How Do You Engage Neutral w/Dead 12v Battery

[johnheddy]

The posts I read seem to indicate the batteries (at least the ones through BMW) arrive at ~ 50% SOC, not fully discharged or fully charged.
That matches up well with what I understand about sitting at high SOC for long periods of time damaging a battery and sitting at 0% SOC for long periods of time also damaging a battery. You wouldn't want either of those cases if the battery was to have any significant shelf life.

The descriptions on replacing the batteries yourself say to put them on a trickle charger for 2 days so they are at 100% SOC before installing them.
I didn't have a trickle charger, a battery, or 2 days, so I went to a dealer.

The dealer quoted pulling a battery off the shelf, unboxing it, installing it, registering it, and sending me on my way in the space of an hour, while I waited. That leaves no time for a trickle charge.
It didn't turn out that way, as they sold the battery listed in their system before I got there.
They apologized nicely, and offered me a nice new BMW to drive until they could make it right.
The final bill came in about $150 less than they quoted ($450) all in very round numbers.

I know they left my car "running" for an extended period of time because of the amount of condensation on the inside of the windshield when I picked it up.
I wiped it as best I could, as I couldn't see to drive it without wiping the inside down.
But I still have water marks in the lower forward section of the windshield where it is hard to reach.

 

[ME2SC]

I have a Schumacher 2500A portable booster pack that I keep in the i3 (left over from my old MB 300TD years). I figure I can help myself out of a potential jam like this, and also I like to be able to help stranded ICE folks that need a jump still ... It's a good investment.

 

[nickp]

I hesitate to recommend my LFP 12V battery because LFP batteries don't charge or discharge well when cold (can't recall which). I live in the tropics, so that's not a problem for me. The manufacturer of my LFP battery claims that its LFP automotive batteries include automatic battery cell heaters that overcome the cold weather problem, but I cannot verify that this works well.

Also, like most (all?) lithium chemistries, LFP battery cells degrade more rapidly at a full charge with an average charge level of 50% recommended for the lowest degradation rate. LFP battery cells differ from other lithium chemistries in that it is not so harmful to charge them fully, so no high charge level unusable buffer is needed. A problem with using a LFP battery in an i3 and probably in most vehicles is that its charge level remains almost always at or near 100%, so this would shorten its useful lifetime.

I agreed to test a LFP battery in an i3 for its manufacturer. I had assumed that they would offer a LFP battery for an i3, but that hasn't happened. The LFP battery designed for a Rivian truck is the same physical size and has a capacity at least as high as the OEM AGM battery. However, it needs screw-on SAE battery posts to work with an i3. The OEM screw-on posts don't fit and would require a lot of modification to fit. The SAE posts that the manufacturer sent to me are too large to fit into the recess in the battery case surrounding the terminals, so they require some grinding to fit snugly. Brass washers must be installed between the posts and the terminals to provide good electrical contact.

One of the nice features of this manufacturer's LFP batteries is that it includes a Bluetooth radio that communicates with a smartphone app that displays all sorts of data about the state of the battery as well as supporting enabling/disabling charging or discharging. So I am currently testing disabling charging until the battery's charge level decreases to ~40% which corresponds to an output voltage of ~13.1V. Its full-charge voltage is ~0.5V higher than an AGM battery and its voltage doesn't decrease as rapidly as its charge level decreases compared with an AGM battery, so it should work well in an i3 within a 40% to 60% charge level range. It should occasionally be charged to 100% so that cell balancing can occur (it contains 4 cells). The manufacturer told me that he could program the battery to automatically enable and disable charging to keep it within a suitable charge level range if my managing its charge level manually works well.

If this manufacturer decides to offer a plug-and-play battery for an i3, I'll post information about it. As it is now, the Rivian LFP battery plus screw-on SAE posts is more than twice the cost of the Remy AGM battery with no guarantee that it would last any longer.

As always, appreciate the detail in your posts.

 

[Gonville]

The parking pawl is bolted on to the top of the gearbox at a relatively accessible location so I wonder how readily it can be removed while engaged. I haven't explored it to see if there's a way to manually manipulate it to disengage it, but that would certainly be ideal.

Parking pawl diagram.

 

[RhodyDave]

I've just discovered a great YouTube source. Styles himself "BMWi3diy". Has some great videos, including one on using a "permanently" installed digital voltmeter plugged into cigarette lighter to monitor health of 12v. battery. (Yes, I just ordered one.) Also, swapping lead-acid AGM battery for lithium LiFePO4. Another one (not relevant here) is about monitoring health of 90 kWh and 120 kWh traction batteries vs. time. This guy is astute and clearly has some engineering / scientific chops. Returning to topic: my local indie says he will install new battery in my 2018 i3 including registering it. Also said that typical service life of AGM battery is 6 years. Interestingly, my i3 was manufactured August 2018, so the sand is almost out of the hourglass.

 

[alohart]

I've just discovered a great YouTube source. Styles himself "BMWi3diy". Has some great videos, including one on using a "permanently" installed digital voltmeter plugged into cigarette lighter to monitor health of 12v. battery. (Yes, I just ordered one.)

