Aux battery charging during long term storage

I have to leave my 2018 i3 REx in storage for about 6 months. The storage will be under cover, and the storage people say they can plug in my 120v charger periodically. I assume that the lithium battery bank will be fine anyway since they have such a low auto-discharge rate, and the storage people will have a key so they can start the car once a month and check on the SOC.

The AGM auxiliary battery is another story though - Iʻm afraid itʻll self-discharge enough so that the car will be dead when I get back to pick it up. I know thereʻs a built-in DC-DC charger that will charge it from the high voltage battery, but I canʻt find solid information on the circumstances that allow that charging to take place. So Iʻm looking for suggestions - how can I maintain the charge on the aux battery over 6 months? Is there a way to activate the built-in DC-DC charger? Should I consider an AGM trickle charger plugged into the cig lighter? Itʻs rated at 140W so the trickle charger wouldnʻt be anywhere near itʻs max rating.

Any suggestions? I have to sort this out before the end of June, and Iʻm on Maui so parts arrive here very slowly if I canʻt find them on island.

If they'll turn the car on periodically, when it is in the ready condition, it will be charging the 12vdc battery. Not sure how long that would be needed to recoup the charge lost, but a bit each month may be enough.

The alternative would be to remove the 12vdc battery and put it on a battery maintainer. I think that if you pulled the high voltage disconnect switch, you could leave it in the car and charge things. I'd put it into storage with about 80% charge or so on the main batteries.

I have stored our 2014 BEV in our Honolulu condo parking garage parking space 4 times since buying it new, twice for 6 months, once for 3 months, and once for 9 months. I do not want to depend on anyone maintaining it while we are away. The self-discharge rates of both Li-ion and AGM cells are very low so that self-discharge isn't a concern. No high-voltage vampire loads exist but significant 12 V vampire loads would discharge the 12 V battery completely which would likely damage it. So I simply follow the recommendation of the BMW of Honolulu service manager to disconnect the 12 V negative battery cable as I have done with other vehicles that I have stored.

Before I disconnect the negative cable, I back up the driver profiles to a USB drive using a standard iDrive command, ensure that the burglar alarm isn't activated by unlocking the doors, and disconnect the high-voltage disconnect. I connect a battery charger to top up the 12 V battery and lock the doors using the physical key in the fob. When the 12 V battery is fully-charged, I close but don't latch the frunk to avoid having to use the emergency pull cable to unlatch the frunk when I return.

When I return, neither the 12 V nor high-voltage battery is significantly discharged. The high-voltage battery pack appears to self-discharge about 1% per month. The 12 V battery discharges enough that I attach a battery charger before reconnecting its negative cable. When the 12 V battery is fully-charged, I connect the high-voltage disconnect, reconnect the negative cable, and restore the driver profiles from the USB drive backup using a standard iDrive command. The entertainment system date and time should sync with the cellular or GPS signal (not sure which). There might be a couple of settings that must be reset, but nothing major.

I do not leave an EVSE connected because after the high-voltage battery pack is full, the high-voltage system and DC-DC converter shut down which stops 12 V battery charging. With the low Li-ion self-discharge rate, topping up the high-voltage battery pack charge might not occur more frequently than once per month, and not for very long each time. It is likely that this would be insufficient to prevent the 12 V battery from becoming very discharged, especially if the 12 V battery isn't new. Some i3 owners have reported dead 12 V batteries even though EVSE's had been plugged in.

The Li-ion battery cell degradation rate increases with the amount of time it is left at a high charge level. When new, the battery management system allows the maximum charge level of a 60 Ah battery cell to be ~95% which is too high for long-term storage if one cares about minimizing the cell degradation rate. I store our i3 with its battery pack charge level between 40% and 60%.

To prevent the wiper blades from possibly deforming after remaining pressed against the windows in the same position for months, I remove all 3 of them. When I store our i3 for at least 6 months, I lift it onto jack stands to prevent potential tire flat-spotting and rotting around the edges of the contact patch. I wonder whether allowing the shock absorbers to remain fully-extended holding the weight of the wheels and tires for months could cause problems, so I don't know whether I'll store our i3 on jack stands in the future. I cover our i3 with a car cover.

