MarkH
Posts: 45
Joined: Mon Jun 10, 2019 1:17 pm

12V battery

Sat Dec 07, 2019 9:43 pm

It hasn't happened (yet) but I'm wondering ....

What happens when the 12V battery fails?
Does it leave you unable to start the car (and drive to a dealer) - or what?

jadnashuanh
Posts: 4766
Joined: Thu May 22, 2014 2:07 pm
Location: Nashua, NH USA

Re: 12V battery

Sat Dec 07, 2019 10:35 pm

All of the computers, lights, etc. are powered by the 12vdc system. The 12vdc battery gets recharged during operation once the high voltage circuit turns on. If the 12vdc battery dies, and you plug in an EVSE, the car won't immediately start to charge since the controls won't be active, but there's a backup trickle charger to the 12vdc battery that will try to bring it back up to a high enough value to then run the car. But, in the interim, you have a big brick unless you change the battery. There's a very special procedure to try to recharge the battery...you must disconnect the high voltage first, or you'll end up damaging things or yourself.

Note, the charging logic in BMWs adjusts based on the type, size, and age of the battery installed. When you change to a new one, even if it's the same in every way (except being new), it will be treated as if it were an old one based on the timer. So, you need to reprogram the logic to restart the timer, and treat the battery as a new one. Should you change the type or size, that would need to be updated as well, or you could damage the new battery.

Some time, the car is too smart...
Jim DeBruycker
2011 535i x-drive GT, 2014 i3 BEV

MarkH
Posts: 45
Joined: Mon Jun 10, 2019 1:17 pm

Re: 12V battery

Sun Dec 08, 2019 3:56 pm

So, if the trickle charge gets it back up so I can start the car, will the car run OK until the next time I stop and try to re-start it?
ie, do I have just one shot at driving to the nearest BMW shop - 100kms away for me?

How long are people getting out of their 12v batteries before they die?
What sort of warnings do they give if they are on their way out?

alohart
Posts: 1855
Joined: Sat Nov 01, 2014 7:36 pm
Location: Honolulu, HI

Re: 12V battery

Sun Dec 08, 2019 4:36 pm

MarkH wrote:
Sun Dec 08, 2019 3:56 pm
So, if the trickle charge gets it back up so I can start the car, will the car run OK until the next time I stop and try to re-start it?
ie, do I have just one shot at driving to the nearest BMW shop - 100kms away for me?
I suppose it depends on why the 12 V battery was so discharged. It's pretty difficult to discharge the 12 V battery because the DC-DC converter is very aggressive about charging it, and most 12 V accessories seem to turn off automatically a few minutes after shutting down an i3. I suppose leaving parking or clearance lights on could discharge a weak 12 V battery unless leaving these lights on could cause the DC-DC converter to maintain the 12 V battery's charge. Also, parking an i3 for several weeks without driving it could discharge the 12 V battery too much to start the car.

I suspect that most cases of the 12 V battery's charge level being too low to boot an i3's controllers are due to a failing or failed 12 V battery. If the failure mode prevents the 12 V battery from holding a charge or reaching full charge voltage due to a bad cell, the trickle charge that occurs when an EVSE is plugged in and actively charging would likely be insufficient to allow an i3 to start.

Personally, if our 12 V battery discharged too much to start our i3 while it was parked in our apartment parking space, I would disconnect the high-voltage disconnect and the 12 V negative battery cable so that I could attach a 12 V battery charger for a few hours in case the 12 V battery had gradually become discharged from being parked so much and driven so little which could occur in our case.
MarkH wrote:
Sun Dec 08, 2019 3:56 pm
How long are people getting out of their 12v batteries before they die?
What sort of warnings do they give if they are on their way out?
I believe that our original 12 V battery was manufactured in May, 2014. Our BEV was manufactured on 1 August 2014. We bought it on 5 November 2014. Its original 12 V battery continues to work normally with no warning that it's about to fail. However, it has never been exposed to extreme temperatures and has led an easy life considering that it has been disconnected for a total of 2 years while our i3 was in storage.
Aloha,
Art

2014 BMW i3 Arravani Grey, Giga World, Tech + Driving Assist, Parking Assist, DC Fast Charging, JuiceBox EVSE

eNate
Posts: 196
Joined: Mon Sep 09, 2019 5:33 pm

Re: 12V battery

Sun Dec 08, 2019 4:39 pm

This is the BS thing with these "smart" coded battery systems -- they don't work!

From what I've been reading about the 12v battery on this and other i3 forums, you can expect 3 to 4 years of service from it.

Rather than the system monitoring the battery's performance and providing a warning that it's nearing time for replacement, all the odd symptoms that are topics of the other "dying 12v battery" posts begin making appearances.

