12V battery

[MarkH]

Hey Art.
Thanks very much for all that advice.
I know I would be a nervous wreck to find out all of that while doing it for the first time.
Very very helpful.

 

[jadnashuanh]

If the car was going to be in say my garage, I'd just leave it plugged into the EVSE. What's your time worth? Probably a lot more than whatever amount of power it might use in two months bumping things back up (which may only be a few times a month). It would also give you the opportunity to remotely monitor things, assuming where it's parked has cellular service. That has to be worth something, too.

 

[MarkH]

I probably would do that, if I had an EVSE.
I'm assuming that an EVSE has more smarts than just a domestic AC outlet.

I also assume that the in-line charging unit that I use to recharge the vehicle from my domestic supply will automatically stop charging once the battery is at 100%.
What I don't know is: does it automatically start charging again once the battery SOC drops to some threshold?
If it does, then that is probably my best solution.
(I say this because when I look at the instructions for disconnecting the 12V battery for charging, I find that the device (in the frunk) for isolating the 12V from the HV on my car is NOT as is shown in the photographs :roll: )

My HV battery is 33kWh or 94AHr.
I understand that there is a process within the car whereby the HV battery keeps the 12V battery topped up through a DC-DC charger.
If the only drain on the batteries is phantom current being drawn from the 12V battery, how big is this current and, thus, how long will it take to drain the HV battery down to, say, 15% - assuming that the 12V drain will always be replenished from the HV?

 

[vreihen]

The OUC cord is an EVSE! The i3 has an intelligent charger built-in. Leave it plugged in, and let the electronics do their thing!

(Says the guy who parked his i3 unplugged on the Tuesday before Thanksgiving with less than half a charge, and is praying that the 4-year-old 12V battery will still have enough charge to pop the frunk if/when the snow bank it is buried under ever melts.....)

 

[alohart]

I understand that there is a process within the car whereby the HV battery keeps the 12V battery topped up through a DC-DC charger.
If the only drain on the batteries is phantom current being drawn from the 12V battery, how big is this current and, thus, how long will it take to drain the HV battery down to, say, 15% - assuming that the 12V drain will always be replenished from the HV?

Your assumption that the DC-DC converter always charges the 12 V battery or will be charge it when its voltage drops below a certain level is an incorrect assumption. The DC-DC converter is a high-voltage device. The high-voltage system disconnects several minutes after an i3 is turned off and doesn't turn on until a door is unlocked or opened, the frunk or hatch is opened, or charging or preconditioning is active. So an i3 parked for a month or more risks having its 12 V battery discharge so far that it has insufficient voltage to start the high-voltage system which is required to drive.

One 2014 REx owner reported that his i3 was able to start after being parked for more than a year without having been charged. He reported that the high-voltage battery pack had discharged considerably while parked. The high-voltage battery pack in our i3 self-discharges only ~1% per month while our i3 is in storage with the 12 V battery disconnected. The only explanation is that the high-voltage system had either remained active so that the 12 V battery didn't discharge or had become active when the 12 V battery needed charging which discharged the high-voltage battery pack more than 1% per month. This behavior hasn't been reported by any other i3 owner to my knowledge, so I assume that something malfunctioned. If this behavior were normal, BMW wouldn't recommend that an i3 parked for an extended period of time be connected to an EVSE.

The i3's 12 V battery stores 12 V x 20 Ah = 240 Wh = 0.24 kWh. A 60 Ah battery pack contains a nominal 18.8 kWh of usable energy, about 78 times as much energy as the 12 V battery. I don't know the current being consumed by vampire loads. However, a full high-voltage battery pack could almost certainly replace the energy consumed by the 12 V vampire loads for a few years.

 

[alohart]

The OUC cord is an EVSE! The i3 has an intelligent charger built-in. Leave it plugged in, and let the electronics do their thing!

Some i3 owners have reported a dead 12 V battery after parking for an extended period with an EVSE plugged in. This makes sense because the 12 V battery is charged only when high-voltage charging is active; i.e., when the high-voltage battery pack is full, charging ceases, the DC-DC converter turns off, and 12 V battery charging ceases until the high-voltage battery discharges sufficiently for the EVSE to begin charging again. This could take a few weeks.

However, there is a workaround. When the doors are unlocked remotely via the BMW Connected app, the high-voltage system turns on and 12 V charging begins (I just verified this). However, the high-voltage system will turn off after a few minutes. Locking the doors remotely should start the high-voltage system again. Therefore, remotely unlocking and then locking after 15 minutes would charge the 12 V battery for maybe as long as 30 minutes. If this were done weekly, the 12 V battery would probably be charged enough to allow an i3 to be driven.

