12V (AUX 18L) new battery voltage shows 11.9V

[alohart]

Older batteries lose capacity and increase internal resistance. It would therefore only make sense to adapt to a more gentle charging profile. A gentler charging profile doesn't kill new batteries.

Aging batteries also lose capacity such that they must be charged more often than a new battery to avoid discharging too much. I've read that to satisfy a certain fuel efficiency requirement, ICE BMW's turn off their alternators when their 12 V batteries don't need charging. Not needing to satisfy any fuel efficiency requirement, it wouldn't be necessary to turn off the i3's DC-DC converter or maybe reduce its output power when the 12 V battery doesn't need charging.

It might be that the life expectancy of an AGM battery is longer if its charge level isn't maintained near full at all times. If so, then charging it more often might shorten its life.

I've migrated to an LFP 12 V battery in hopes of not having to replace the 12 V battery as often. However, the i3's 12 V battery charge controller maintains the LFP battery at near full charge at all times. The battery's manufacturer told me that this would reduce its life expectancy. I now disable 12 V battery charging most of the time, a feature of this LFP battery, enabling it for a few minutes every couple of weeks so that its average charge level is closer to 80%. Most people wouldn't want to bother managing their 12 V battery like their HV battery pack should be managed. The LFP battery's BMS doesn't seem to be as sophisticated as that for the HV battery pack.

 

[eNate]

So maybe I'm not completely following you on this one. The most reliable way to know whether a battery needs charge is to check its voltage. I can't imagine a system that applies a charge to a battery just because it is old. And to stop a charge when a target voltage is achieved (or a threshold internal resistance, or a temperature). And, as we understand with the i3, the 12 volt DC rail is powered by the HV battery whenever the car is powered up, so it's not as if that system is being cycled on and off.

 

[alohart]

So maybe I'm not completely following you on this one. The most reliable way to know whether a battery needs charge is to check its voltage.

True if its voltage is its open-circuit resting voltage a few hours after it has been charged or discharged, However, the voltage of any battery isn't its open-circuit voltage when current is flowing in or out, so it's difficult to determine a battery's charge level while its being charged or discharged. This is a reason why a BMS counts coulombs in and out of the HV battery pack to calculate its charge level rather than using its instantaneous voltage.

The i3's Intelligent Battery Sensor (IBS) measures the 12 V battery's voltage, current, and temperature frequently. I believe that it uses these data plus the battery's new capacity, type, and age to determine whether it should be charged rather than just using the voltage which varies considerably while a vehicle is operating.

I can't imagine a system that applies a charge to a battery just because it is old.

Charging algorithms include an aging factor that varies by battery cell type. The mi3 app displays a 1.1117 aging factor for the HV battery pack in our 2021 i3. I don't know whether the aging factor was 1.0000 when it was new. It's reasonable to assume that the age of a battery cell might be a factor in determining charging behavior.

I had thought that registering a new 12 V battery in an i3 would record the registration date to use for battery cell aging calculations. However, it seems to record mileage instead of date, so I don't know whether aging is considered or what value knowing the mileage would have.

And to stop a charge when a target voltage is achieved (or a threshold internal resistance, or a temperature).

The current into the battery would decrease as its charge level increases assuming that the DC-DC output voltage is constant. However, my always-on 12 V system voltmeter shows its output voltage varying between ~14.0 V and ~14.3 V which is lower than many other i3 owners report. I'm guessing that the DC-DC output voltage increases as the ambient temperature decreases below temperatures that our i3 experiences.

And, as we understand with the i3, the 12 volt DC rail is powered by the HV battery whenever the car is powered up, so it's not as if that system is being cycled on and off.

Unlike with our 2014 i3, the DC-DC converter doesn't always turn on when I unlock the doors. I can't explain the behavior difference. However, as you wrote, it's always on when any of our i3's have been in drive ready state, probably because of increased 12 V system consumption when all controllers are running. I've never observed any DC-DC converter behavior difference as our 12 V batteries have aged. However, if the DC-DC converter's output voltage were varied with battery age, I wouldn't have noticed it.