It's not an i3-specific problem. My neighbor's BMW Z4 stranded him without warning in exactly the same way. And after dropping in a brand new 12v battery, his car was relegated to limp mode until he took it in to get it coded. Bunch of malarkey. Hooey, I tell ya!

With my Ford, I can accept that when the battery dies without warning, that's that, because the drop-in replacement doesn't require any back-end programming or resetting. Unfortunately with HID and LED headlights and pushbutton starting, all of the tell-tale signs of a dying battery are gone.

And with my previous car, an Audi with a coded battery, I skated around all the shenanigans by connecting a 12v power supply to the car to keep it alive while I swapped batteries. Didn't code it, and the car didn't skip a beat because of it.
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MarkH
Posts: 45
Joined: Mon Jun 10, 2019 1:17 pm

Re: 12V battery

Sun Dec 08, 2019 8:50 pm

I would disconnect the high-voltage disconnect and the 12 V negative battery cable so that I could attach a 12 V battery charger for a few hours
Since I have a 12V charger, this sounds like a reasonable solution to getting going again - even if it's just so I can get the car to a dealer to replace the battery.

Is there a link to how to disconnect the high-voltage??

alohart
Posts: 1855
Joined: Sat Nov 01, 2014 7:36 pm
Location: Honolulu, HI

Re: 12V battery

Sun Dec 08, 2019 10:25 pm

MarkH wrote:
Sun Dec 08, 2019 8:50 pm
Is there a link to how to disconnect the high-voltage??
This describes how to disconnect the high-voltage disconnect which is a step in this description of how to charge the 12 V battery.

A note at the top of the battery charging procedure states that disconnecting the high-voltage disconnect isn't strictly necessary for i3's whose system software isn't a very early version. However, disconnecting the high-voltage disconnect is so easy that I always do it before disconnecting the 12 V battery which I have done at least 4 times preparing our 2014 BEV for storage. I have never had any problems after reconnecting the 12 V battery.
Aloha,
Art

2014 BMW i3 Arravani Grey, Giga World, Tech + Driving Assist, Parking Assist, DC Fast Charging, JuiceBox EVSE

MarkH
Posts: 45
Joined: Mon Jun 10, 2019 1:17 pm

Re: 12V battery

Sun Dec 08, 2019 11:52 pm

Thanks for that reply, Art.
Woah! It all looks pretty intimidating :?
My BEV is 2016 - maybe we don't need to disconnect the HV?

You say you disconnect the 12V before putting the car in storage.
We will be leaving the car unused for two months next year. Is that long enough to warrant doing this?

alohart
Posts: 1855
Joined: Sat Nov 01, 2014 7:36 pm
Location: Honolulu, HI

Re: 12V battery

Mon Dec 09, 2019 11:33 am

MarkH wrote:
Sun Dec 08, 2019 11:52 pm
Woah! It all looks pretty intimidating :?
It's really not. The high-voltage disconnect is easily accessible under the cover to the right of the frunk box.
MarkH wrote:
Sun Dec 08, 2019 11:52 pm
My BEV is 2016 - maybe we don't need to disconnect the HV?
I don't want to risk potential damage that might occur to the DC-DC converter when a 12 V battery charger is attached to the battery terminals (some sparking is difficult to avoid) and turned on. The procedure I referenced does suggest that disconnecting the high-voltage disconnect isn't necessary when the system software version is later than that installed on early 2014 i3's.
MarkH wrote:
Sun Dec 08, 2019 11:52 pm
You say you disconnect the 12V before putting the car in storage.
We will be leaving the car unused for two months next year. Is that long enough to warrant doing this?
Parking an i3 for 2 months without disconnecting the 12 V battery or leaving an EVSE plugged in would almost certainly result in a very discharged 12 V battery, maybe permanently damaged if completely discharged. This isn't unique to an i3 or even an EV. All modern vehicles have vampire 12 V loads that will discharge the 12 V battery over time.
Aloha,
Art

2014 BMW i3 Arravani Grey, Giga World, Tech + Driving Assist, Parking Assist, DC Fast Charging, JuiceBox EVSE

jadnashuanh
Posts: 4766
Joined: Thu May 22, 2014 2:07 pm
Location: Nashua, NH USA

Re: 12V battery

Mon Dec 09, 2019 2:31 pm

If you left it plugged into an EVSE, over that two months, it might activate the EVSE 3-4 times, if that, but enough to keep both batteries charged. Unlike a cellphone or most other consumer battery powered devices, the car is not getting a constant charge while connected...it only turns on once the levels drop enough to warrant it while preserving the batteries.
Jim DeBruycker
2011 535i x-drive GT, 2014 i3 BEV

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