12V battery - how is it charged and how do I charge it?

[Rambutan]

So I've been reading and trying to figure this out. Can anyone confirm or correct my findings?

• The 12V battery is charged from the HV battery when driving.
• The 12V battery is charged from the HV battery if I press the start button two times, without pressing down on the brake pedal.
• It's okay to charge the 12V battery with a 12V charger without disconnecting the negative pole.

Regarding the last entry, I did this with no ill effect. I've since read that some people recommend disconnecting the negative pole first.

My car broke down, refused to start and I was picked up by my local roadside assistance. They used a 12V jump starter without disconnecting any poles.

 

[Coulomb]

The 12V battery is charged from the HV battery if I press the start button two times, witho

Are you sure? I'm 90% sure that this doesn't happen in my Mark 1 ZS EV.

I agree with your other points.

I've long suspected that my ZS Mark 1 never charges the 12V battery when parked. Or if it does, it's only ever at quite low state of charge, where it rarely goes for me. It's supposed to occasionally charge the 12V when parked.

Its 12V battery is likely to fail soon, not helped by this lack of charging. Just recently my wife could not start it, though she had the blower fan fairly high. I switched off the air con and it started fine. I gave it a charge (no disconnection) for a few hours just in case.

 

[Rambutan]

Are you sure? I'm 90% sure that this doesn't happen in my Mark 1 ZS EV.

I'm not sure, if I remember correctly I read it on this forum.

 

[fred12345]

I'm not sure, if I remember correctly I read it on this forum.

if you press the start button twice (on Mk 2 ZS) , it should go into yellow then green light on the button so it's getting to ready mode in two stages ..

but - when you start having problems with 12V battery that it needs charging, best to get a new one rather than risk it ...

 

[Spinynorman]

Pressing the starter puts the car in what used to be called ‘accessory mode’ and will show the SOC for the 12V battery along the bottom of the Speedo. Usually around the 12V mark.
If you hit the brake and then turn on, you go into ‘Ready mode’ - waiting to move off - and the indicator will change to just above 14V - this indicates the HV battery is charging the 12V.
It’s not like ICE, where an inbuilt ‘dynamo’ generator is activated when you drive and it charges the 12V.
You just need to be in ready mode - so you can park up for 15/20 minutes and still be charging your battery.
The Zappi 2 charger I have is also supposed to charge the 12V when you charge your car but I’m not sure this works, as when I charge my car, it charges the HV at 7kW or so for a few hours then drops dramatically - as the car goes into battery balancing (equalising mode)for a couple of hours and finally stops when full.
I have one of these ‘dodgy’ battery’s - 4 year old Mk 1 20 plate but I do very few miles now as partially disabled. I have a NoCo maintenance charger somewhere (can’t find it - another long story) which worked a treat on my previous car - a 28 year old Toyota RAV 4. It charged the battery then kept it up to scratch. I’ve seen other posts where ppl have done the same and been able to keep the 12V in working order but I think the overall solution is get a new good quality battery and replace it.

 

[Lovemyev]

When standing / parked, it is safer to have the car in the READY mode.
In this mode, the 12 volt battery IS receiving a charge.
If your 12 volt battery charger is a few years old, although it may work okay, but a better option would be to use one of the new smart units.
These units have the ability to interrogate the condition of the battery first, before then applying the correct level of power required.
Maybe starting off slowly at first, then increasing the charge rate later.
I have charged my car without disconnecting it from battery, as I do have a CTEK smart charger.
In fact, the unit came with a quick connect loom that you attach to both sides of the battery.
This then has a quick connect / disconnect connector that plugs into the loom of the chsrger itself.
This makes its use very easy and there is no requirement for using the large crocodile clamps that you usually get with a battery charger.

 

[Coulomb]

The Zavvi 2 charger I have is also supposed to charge the 12V when you charge your car but I’m not sure this works,

Do you mean Zappi?

Charging the 12 V battery has nothing to do with the EVSE, unless it has something special that connects directly to the 12 V battery.

When charging the main battery, the contactors will be closed, which I believe means that the DC-DC will be running, charging the 12 V battery. What makes you unsure that this is happening?

 

[evnick]

So I've been reading and trying to figure this out. Can anyone confirm or correct my findings?

• The 12V battery is charged from the HV battery when driving.
• The 12V battery is charged from the HV battery if I press the start button two times, without pressing down on the brake pedal.
• It's okay to charge the 12V battery with a 12V charger without disconnecting the negative pole.

Regarding the last entry, I did this with no ill effect. I've since read that some people recommend disconnecting the negative pole first.

My car broke down, refused to start and I was picked up by my local roadside assistance. They used a 12V jump starter without disconnecting any poles.

I've had my 12v battery go flat a couple of times after leaving the car unused for a few days - not more than a week. What can I do to avoid this happening?

 

[Rambutan]

I've had my 12v battery go flat a couple of times after leaving the car unused for a few days - not more than a week. What can I do to avoid this happening?

This is not normal behaviour, a battery should last much longer than that. You have to test the battery, a dealer can do this if you don't have the equipment. If the battery is ok you may have an electrical fault that is slowly draining the 12v battery.

