Under what circumstances can a car battery be too dead allow a jump start?

[tom66]

In my GTE I had a worse failure, the 12V fuse on the battery failed. So it cut out while driving because the DC/DC converter can only supply the average current and as soon as you exceed the limits the DC-DC shuts off immediately.   Since the power steering is electric from the 12V supply you only need to demand a lot of torque from that and it'll die.  Then you're stuck steering a car which coasts but with no power brakes or power steering, and no hazard lights, or way to wind the electric windows down to flag other motorists ...

Bad design IMO - would be extremely unsafe on a motorway!   Luckily the road this was on was not too busy.  The car had been warning me to service the 12V system before the failure but I thought I could drive it to the mechanics safely enough.  Little did I know...

[SilverSolder]

Not everything in the car is tested to that standard.  A friend had his alternator fail in a newish Ford...  on the way to the dealership to get it fixed, things started to fail as the battery got depleted...  the radio turned off,  the instruments stopped working, by the time he got to the destination, the ONLY thing still working was the engine - and it quit just as he rolled up to the garage door!  (At least Ford had the priorities right with that design.)

Pretty much the same thing happened with my old Peugeot.  The alternator would crap out on that car until you could restart it.  I was stuck on a motorway with miles to the next exit. First, the rear window demist function is disabled, then the cabin fan and A/C clutch, then the alternator light comes on, then the radio goes off, then all the instruments died.  By the time I found somewhere to safely pull over the only thing running was the engine and the single check-engine light on the dashboard.

This is one of the interesting things about owning a plug in hybrid (I have a Golf GTE now.)  The car doesn't have a conventional starter.  It uses the 400V battery and electric drive motor to spin the engine up. Consequentially, I've got in the car when it was -4C out and started the engine from stone cold.  It didn't even cough, because it can put 10kW straight into the engine to get it to turn over. The diagnostics show that the car engine is at 800 rpm before any spark is fired so it starts pretty much at the speed it idles at.  This contrasts to most starters which don't get over 200-300rpm because they can't get enough current from the 12V battery.  But a disadvantage is that VW tells me that the car cannot be used below -30C as the li-ion battery cannot safely provide that much power when cold. So if I move to Norway I might have trouble!

I have an older hybrid car in a climate that on rare occasions can approach -30C - and it is noticeably unhappy until the battery warms up!