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Losing all electrical power in a car - modern things that should not happen

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tom66:
The other week, I purchased a 2015 Volkswagen Golf GTE, a plug-in hybrid car.

Today I was driving along and the car popped up with a message, "12V battery not charging.  Car may not restart.  Visit workshop soon!"

This isn't great, but I decide to continue driving. A mile later everything dies.  Lights, instrument cluster, brake assistance,  power steering.  Everything.  At 40 mph.  And the worst part is that the electronic parking brake wouldn't engage, and I'm on a hill, so I'm left pressing the brake pedal down continuously whilst I'm completely unlit on a dark road. 

My girlfriend was sitting in the car.  Luckily, she was able to hold the brake down while I jump started the car.  After doing this,  the car drove OK. 

So, on examining the fault, when I got the car home, as soon as I turned off the ignition,  the car died immediately. All power lost.  Nothing worked any more.

I checked the battery and it was measuring just 1.8 volts.  So,  the battery has clearly failed.  Here's what gets me.  This happens on petrol cars too,  but they can continue running as long as the engine is still going.  This car cut out while driving.  Presumably the DC-DC converter in the hybrid system briefly turned off, perhaps when switching between petrol and electric, but there was no battery to "catch" it.  If the car knew the battery was potentially faulty, why did it shut down the DC-DC? 

In my mind, this shouldn't be a fault that can happen (and from reading about it, it's not uncommon on EVs and hybrids.)

thm_w:
EV/hybrids like to run everything critical off of the 12V lead acid battery, its probably cheaper/simpler/etc. Even tesla has one: https://teslamotorsclub.com/tmc/threads/model-3-12-volt-battery-location.107340/
Seems so ridiculous that they haven't been replaced by using the main li-ion pack, which would eliminate the problem you had, but they must have their reasons.

There are *some* gas cars that will have problems when the batteries dies mid drive as well. In the future you could check the battery health more often.

Ian.M:
You don't *know* the battery is bad though if its been discharged to 1.8V its shortened its life a lot even if you recharge it within 24H.   It could be 12V charging system failure, which is probably a buck converter off the main DC bus.

Try to recharge the 12V battery out of the car - if it holds a charge, you'll probably need to take the car to a dealer 's garage who can access the computerised diagnostics and error logs.  If it doesn't, a new battery has 50/50 odds of curing it.  If it doesn't charge the new battery, again its a job for the dealer.

Gregg:
Tom66, did this incident happen as you drove past one of the many haunts of Lucas, Prince of Darkness? 
This sort of thing used to happen a lot with my 1957 MGA until I replaced the Lucas generator with a US alternator, replaced all of the wiring which was getting low on magic smoke, got rid of the nasty dual 6 volt battery system and installed a PerLux Hall effect module to eliminate the points in the distributor.  The only Lucas parts remaining are the starter, distributor and turn signal switch.   It has been much more reliable but the ghost of Lucas occasionally teems up with Murphy to make me question my sanity.

grythumn:
I had this happen in an old Honda Civic Hybrid... doesn't use an alternator, and if the traction battery acts up, the DC-DC converter won't charge the 12V system.

Replaced the 12V battery to get home and then the car... wasn't worth putting a new traction battery in.

-R C

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