 

[Mondomensch]

I have noticed over the past decade and a half that the quality of the AGM batteries I used in my conventional cars has declined, leading to much shorter lifetimes (say 3 years instead of 7). That's on non-BMWs without any fancy registration, and I wonder if any i3 battery lifetime reduction might just be a battery quality issue.

In a recent post here, an i3 owner also noted that the dealer-purchased BMW i3 replacement battery he installed was rated at only 16Ah rather than the 20Ah of the one it replaced.

It's tough to figure out causation with multiple variables and no controlled experiment.

 

[eNate]

This is a reason why a BMS counts coulombs in and out of the HV battery pack to calculate its charge level rather than using its instantaneous voltage.

Given all that you wrote, I'm homing in on this in particular because...

(...I've written this before...)

... don't you think that if BMW put this much attention in to 12-volt battery maintenance, that it would be able to reliably tell us when to replace our 12-volt batteries instead of regularly surprising us and leaving us out in the cold?

 

[alohart]

... don't you think that if BMW put this much attention in to 12-volt battery maintenance, that it would be able to reliably tell us when to replace our 12-volt batteries instead of regularly surprising us and leaving us out in the cold?

Good point! Both i3 battery failures I've experienced included the "excessive discharge while parked" (or whatever) message before either i3 wouldn't start, so I wasn't stranded. These batteries both gradually lost capacity with their voltages decreasing more and more rapidly while parked. So some advance warning was given, but the warning message certainly wasn't as informative as it could have been.

Lead-acid batteries can fail in various ways, so maybe some of the failure modes are so sudden (e.g., an internal short circuit) that no warning is possible. BMW's Intelligent Battery Sensor wasn't introduced with the i3 and has been included on BMW vehicles for many years. However, with all of the data that it provides, I would think that improved warnings would be possible.

 

[mojo]

If Wisely is making that claim, they ought to back it up with dualing data from battery monitors.

I don't think Wisely is really making such a clear assertion. I think the quote that Gonville refers to is one by Wisely's Arthur Kaminski on the UK i3 Driver's FB Group two weeks ago, where he says this:

We have a video on this soon, though I suspect many here won't find it helpful as it won't align with their stance.
Anyway, I'm staying out of it but just wanted to chime in with a thought: what would be MUCH more useful than yet another speculative post on this subject, is some actual data/facts with which people can then make up their own minds. We read many, many posts from people who have just replaced their 12v without registration and shout from the rooftops about how the car is working absolutely fine without it, don't be a mug, money down the drain, stealer, etc etc. This is not data, this is just an inevitability (yes, the battery will work fine unregistered for several years). Their enthusiasm for non-registration continues and gets reaffirmed for months, others read this and the stance snowballs. Then, case by case this fizzles and they eventually disappear (usually because they've moved onto a different car). As such, we rarely hear from them again and certainly not approx 6-7 years later when they'd be at their most useful and could confirm that everything is still fine and dandy (as it would be, on average, using actual data we have for registered battery replacements).
Where are you all, long-term non-registrators!? Speak up! Give us your data! Prove / disprove the myth!

I interpret this rather differently: he does not seem to be clearly committing to either point of view. It seems (to me) to be more of a question - a desire for factual data rather than speculation.

 

[eNate]

I wish this forum had a " Helpful" reaction emoji! Yeah, that's pretty clear.

However the i3 is notorious for eating batteries in 3 to 4 years. OE batteries, not replacements.. Not all, but lots. So that doesn't really square up.

 

[alohart]

I encourage everyone to license BimmerLink with an OBD dongle. BimmerLink could be helpful when a mysterious problem occurs.

And since everyone has licensed BimmerLink, they can register their replacement 12 V batteries at no additional cost so they'll sleep soundly not worrying about their new battery dying prematurely (or not) because they didn't register it. The cost of one battery registration would probably exceed the cost of BimmerLink plus an OBD dongle. The value of sound sleep … priceless.