Nicksey]

I do find it odd though that a manufacturer like Honda do not have a cut out system built in, that when it detects a degeneration of the battery kicks in and turns everything off.

[CB72]

You may interested in my experience when parked,  the car had full charge on 12v battery,  when I switched on the heating,  the engine started, when I turned off the heating the engine stopped. I tried this several times,  always the same, still full charge on battery. This must be how the Jazz system is designed, the heating doesn't use the 12v battery if not necessary.

[Jazzik]

You may interested in my experience when parked,  the car had full charge on 12v battery

How/where do you know/see that the car has full charge on the 12v battery?

[HertsHonda]

@Jazzik,
Re:How/where do you know/see....

On Mk3 - If the Auto Idle Stop doesn't cut in & round window shows message "AIS not available + battery symbol"!?   
But beware, mine shows this message, all other battery dependent functions work ok and have done so for 9 months and several 1000 miles!!