The original 12 V battery shows no sign of weakness after 5.5 years, so this storage procedure must not have reduced its life. Our high-voltage battery pack has apparently lost ~15% of its capacity despite our i3 having been driven only 10k miles. I can't be certain that this isn't mostly due to the battery management system reducing the usable capacity to reduce the number of battery pack capacity warranty claims. However, battery cell experts recommend storing a Li-ion battery cell at a charge level between 40% and 60%, so it seems certain that my storage procedure hasn't increased the battery cell degradation rate.

Another alternative that I have been using lately because of infrequent driving during the pandemic is to remotely unlock/lock the doors using the BMW Connected app. This starts the high-voltage system and DC-DC converter, so 12 V battery charging commences. After 30 minutes, I gave up timing how long the high-voltage system would remain on after an unlock or lock event, so I know that it's at least 30 minutes. So I send an unlock immediately followed by a lock command once per week so that the 12 V battery will be charged for at least 30 minutes each week. If one is confident that the BMW Connected app can reliably connect to its server which can reliably connect to one's i3's telematics module, it would be possible to charge the 12 V battery remotely while anywhere with an Internet connection. The 12 V battery's energy capacity is only 20 Ah x 12 V = 0.24 kWh compared with a 60 Ah battery pack's 18.8 kWh energy capacity when new. Even at a 50% charge level, the high-voltage battery pack could charge the 12 V battery from empty to full 39 times assuming no charging losses, so there's no concern about discharging the high-voltage battery pack while charging the 12 V battery remotely.

Aloha Art -

Mahalo nui loa - thatʻs quite an interesting post! Iʻm not so far from you - weʻre on Maui. Iʻve never played with the BMW Connect app before. Took a while to get it figured out - not totally intuitive and not well documented - but I got it working. With the car parked outside, I can unlock and relock it from inside, and I can trigger the climate controls to cool it down - very happy to find that little trick <g>...

So I guess that in order to try out your last paragraph suggestion, Iʻd need to take the car to the storage lot where Iʻm going to be leaving it, and make sure I can connect with it remotely. Iʻm not quite sure how to do that, but I expect that as long as I can park it, walk away some distance and trigger the doors and climate control, I can assume that itʻll work from long distance.

What does the car need in order to receive/send data? Iʻve never thought about how the car handles data - is it using internet or cell connectivity?

Mālama pono - Peter

The car uses its internal cellular connection when you communicate with it via the app, so, if the place has reasonable cellular coverage, the app should work. Now, FWIW, I've not been able to communicate with my car for a couple of months...it does report it's status, but it always fails with the 'sending to car' timing out. It's somewhat annoying, but not worth delving into much at this time.

ptourin said:

So I guess that in order to try out your last paragraph suggestion, Iʻd need to take the car to the storage lot where Iʻm going to be leaving it, and make sure I can connect with it remotely. Iʻm not quite sure how to do that, but I expect that as long as I can park it, walk away some distance and trigger the doors and climate control, I can assume that itʻll work from long distance.

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Your phone would need an Internet connection, either WiFi or cellular. As Jim stated, your car would need a good cellular data connection.

U.S. i3's use AT&T's cellular data network which is strong enough in our parking space for remote commands to work despite our parking space being on floor 2 of 6. The open sides of our parking garage might help. My iPhone also uses AT&T's network, so I can see its signal strength on my iPhone while standing near our parked car.

I suggest verifying that your remote command has succeeded rather than relying on the "success" message displayed by the BMW Connected app. The i3 telematics software seems buggy because it can sometimes transmit charge level information correctly yet fail to execute remote commands. Many i3 owners have rebooted their telematics modules by briefly removing fuse 115 in the fuse panel below the glove box which has restored remote command execution. I have not had to do this in since buying our i3 in 2014, so I feel lucky. However, this isn't a permanent fix.

Another recurring problem has been BMW's apparent inability to prevent server outages which would also prevent remote commands from being executed. This plus the buggy telematics software has helped me decide to merely disconnect the 12 V battery so I don't have to worry about it discharging and I don't have to be reminded to remotely execute unlock/lock commands weekly while away.

If your i3 quits executing remote commands while you're away, you'd have to ask someone to unlock and lock your doors weekly to prevent the 12 V battery from discharging too much.

