Jump starting a hybrid…

My Renault has slightly similar behaviour with the handbrake, if it has applied it abnormally[1], ie not by you switching the "engine" off or manually applying the brake using the switch, it then requires you to manually release the brake - although it does say this on the display if you try to drive off.

I suppose the designers have worked through what could go wrong when the car is not sure about the recent sequence of events.

nib

Regular (non-'mild') hybrids have their 12V battery as essentially an accessory battery to power the electronics and ancillaries (lights, radio, wipers). On starting it has to move the high voltage battery contactors to de-isolate the traction battery. This can take a big gulp of current, which a nearly-flat battery can't provide. Hence the lights come on but it refuses to 'start' - you don't hear the contactors click.

When you 'jump' them, all you're doing is assisting the existing battery in smoothing out the voltage dip when those contactors take their big gulp of current - but the accessory battery doesn't get to take much charge in those few seconds. The high voltage traction system is now live and starting to charge the 12V but if you remove the 'jump' battery you're now depending on the flat accessory battery again.

I don't know the electronic handbrake systems but it's possible it uses the

As you say, a 'graceful' shutdown is not to just disengage the handbrake and call it a day - but you do need battery power to disengage it. When the battery is flat there's not much it can do.

I replaced my Prius 12V OEM battery with an AGM one from a UPS, which is much better able to handle the low states of charge it gets when it doesn't get driven for long runs and so the 12V gradually runs flat (the charging system only gives it 4A so you need to drive for hours to recharge it fully).

Theo

I had a Prius during the pandemic and had to call the AA twice, because the battery had gone flat and I hadn't been out much. Afterwards, coincientally, the dealership emailed offering small solar panels that would trickle charge the battery. Knowing that whatever they were offering wouldn't be cheap I went on Amazon and bought one for about £20. Coincidence or nor, I don't know, but I haven't had a flat battery since.

That will likely help a little, especially if you are also driving it regularly. On mine I set up a solar panel to fit in the quarterlight. Problem was the 'solar charge controller' I was using would take more current than the solar was generating and flattening the battery further.

Seems it was possibly the 5V regulator dragging it down, it was one of these trashy units:

Theo

I used one even before the hybrid, as my use of the car was intermittent. That was after the pandemic when it went completely flat!

I now have a hybrid (same model of car though) and the battery is wedged into the rear wing and of course tiny. The touch locks on all but the driver's door are shut down if the battery is low.

It was noticeable aftter a week of hot weather (and the panel facing south because of where the car had been parked) that the locks started to work!

I had an Outlander hybrid. Every day at 2pm, it checked if it had been driven in the previous 24 hrs and, if not, it would ‘top up’ the 12v battery using the drive battery and an onboard 12v charger. I could leave it for 6 weeks at least and it would start, on electric if there was drive battery * available. I never tried longer than I recall.

I now have a Rav 4. I have yet to determine if it has a similar system. If it has, I can’t find a reference to it. The drive battery is larger - just over 50 miles on a good day. For local trips, we rarely use petrol. Toyotas are great but their handbooks less so.

I don't think this is *universally* true...be warned

Thanks all, some interesting and informative stuff here.