How to keep hibernating i3 battery healthy

My neighbor, (the i3 owner) will be out of town for months and left me to babysit her car.
Our apartment has no plug in charging options. If the indicator warrants it, I'll charge it at a public site.

Until then, will starting the car bi-weekly be of any benefit to keeping the batteries conditioned?
I realize this is not a combustion engine with an alternator to charge the battery while driving.

Thanks.

We store our i3 for 6 months every year. There are 2 batteries that need to be maintained during storage: the Li-ion battery pack that propels the car and the convention 12 V battery whose function is similar to that in any other car.

The Li-ion battery pack is electrically isolated from the car when the car isn't on or being charged, and it has a very low self-discharge rate, so the charge level of the Li-ion battery pack doesn't drop much in 6 months. However, It is best for this battery pack not to remain at full charge during several months of storage. Battery experts recommend storing Li-ion battery packs at ~50% charge level.

The problem is the 12 V battery. There are parasitic loads active at all times discharging the 12 V battery (e.g., keyless entry system, burglar alarm, entertainment system, mobile phone transceiver). So the 12 V battery will discharge completely which could ruin it if it is not charged periodically or disconnected. Without disconnecting the 12 V battery and charging it with a battery charger, the only ways to charge the 12 V battery are to turn on the car or charge its Li-ion battery pack, either of which would charge the 12 V battery at the same rate. I don't know how fast the 12 V battery is charged in either case, so it is difficult to recommend exactly how long the car should be on to increase the charge lost by the parasitic loads.

If the Li-ion battery pack's charge level is >50%, the simplest thing to do would be to turn on the car periodically. If the charge level is >80%, drive the car instead to reduce the charge level to ~60%. As the 12 V battery is charged, the Li-ion battery pack will be discharged slightly. Unfortunately, the car will turn off automatically when one exits through the driver's door, but it doesn't when one exits through the passenger's door. So maybe weekly, you could turn on the car, exit through the passenger's door, and return 30 minutes later to turn off the car.

If the Li-ion battery pack's charge level is <40%, then drive to a public charging station and charge to ~60% which should not take long.

Because our i3 is lifted onto jack stands and placed under a car cover during storage, it would not be easy to do as I have suggested above. So I simply disconnect the negative 12 V battery cable. I think that this is the best solution because the 12 V battery cannot discharge when disconnected. It, too, has a very low self-discharge rate. With the 12 V battery disconnected and the Li-ion battery pack's charge level ~50%, an i3 could remain in storage almost indefinitely without any intervention required.

One disadvantage with disconnecting the 12 V battery is that driver profiles are lost. These can be backed up to a USB thumb drive before disconnecting the 12 V battery and then restored after reconnecting the 12 V battery, but you might not want to bother with this. The driver profiles are the preferences specified by each driver, so if they are lost, each driver could reset his/her preferences in iDrive, set the side mirror positions, etc. I could tell you how to do disconnect the 12 V battery if you want to do this.

BMW's recommendation is to leave it attached to an EVSE...the thing disconnects itself once the car is charged, and if, over typically many weeks, the LiOn battery pack discharges enough, it will reactivate the charge. Not all LiOn cells are created equal. My preference is to follow the instructions of the manufacturer of those in use, not generic guidelines.

As I understand it, the 12vdc battery only gets charged when the car is connected to an EVSE (or CCS unit), or while it is 'running', i.e., in ready state. The battery is small, but I'm not sure how big the 12vdc charging circuit is in the car...that would determine how long, as well as how discharged, it would take to recharge the 12vdc battery. If that battery does discharge far enough so that it won't power the vehicle's computers, there's an emergency circuit in the charging pack that will charge only that battery until it has enough power to turn on the computers, and then, the 'normal' charging process will continue. Lead acid batteries don't like to be deep discharged, but an AGM battery is better at it than some other types. That certainly won't help if there's no place to plug in! BMW says to not try to jump start the car with a 12vdc battery. You'll probably fry something. You need to use the onboard charging circuits, and that requires it to get to the ready state or be connected to an EVSE (or CCS) unit.

Thanks Art and Jim,

Art:

Nice to hear at what % levels I am safe. At some point I may just unplug the 12 volt - so if it is not intuitive, please share the steps.

Jim:

Thanks for the warning not to jump start the battery. No powering options at my apartment, so leaving connected to EVSE not possible.

Thanks again,

Elliott

Have a look here: http://www.mybmwi3.com/forum/viewtopic.php?f=11&t=2438&p=39178#p39178

My i3 was 90% charged when we went away for a 6 week trip. I completely forgot to plug it in before we left.

When we returned, it was still operational. I plugged it into the standard charger at the low charge rate (set on the dashboard) so the 12v battery got as long a charge as possible. No problem.

If I was looking after a friend's i3, I would take it for a drive to consume more than 50% of the battery within 2 weeks and charge it back up to 100% and place it back at your friend's residence. Are you able to charge it at your own home, or run an extension lead to the car?

elliottcowand said:

Nice to hear at what % levels I am safe. At some point I may just unplug the 12 volt - so if it is not intuitive, please share the steps.

Click to expand...

I am half way around the world from our i3 which I stored in January, so I'm writing from memory.

• Follow the procedure in iDrive to back up each driver's profile to a USB thumb drive inserted into the USB port in the center console.

• Remove the covers on either side of the frunk box. Lift the finger grab areas on the front edge to free the Velcro attachments.

• Remove the frunk box which is attached by 6 Allen screws.

• Find the orange high-voltage safety connector to the right of the frunk box's location. Unfortunately, I cannot include an image of this connector. The plug part of this connector slides out <½" from its socket part, but a lock tab must be pressed to free the plug from its socket. While the lock tab is pressed, pull the connector a few millimeters from the socket. Do not pull any further or harder when resistance is felt. The plug cannot be pulled completely out of the socket. When the plug is pulled out far enough, "OFF" is visible on the plug. The high-voltage system is now in a de-energized state.

• Unbolt and lift off the cable from the negative battery terminal (on the left side of the battery when the battery is viewed from the front of the car).

• Follow the above steps in reverse to reconnect the 12 V battery.