12v Battery Charging

[tsedge]

EVs can drain the 12V battery much faster due to:

• Smaller capacity 12V batteries.
• Powering various electronics on the car (standby draws).
• Bugs / faults where components get stuck in a power-sapping loop.
• External USB/OBD2 devices plugged into the car.

If the main battery is not charging the 12V battery enough (or at all), then it can discharge.

It's a silly weak point that everyone (including Tesla) has encountered and we should have a redundant 12V design so the worst that happens is you get a partial failure and need to charge up or replace that, but the car is always usable.

Here's one of the components needed alongside two 12V batteries. It can be done.

https://www.analog.com/en/analog-dialogue/articles/finally-a-12v-12v-dual-battery-automotive-bidirectional-dc-dc-controller-for-redundancy.html

 

[siteguru]

FWIW I'm sure my car's 12V battery is being charged when the car is charging on my 7kW wall box. ?‍

 

[tsedge]

FWIW I'm sure my car's 12V battery is being charged when the car is charging on my 7kW wall box. ?‍

Yes, the issue is when it is being charged when not charging - eg when being left in an airport car park for a couple of weeks.

 

[siteguru]

Yeah, I know what the thread topic is about. I was addressing an earlier comment (perhaps I should have quoted it) that charging the LV battery doesn't happen (on the Taycan at least) when the HV battery is being charged. (Unless I've misunderstood that earlier comment).

 

[tsedge]

Yeah, I know what the thread topic is about. I was addressing an earlier comment (perhaps I should have quoted it) that charging the LV battery doesn't happen (on the Taycan at least) when the HV battery is being charged. (Unless I've misunderstood that earlier comment).

Ah, that wasn't clear to me.

 

[Rolfe]

I thought the issue was that the 12v wasn't being charged when the car was plugged in but not charging.

 

[tsedge]

I thought the issue was that the 12v wasn't being charged when the car was plugged in but not charging.

Ian's original post was about disconnected 12V charging, I believe.

 

[Rolfe]

The whole thing is confusing the life out of me.

 

[tsedge]

The whole thing is confusing the life out of me.

Yes, I know what you mean - there have been several X-not-working-when-plugged-in-but-not-charging bugs.

 

[BrianHere6]

This is 6 years old - but remains a basic, clear explanation of EV's and the 12v battery and shows it 's not a new problem, or unique to MGs.

It's a new way of thinking about living with EVs. Change of mindset. With ICE's the battery is in a different league.
What has gone in the wrong direction with EVs is that they came from "oh, we don't need anything like battery used in ICEs" when it should have been "hmmm, 12v battery is a critical item to EVs".
Army has a saying "one is none, two is one" - should apply to EVs and the 12v power supply should have been a completed new design.

Anyways, now have 12v booster in glovebox.

 

[Martinonline]

I've had no trouble with my 12v battery since I bought the car in April but today I had a dash battery warning light with an indicated voltage of 11.8v. Earlier I had put the HVAC high for the full 10 minutes as it was a bit nippy: 1°.

Over the next few driving minutes the voltage went up to 11.9v until I arrived home. I powered down the car and locked it letting the HV battery (at 61%) charge the 12v battery. After 7 hours the battery was at 12.2v which had not changed in the previous 3 hours as presented in the App.

So, I powered up the car and the battery indication came up as 13.8v but then jumped to 14.3v before dropping to 14.2v. Leaving the car powered I checked the App and that showed 14.2v. Immediately after powering down the App showed 12.8v. After 5 minutes it was down to 12.7v. After 10 minutes the voltage was 12.6v. After 15 minutes it is at 12.6v. I won't try the App again until tomorrow.

So, can I assume that:

The App gives the actual voltage reading;
The driver's display normally gives the charging voltage;
The driver's display gives the actual voltage if a low voltage is detected?

 

[tsedge]

I believe the car and App show the exact same measurement which is the current battery voltage, which is >13V if being charged.

 

[Martinonline]

That I can understand but why did the car show below 12v when in warning when it should be charging at >12v as it did later when I powered up and it was >14v?

 

[tsedge]

That I can understand but why did the car show below 12v when in warning when it should be charging at >12v as it did later when I powered up and it was >14v?

I think because you had the HVAC high it was draining the 12V battery and the car wasn't charging it for some reason - was it connected to a charger after high voltage pack charging?

The are some scenarios where people are ending up with flat 12V batteries but I am not sure what they are.

 

[Martinonline]

@tsedge, you may well be right. The car was not connected to a charger. I did not notice the warning light when I drove the car half a mile and parked it for an hour. It was only when I got back in the car to return home that I noticed the warning light.

To learn more I experimented:

First think this morning the battery was 12.4v on the App;
I activated the HVAC on Hi from the App and the battery went to 13.9v;
When the heating stopped the battery went down to 12.7v;
Half an hour later the battery was down to 12.5v;
I unlocked the car from the fob and the battery went down to 12.3v;
I powered up the car and the car indicated a battery voltage of 13.9v and then 14.2v;
The App indicated 13.9v (which probably would have gone up if I had refreshed);
I locked the car from the Fob and the App indicated 12.5v;
After 1 hour the App showed 12.2v;
After 2 hours the App showed 12.4v.

I will monitor the battery condition more regularly, particularly when pre-heating, but it does appear that the charging circuit is working, so perhaps it was just a blip. However, I will investigate a booster for when I am away from home.

 

[tsedge]

Just done a test, left the car in the airport car park for 10 days whilst on holiday. No problems at all with 12V battery, everything was fine.

Before:

After:

So we lost 1% in the main battery over those 10 days.

 

[fnegroni]

@tsedge, you may well be right. The car was not connected to a charger. I did not notice the warning light when I drove the car half a mile and parked it for an hour. It was only when I got back in the car to return home that I noticed the warning light.

To learn more I experimented:

First think this morning the battery was 12.4v on the App;
I activated the HVAC on Hi from the App and the battery went to 13.9v;
When the heating stopped the battery went down to 12.7v;
Half an hour later the battery was down to 12.5v;
I unlocked the car from the fob and the battery went down to 12.3v;
I powered up the car and the car indicated a battery voltage of 13.9v and then 14.2v;
The App indicated 13.9v (which probably would have gone up if I had refreshed);
I locked the car from the Fob and the App indicated 12.5v;
After 1 hour the App showed 12.2v;
After 2 hours the App showed 12.4v.

I will monitor the battery condition more regularly, particularly when pre-heating, but it does appear that the charging circuit is working, so perhaps it was just a blip. However, I will investigate a booster for when I am away from home.

Many 12v batteries require to be kept near 100% as much as possible.
Therefore, I suggest you condition your 12v battery with a trickle charger so that it will top it up to properly full and that will ensure it lasting longer.

 

[Ian Key]

So we lost 1% in the main battery over those 10 days.

That could be down to the cooler weather.

 

[Rolfe]

I left mine at 69% for just over a fortnight, and it was still at 69% when I got back.

 

[tsedge]

There's an awful lot I could say in response to that.... but given that you are a moderator, I'll just say your car must like that SOC. ?