If you decide to disconnect your 12 V battery, I can tell you how to avoid being suddenly shocked by the very loud burglar alarm siren sounding very near your head while disconnecting the 12 V battery :shock: Nice to learn that our i3's alarm sounds when the alarm is activated (i.e., the doors are locked) and the 12 V battery is disconnected. I never imagined that the siren has its own battery.

Thanks guys, this is tremendously helpful. Thereʻs good AT&T cell service all along the highway from Wailea to Kahului, and the car storage place is just off the highway. Itʻs a warehouse with roof and open sides so the car will probably see good cell service - I can of course stop over there with my phone and check. The guys who run the place are very obliging - they offer to drop you at the airport and take the car back to store it, then come and pick you up when you get back on the island. They said theyʻd watch the car and trickle charge as necessary - so if I find that I canʻt trigger the locks remotely, Iʻm sure theyʻd be happy to do it once a month - itʻs just 2 clicks of a remote, I think they can handle it <g>... They also do car repair there, so none of this is going to be difficult for them.

So this is a lot better than setting up a trickle charger for the chassis battery, or attempting to disconnect batteries. I think Iʻll have them turn the car on once a month and see what percent SOC the big battery displays, and I can stick a small digital voltage readout in the window and plug it into the auxiliary power receptacle, tell them that if it starts dropping down to 12.0 weʻve got a problem that Iʻll want to know about.

Big mahalo !! - Peter

ptourin said:

So this is a lot better than setting up a trickle charger for the chassis battery, or attempting to disconnect batteries. I think Iʻll have them turn the car on once a month and see what percent SOC the big battery displays, and I can stick a small digital voltage readout in the window and plug it into the auxiliary power receptacle, tell them that if it starts dropping down to 12.0 weʻve got a problem that Iʻll want to know about.

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You shouldn't worry about the high-voltage battery pack's charge level because it won't drop noticeably. There are no high-voltage vampire loads when an i3 is parked, not charging, and not preconditioning. Of course, having someone look at the charge level won't hurt and would turn on the DC-DC converter which will charge the 12 V battery for 30 or more minutes.

Unfortunately, measuring the voltage at the auxiliary power port won't provide much information. I had hoped that it would and have had a voltmeter in the power port for over 2 years. The power to the auxiliary power port automatically turns off within 15 minutes of turning off an i3, so the voltmeter has no voltage to display. When an i3's doors, frunk, or hatch are unlocked or opened, the high-voltage system including the DC-DC converter and the auxiliary power port turn on. The voltage at the power port will immediately begin falling quickly as various modules boot so the resting voltage of the 12 V battery isn't displayed. Within a few seconds, the voltage drops to maybe 11.9 V before the DC-DC converter turns on which will then increase the 12 V system voltage to ~14.3 V, the output voltage of the DC-DC converter.

If I were you, I'd send a remote unlock message immediately followed by a lock message weekly rather than monthly. I don't know whether sending a headlight flash or horn honk message would turn on the DC-DC converter, but doing so might surprise anyone nearby . The 12 V battery might discharge too low within a month to boot the high-voltage system and thus the DC-DC converter. Should that happen, you would no longer be able to charge the 12 V battery by sending remote unlock/lock commands and the 12 battery would continue discharging potentially to the point of damaging it.

An alternative method that I have used requires that you can arrange a solar panel reasonably close.
I have installed a small (10-15 watt) solar panel on the roof of my garage.
This runs through a small solar controller.
I have wired an electrical 2 pin socket into the 12V terminal under the engine cover in the back.
The socket resides on the left hand side wall of the boot.
Before I go away and leave the i3 unused for several weeks, I just plug the controller supply into the socket.
Done.

See below for details. But basically I left my 2017 i3 unplugged in a garage for 6 full months without
any accessory battery issues. Would any bmw engineer with intimate knowledge of the inherent
charging abilities of the i3 unplugged please comment.

