Does HV battery charging charge up a flat 12v battery?

[Cocijo]

I wondered. If your 12v battery is flat does connecting to a HV charger charge it up or does the 12v need to be charged up first before the HV charger will work?

 

[Kithmo]

Good question, will the 12v signal from the charger wake up and feed the onboard charger or will it need power from the 12v battery?
At a guess I'd say either a jump starter pack or a little bit of a charge up of the battery would be needed.

 

[Cocijo]

My friend asked me if plugging in her home HV charger would charge up her flat 12v battery. I didn’t know if this would work, hence the post. I guessed not, as the 12v would need some power for it to work?

 

[Coulomb]

If your 12v battery is flat does connecting to a HV charger charge it up

It depends. If it's so flat that it can't power the computers and pull in the contactors, you are like an ICE owner that needs a jump start.

But as long as it has enough for that, as soon as the contactors are pulled in, the DC-DC will start charging the 12 V auxiliary battery.

Note however that this will only gently charge it, at about 13.5 V, not at the 14 V or more when the car is in ready mode. I think that the reason for that is that a charge might be 10 hours or more, so that there is actually a chance of over-charging the battery, whereas when driving, it's often much less time. I think when driving, a computer also monitors the auxiliary battery charge current, and if it falls below a certain level for a certain time, it deems the battery full and drops back to the ~13.5 V float voltage. The auxiliary battery can be charged indefinitely at the float voltage without risk of damage from over-charging.

So if you find yourself with a flat or nearly flat auxiliary battery, and you are at home where you might have a 12 V battery charger, it's a good idea to charge the auxiliary battery while the main battery is charging. That way, the auxiliary battery gets away from the dangerously low state of charge the quickest, and therefore has the best chance of recovering fully.

 

[Lovemyev]

It depends. If it's so flat that it can't power the computers and pull in the contactors, you are like an ICE owner that needs a jump start.

But as long as it has enough for that, as soon as the contactors are pulled in, the DC-DC will start charging the 12 V auxiliary battery.

Note however that this will only gently charge it, at about 13.5 V, not at the 14 V or more when the car is in ready mode. I think that the reason for that is that a charge might be 10 hours or more, so that there is actually a chance of over-charging the battery, whereas when driving, it's often much less time. I think when driving, a computer also monitors the auxiliary battery charge current, and if it falls below a certain level for a certain time, it deems the battery full and drops back to the ~13.5 V float voltage. The auxiliary battery can be charged indefinitely at the float voltage without risk of damage from over-charging.

So if you find yourself with a flat or nearly flat auxiliary battery, and you are at home where you might have a 12 V battery charger, it's a good idea to charge the auxiliary battery while the main battery is charging. That way, the auxiliary battery gets away from the dangerously low state of charge the quickest, and therefore has the best chance of recovering fully.

Great advice !.
I have purchased a battery jumper pack that rides along with us on our trips.
Also, I have purchased a 12 volt Smart charger that I offer to the car about three of times in a year.
No particularly reason why I do this, I feel it is offering a bit of a helping hand to the 12 volt battery, that’s all.
These items forms part of our what I call “My Standby” kit.
A little few extras I have decided to purchase as part of your EV life .
A nice little bundle of confidence builder goodiesthat includes the following items.

A type 2 public charging cable.
A space saver wheel / jack & brace.
A 12 volt battery jumper pack.
A 10mm spanner for the 12 volt battery.
And A.A. cover of course if all the above fails of course .
Oh …… And a cuddly toy in the shape of Morris the bear !.
Again, not totally necessary but I find it does offer a bit of a small warm feeling of safety to get you out of a small situation that MAY save a long wait from our friends on the recovery services.
A two hour wait for a 30 second jump start pack on a 13 volt battery is worth every penny in my book.

 

[Alb]

How are people flattening their 12V battery in the first place?

 

[Kithmo]

How are people flattening their 12V battery in the first place?

Some have had a duff 12v battery and some charging in ACC mode with stuff on like HVAC etc.
Also the usual suspects on any car, leaving the parking lights (AKA sidelights) on.
I left an interior light on once but they turn themselves off after a while (30 minutes ?) no problem.
I've also sat in the car in ACC mode, fiddling with the dash cam and the car turned itself off after 10 minutes, again no problem.

 

[Phil Hayward]

Plug one these into the cigar lighter socket, you can see how the battery is charging.

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[Kithmo]

Plug one these into the cigar lighter socket, you can see how the battery is charging.

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A bit difficult when the 12v socket is hidden under the centre console

 

[Deleted member 2439]

There’s a fair bit of guesswork going on here. Why not ask on one of the Monday sessions and get a definitive answer. My view is that the system should look after itself and that topping up with a charger in normal use isn’t necessary- in the same way it isn’t with an ICE car.

 

[Kithmo]

There’s a fair bit of guesswork going on here. Why not ask on one of the Monday sessions and get a definitive answer. My view is that the system should look after itself and that topping up with a charger in normal use isn’t necessary- in the same way it isn’t with an ICE car.

Agreed, topping may only be necessary when the 12v ages a bit.