I'll continue to disconnect the 12 V negative cable just before parking for an extended period so I don't have to worry about whether the remote unlock/lock were successful. I'm not comfortable leaving our EVSE powered for an extended period of time in our condo garage parking space. Not only would it waste some electrical energy while idling, but if it malfunctioned or overheated, it could cause problems. I always turn it off when charging has completed using an in-line on-off switch so that it doesn't remain powered.

To avoid the need to disconnect the negative cable, I am considering installing a battery disconnect like this assuming that its battery post clamps are large enough.

 

[MagicCarpet]

Also when the 12v battery gets charged it happens very rapidly. I have a little voltage display across the lighter socket and notice that the battery voltage can be down to 11.8 after a few days sitting and the moment a door is opened or the car is woken up there is a rush of charge and the battery becomes 14.4v within a few seconds.

The BMW technical training info states the DC DC charging inverter runs around 200 Amps. This may be why any sort of external charger needs thinking about, the onboard system could easily ruin an unsuitable charger.

Remote unlocking would only need to be done for a minute or so every 2 weeks to keep it up.

Ralph

 

[vreihen]

That jump of voltage is the DC-DC converter powering on and taking over 12V duties IIRC, not the AUX battery charging in seconds.....

 

[alohart]

That jump of voltage is the DC-DC converter powering on and taking over 12V duties IIRC, not the AUX battery charging in seconds.....

This is exactly what is happening. The 12 V battery is slowly being charged. A 12 V battery cannot be charged very fast because it would overheat and be damaged.

 

[jadnashuanh]

Maybe overkill, but set a departure time for say every MOnday. The load should trigger the EVSE to be turned on.

 

[vreihen]

For the archives...Programmed departure times will disable themselves after a few missed departures.....

 

[frictioncircle]

alohart –

I used to use a different style of quick disconnect on an ICE car years ago. It had an aggressive idle draw which would kill its full-size battery in about a week and a half.

After I sold that car I thought about installing the quick-disconnect on the replacement ICE car but decided against it. I was concerned about the potential for corrosion at the connections and any resistance increases that might subsequently occur.

 

[MarkH]

From the manual:

For idle phases that last several weeks, park
the vehicle with a fully charged battery if possible.
Do not park the vehicle for longer than 14 days
if the electric range is less than 6 mls, approx.
10 km.
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.

Given that the HV disconnect switch on my i3 is not the same as that displayed in any online instructions that I have seen (and which appears to be a prerequisite for disconnecting the 12V battery, to prevent it going flat, my intention, at this stage is to leave the HV battery fully charged; leave the frunk and doors unlocked. Maybe, with the doors unlocked, the HV battery will charge the 12V battery as needed.
I, also, am reluctant to leave the car plugged into the mains while I am away for 2 months.
This seems a nightmare and a major design oversight by BMW. Why not have the system turn itself on once a month and recharge the 12V: there's plenty of HV capacity?

 

[alohart]

I used to use a different style of quick disconnect on an ICE car years ago. It had an aggressive idle draw which would kill its full-size battery in about a week and a half.

After I sold that car I thought about installing the quick-disconnect on the replacement ICE car but decided against it. I was concerned about the potential for corrosion at the connections and any resistance increases that might subsequently occur.

After searching the Web for auto battery disconnect switches that would be easy to install on the negative battery terminal, I am not impressed with their quality based on reviews. It would just add a potential point of failure. I don't disconnect the 12 V battery more than once per year, so I'll just disconnect the negative cable which is quite easy to do, even without removing the frunk box.

 

[alohart]

Given that the HV disconnect switch on my i3 is not the same as that displayed in any online instructions that I have seen (and which appears to be a prerequisite for disconnecting the 12V battery, to prevent it going flat, my intention, at this stage is to leave the HV battery fully charged; leave the frunk and doors unlocked. Maybe, with the doors unlocked, the HV battery will charge the 12V battery as needed.

The design of the high-voltage disconnect must have changed over the model years. However, it's basically just a plug and socket that connect/disconnect a 12 V line, so there's no high-voltage risk. On early i3's, the plug cannot be separated fully from the socket and can be slid only about ½" to unplug it from the socket. A tab on the plug must be pushed in to move the plug. The plug is embossed with "ON" and "OFF" to indicate its state. Your high-voltage disconnect must operate similarly although it apparently looks different.