 

[fred12345]

I've had my 12v battery go flat a couple of times after leaving the car unused for a few days - not more than a week. What can I do to avoid this happening?

if it went flat (and more than once), time for a new one. you don't want to be stranded somewhere because of that ... how old is that battery? you sure you didn't leave the lights on or something ...?

 

[Spinynorman]

Do you mean Zappi?

Charging the 12 V battery has nothing to do with the EVSE, unless it has something special that connects directly to the 12 V battery.

When charging the main battery, the contactors will be closed, which I believe means that the DC-DC will be running, charging the 12 V battery. What makes you unsure that this is happening?

Yep - now corrected.
I get a message on the panel saying my battery is low and I should start. This happens even after I’ve left it on charge overnight - the Zappi turns off the charge when the car says it’s full.
I get around this by going for a run each week - low mileage user - usually for lunch, which I park up and eat in a car park. With the car in ready mode, the HV battery is charging the 12v so I survive that way. Just need to find my NoCo maintainer and see if a long maintenance charge will recover this or if not, time for a new battery…

 

[Lovemyev]

I get around this by going for a run each week - low mileage user - usually for lunch, which I park up and eat in a car park. With the car in ready mode, the HV battery is charging the 12v so I survive that way

It’s great to get away for lunch once a week that is 100% for sure, but really EV’s don’t benefit in the very same way as ICE vehicles do, when taking them out for a run.
Leaving the car just stationary in READY mode for the same given time period, has the same result ( if not slightly better ).
As you are a low mileage user case, then using your Noco smart battery charger for a couple of hours, is likely to put more energy back into your 12 volt battery than taking the car out for a run.
Although this may benefits for your 12 volt battery, it does not provide YOU with the same personal “down time” benefits of course.
I guess the question of “should I need to provide any additional 12 volt charging requirements, other than that the car is able to provide” is going to be largely based on your user type.
Lower milage users are going to benefit from some additional external charging of their batteries, while higher mileage users are much less likely.
No two user cases are going to be identical.

 

[evnick]

if it went flat (and more than once), time for a new one. you don't want to be stranded somewhere because of that ... how old is that battery? you sure you didn't leave the lights on or something ...?

Battery is apparently fine but losing charge gradually. Nothing obvious like lights. What else could be draining it?

 

[fred12345]

Battery is apparently fine but losing charge gradually. Nothing obvious like lights. What else could be draining it?

Could be anything... something left plugged in? could be something in the car draining it that MG garage would need to deal with ... how old is the battery? they say 12V battery lifespan is 3-5 years ...
You use the car at least once a week?

 

[Spinynorman]

It’s great to get away for lunch once a week that is 100% for sure, but really EV’s don’t benefit in the very same way as ICE vehicles do, when taking them out for a run.
Leaving the car just stationary in READY mode for the same given time period, has the same result ( if not slightly better ).
As you are a low mileage user case, then using your Noco smart battery charger for a couple of hours, is likely to put more energy back into your 12 volt battery than taking the car out for a run.
Although this may benefits for your 12 volt battery, it does not provide YOU with the same personal “down time” benefits of course.
I guess the question of “should I need to provide any additional 12 volt charging requirements, other than that the car is able to provide” is going to be largely based on your user type.
Lower milage users are going to benefit from some additional external charging of their batteries, while higher mileage users are much less likely.
No two user cases are going to be identical.

Yeah of course I take the car for a run cos it gets me out of the house, as I work from home and go stir crazy if I don’t get out for a bit. I have before now just sat in the car in my driveway for an hour and used my laptop from there but it always seemed a bit dodgy in the area I live - east London. I even park my car in a CPZ bay so I don’t get any ‘visitors’ trying to capture my fob signal to steal the car - can’t do it if they don’t know which door to stand in front of with an aerial!
It’s free though so costs nothing. Is just something I think reduces risk a bit.

 

[magicstuff99]

On Amazon I purchase a Bluetooth 12V battery monitor $30Au. It is definitely helping me work out my issues.
I believe:
Yes it 100% charges in Ready mode.
Yes, charges while driving.
Not sure in accessory mode, I would doubt it. as there has to be a difference between ready and accessory.
My drain is caused by a dashcam that runs all the time, I believe.
When reducing the features of the dashcam I can manage the battery.
Still not good as it was brought to do a job and cant.
I hope this helps.

 

[Spinynorman]

You can plug your dash cam directly into a fuse on the fuse box that isn’t always on - my windows don’t work unless I’m switched on so you could piggyback your cam onto that fuse and when you turn off, it’ll stop pulling power.
Bit of a nuisance though if you want a parking mode on your cam, as that requires always-on power

 

[hmadsen]

On Amazon I purchase a Bluetooth 12V battery monitor $30Au. It is definitely helping me work out my issues.
I believe:
Yes it 100% charges in Ready mode.
Yes, charges while driving.
Not sure in accessory mode, I would doubt it. as there has to be a difference between ready and accessory.
My drain is caused by a dashcam that runs all the time, I believe.
When reducing the features of the dashcam I can manage the battery.
Still not good as it was brought to do a job and cant.
I hope this helps.

Does your car not have the USB connector on the side of the plastic behind the rearview mirror ?

This is where my Dashcam is plugged in and this USB port turns on and off with the car.