Unfortunately, this voltmeter won't help much in determining the 12V battery's health. I, too, thought that a voltmeter would help and in 2016 installed one in our 2014 i3's auxiliary power port beneath the center of the dashboard where I could see it easily. However, it almost always displays the 12V system voltage when the DC-DC converter is on with its output of ~14.3V. I learned to look at it through the driver's window while unlocking the doors. For a few seconds before the DC-DC converter turns on, the 12V battery's voltage is displayed. However, several electronic controllers are booting during this period causing the 12V battery's voltage to sag, so it's impossible to read the battery's resting voltage because the auxiliary power port's power automatically turns off a few minutes after shutting an i3 down.



The much better way to monitor the 12V battery's health would be to install a 12V system logger. The attached screenshot shows the original 12V battery in our 2019 i3 failing and the DC-DC converter automatically trying to keep it alive while its output voltage was rapidly decreasing. I already had a LFP battery ready to install, so I was not stranded due to the 12V system monitor.

Also, swapping lead-acid AGM battery for lithium LiFePO4.

I have not had any problems after installing our LFP battery in November, 2023. However, if you live in a climate where temperatures could decrease below freezing, a LFP battery cell should not be charged below freezing. I don't know whether this could damage the cell or whether it just won't accept a charge, but it could cause problems unless the LFP battery includes an automatic battery cell heater.

 

[RhodyDave]

After all the discussion of mysterious errors on ~ 4 year old cars here, I decided to replace the 12 v battery in my 2018 i3 REx at 29,000 miles. Got the battery from Remy for $187, including shipping. Actually, I delegated the job to my local indy. They charged me one hour of labor ($120) and all seems well. I had them read the previously registered date when they registered the new battery. Surprise, there was none. I'm guessing that means it was original, dating from the car's manufacture in August, 2018. (The old one was a BMW battery, so consistent.) If all that's correct, my battery was six years old, about the expected life of an AGM battery. I've had no trouble at all to date.

 

[ksnax]

I have not had any problems after installing our LFP battery in November, 2023. However, if you live in a climate where temperatures could decrease below freezing, a LFP battery cell should not be charged below freezing. I don't know whether this could damage the cell or whether it just won't accept a charge, but it could cause problems unless the LFP battery includes an automatic battery cell heater.

People considering LFP should look for one that has advertised low temperature charge cutoff protection. The one I installed last summer allegedly does and I have not had an issue with it so far, but I never actually drove the car during sub-freezing conditions, though it has endured multiple nights in the low 20's.

I see no reason driving it would have been an issue for that here, as it did warm to over freezing most of those days, so likely would not have depleted much as it was placed on the charger later in the day. It's unclear if multiple days of sustained sub freezing would have been problematic.

 

[alohart]

People considering LFP should look for one that has advertised low temperature charge cutoff protection. The one I installed last summer allegedly does and I have not had an issue with it so far, but I never actually drove the car during sub-freezing conditions, though it has endured multiple nights in the low 20's.

I see no reason driving it would have been an issue for that here, as it did warm to over freezing most of those days, so likely would not have depleted much as it was placed on the charger later in the day. It's unclear if multiple days of sustained sub freezing would have been problematic.

If a LFP battery has low temperature charge cutoff protection rather than a cell heater, I doubt that multiple days of sub-freezing temperature would be a problem. The DC-DC converter would turn on whenever the HV system turned on, so it would provide all the power that the 12V system needs except when your i3 is off, not charging, and not preconditioning. 12V vampire loads would slowly discharge the 12V battery.

The smartphone app for my LFP battery allows me to disable charging at any time. Like all lithium battery chemistries, the LFP cell degradation rate increases at high charge levels although a LFP cell isn't damaged as much when charged fully compared with other lithium battery chemistries as long as it is not allowed to remain at a full charge. A problem with the i3's 12V battery charging system might be that it keeps the 12V battery fully charged while driving which would shorten the life of a LFP battery. If driven regularly, 12V vampire loads wouldn't discharge the battery much, so its charge level would remain very high.

The manufacturer of my LFP battery has asked me to test disabling charging to lower the average charge level of my LFP battery which should increase its life expectancy. The 12V battery's BMS might be able to be programmed to automatically disable/enable charging to lower the average charge level. I disabled charging for 3 weeks before the resting output voltage of my LFP battery decreased to 12.5V and its charge level decreased to ~60%. I had planned to allow the charge level to decrease to maybe 40% before enabling charging but I was concerned about the DC-DC converter turning on automatically to try to charge the 12V battery. I have a 12V system log that shows this occurring when the OEM battery's output voltage decreased to 12.5V. With charging disabled, the DC-DC converter would not have been able to charge the battery, so it would probably have turned on repeatedly which I wouldn't have wanted.

So you would have to experience a continuous sub-freezing period of at least 1 month or maybe more before your LFP battery's charge level might drop too low, depending on its capacity.

I wonder whether the DC-DC converter keeping the 12V battery fully charged while driving might also be causing early AGM battery failure. Alternators have voltage regulators and can be turned off and on by the 12V battery charging system. The i3's DC-DC converter seems to be on most of the time an i3's HV system is on with its output voltage remaining within a 0.3V range, so it might not be maintaining the 12V battery ideally for best life expectancy. This was a problem with the 1st generation Honda Insight which also had a DC-DC converter to maintain its 12V lead-acid battery which failed prematurely. Some Insight owners tried periodically discharging their 12V batteries to ~50% by leaving the headlights and climate control system on with the engine off which seemed to increase the battery's life expectancy. However, the sample size was too small to know whether this had any effect.