There obviously is poorly confirmed theories of exactly what is going on between the accessory
battery and the high voltage battery in storage. The manual just telling people to not trickle charge
and to plug the car in is really not ideal.
I want to share my experience with my 2017 bmw i3 with regards to long term storage.
Here are the facts:
Car unplugged in garage. March 20 123 mile capacity, April 20. 118 mile capacity, June 19 115 mile capacity (91%), Sept 20 105 mile ,
capacity, These measurements were taken by my friend who checked on the car. The car started no problem on Sept 20 ( 6 full months left unplugged) . He took it for a 35 mile drive ( theoretically to charge the accessory battery) and returned to the garage unplugged and store further reading 85 mile capacity (62%).
Before I left, I spoke with BMW service/mechanic in Scottsdale (Penske BMW) who said that the accessory battery will trickle charge by itself from the high voltage battery. Also, the manuals recommendation to leave it plugged in for long term storage is only because leaving it 100 percent charged is the lesser evil than unplugged and risking discharge to 0. Both are actually undesirable for lithium battery permanent capacity loss . The best discharge rate is between 75 % down to 60% for the battery and I was able to achieve this because I had a friend checking on the car to ensure it will not go below 60 (otherwise he would plug it in for me which did not happen).
There was no need to remove the accessory battery and trickle charge it (difficult to do for the average person).
I really however would like a high level BMW engineer to confirm thru this thread that indeed the accessory battery is trickle charged from the high voltage battery when placed just in storage unplugged. Because other than verbal from the mechanic, I cannot for the life of me find anywheres on the internet or in any other forum this statement. This has led to much advice regarding diving into the front of the car and risking high voltage shock and removing the accessory battery to attach a trickle charge to it. So please if anyone who has formal training in the design of this car's electronics, please confirm this mechanic's statement that the accessory battery is indeed trickle charged by the high voltage battery when left unplugged.
P.

pmpleung60 said:

Car unplugged in garage. March 20 123 mile capacity, April 20. 118 mile capacity, June 19 115 mile capacity (91%), Sept 20 105 mile ,
capacity, These measurements were taken by my friend who checked on the car.'

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Every time your friend checked the estimated range, the high-voltage system turned on and began charging the 12 V battery and continued to charge it for at least 30 minutes after he shut off your car. This must have been sufficient to charge the 12 V battery enough to be able to start the car.

pmpleung60 said:

Before I left, I spoke with BMW service/mechanic in Scottsdale (Penske BMW) who said that the accessory battery will trickle charge by itself from the high voltage battery.

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I suppose anything is possible. However, there's little evidence that this occurs. There would have to be a 12 V system voltage monitor that would fire up the high-voltage system to charge the 12 V battery when its voltage dropped below a certain level. I've found no documentation for such a monitor. Those who have installed 12 V battery loggers have not reported periodic 14+ V readings that would occur if charging were occurring.

However, a REx owner claims to have parked his i3 for over a year without an EVSE plugged in and without the 12 V battery discharging enough that the car wouldn't start, but with the battery pack's charge level dropping significantly. That's difficult to explain unless some 12 V charging were occurring. Several other i3 owners have reported dead 12 V batteries even though they stored their cars with an EVSE plugged in. I've never seen evidence that the 12 V battery is charged for more than 30 minutes after the battery pack has reached a full charge and charging ceases because the high-voltage system turns off automatically.

pmpleung60 said:

Also, the manuals recommendation to leave it plugged in for long term storage is only because leaving it 100 percent charged is the lesser evil than unplugged and risking discharge to 0.

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From the i3 Owner's Manual:

"With storage times of up to three months, if possible plug the vehicle into a suitable power source or park it in a nearly fully charged state.
Note
The service center can advise you on what to consider when storing the vehicle for longer than three months."

Prior to storing our i3 for 6 months, I asked our BMW service manager for advice. He told me to disconnect the 12 V battery to prevent it from discharging. I fully charged the 12 V battery and disconnected its negative cable. I stored our i3 with the battery pack charge level at ~50%. When I took our i3 out of storage, the charge level was ~44%. I recharged the 12 V battery. Li-ion and AGM battery cells have low self-discharge rates, and there are no vampire loads on either when the 12 V battery is disconnected, so neither had lost much charge over 6 months. I have since stored our i3 3 additional times for periods of 3 to 9 months each time. I followed the same storage procedure and have never experienced any battery problems. In fact, the 12 V battery is the original battery and still works normally.

pmpleung60 said:

There was no need to remove the accessory battery and trickle charge it (difficult to do for the average person).

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It's not necessary to remove the 12 V battery to charge it. Just disconnect its negative cable prior to charging it in place. This is easy to do. I always disconnect the high-voltage disconnect as well just to be safe.