If you decide to take another look, be sure that the high-voltage system is off when disconnecting the disconnect. This is indicated by the Start/Stop button lighting being off. If it's a pulsing orange light or steady blue light, the high-voltage system is on.

There's no strong evidence that the HV battery pack will charge the 12 V battery as needed. In fact, it's just the opposite.

This seems a nightmare and a major design oversight by BMW.

I wouldn't describe merely disconnecting the 12 V battery as a nightmare. This would be necessary for any ICE vehicle as well. No modern vehicle can be parked for an extensive period without its 12 V battery being discharged.

Why not have the system turn itself on once a month and recharge the 12V: there's plenty of HV capacity?

That would have been a nice feature. However, without it, disconnecting the 12 V battery is the best way to prevent the 12 V battery from discharging. Leaving an EVSE powered and plugged in for weeks at a time doesn't absolutely prevent the 12 V battery from discharging. However, it's slightly easier to do than disconnecting the 12 V battery, so BMW seems to be recommending the easiest but less certain method.

Why does BMW recommend against charging with an EVSE for more than 3 months of parking? Might BMW recognize that leaving the HV battery pack at a high charge level for more than 3 months could lead to enough battery cell degradation to increase the number of battery packs replaced under warranty? That's another reason why I have always stored our i3 with its charge level at 40% - 60% without our EVSE plugged in and with the 12 V battery disconnected.

 

[jadnashuanh]

THe i3 waits until the HV batteries have discharged a couple of percent prior to re-engaging the EVSE. That process can take weeks in between. So, the EVSE, even though it's connected, isn't doing much of anything the vast majority of the time. There's a constant, but normally low, level of discharge on the 12vdc battery that maintains the alarm, the cellular connection (even if periodic), and the sensing, if you have comfort access. The internal clocks are still running, at least some of them. I haven't measured the current draw on the i3 when sitting idle, but would guess it's in the 10's of milliamps.

People have mentioned 'transport mode'. Don't know what that is, or how to activate or exit it, but it might provide some relief WRT battery SOC if someone were to figure that out. Anyone know what the characteristics of that mode are and how to enter/exit it?

 

[leocelli]

Here in Brazil I did like this. I3 Rex with 85.000 km and 3.5 years.

* portuguese

https://youtu.be/VX5AqfhLdtk

 

[MarkH]

Thanks for all the replies.

Art, I described disconnecting the battery in an i3 as a "nightmare" only because in an ICE there are no issues whereas with an i3 there are multiple warnings of dire consequences should you miss a step or do something wrong.

One of the references given in the posts, above, states that if charging the 12V battery then:
"If the integration level in the vehicle is 14-03-505 or more recent, then it is not necessary to deactivate the high-voltage system."
I have found in my i3 profile download a line that reads <i-step> I001-16-07-503 </i-step>

Can I safely assume, then, that, if I want to charge the 12V battery, I do not have to isolate it from the HV system?

It seems to me, from what I have read, that it may be satisfactory for me to do the following:
1. Permanently wire an Anderson socket into the 12V connections on the motor under the boot of the i3.
2. Purchase a small (10W - 20W?) solar panel with a PWM controller, terminated with an Anderson plug.
3. Whenever leaving the car unused for any length of time, put the car in "sleep" mode and connect the solar panel.
4. Leave a "safety notice" on the Start/Stop switch to remind me to disconnect upon my return - before starting it.

Can anyone see any "gotcha's" with this?

 

[AndySwan]

I left my 2015 REX (I001-16-11-502) for 3-4 months a few times just parked in the garage without plugging it in or disconnecting the battery, at different levels of SOC (about 60-90%), it has always started as usual afterward.

I'm not saying everyone should follow my lead, just my 50¢.

 

[MarkH]

Yeah, I think I may have been scared into taking unnecessary precautions.

Reading back through the advice in the manual, it seems that it is aimed at ensuring the HV battery comes through a period of storage unscathed.
If the DC-DC charger (of 12V battery) never activates unless the car is turned on, then either leaving the HV system fully charged or leaving the car plugged in does nothing at all for the 12V system.

As it happens, however, I have installed a 10W solar panel on the roof of the garage and today will connect it to a PWM solar controller. This can be plugged in to a socket that I have installed in the car, connected to the 12V charging terminals on the motor.

The system will be completed today and will, at least, give me peace of mind whenever I have to leave the car unused for several weeks/months.