12V battery. Should a trickle charger be used

I believe the 12V battery drains over time if the car isn't driven enough miles for the alternator to keep it fully charged. The battery constantly uses some power for communicating, monitoring for theft, responding to unlock, etc This raises 3 important questions:

1. Is there any way to know the state of the 12V battery?
2. Can/should a trickle charge into the cigar lighter be used to keep it fully charged if the car doesn't get much use
3. Can the owner charge a dead 12V battery, or does it have to be done by BMW service?

With an ICE car, it's no problem, but I'm wondering if an electric car has circuitry that advises against either a trickle charger or charging with a Sears battery charger.

Ron

There is no alternator. The 12v battery is charged from the HV battery. If you check the owner's manual it says the 12v battery must not be charged.

The 12 v. battery is charged when the main battery pack is charged, so if one doesn't drive regularly, charging the main battery pack occasionally should keep the 12 v. battery charged. One could measure the voltage at one of the 12 v. accessory ports to determine the state of the 12 v. battery.

I will be disconnecting my 12 v. battery when I store my i3 for ~5 months soon. With my 12 v. battery disconnected, it shouldn't self-discharge fully. When I return, I will reconnect my 12 v. battery and charge my main battery pack which should recharge my 12 v. battery so that my i3 will be ready to drive.

Art, have you run that by an i-genius yet?

I thought the long term no driving option was to leave the car hooked up to an evse?

I33t said:

Art, have you run that by an i-genius yet?

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Actually, BMW of Honolulu's repair shop manager. In any event, I don't really have any reasonable choice.

I33t said:

I thought the long term no driving option was to leave the car hooked up to an evse?

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That's what the manual recommends, but our electricity will be disconnected, so our EVSE will not be powered. Besides, I still believe that Li ion batteries are happiest when stored at ~50% charge, so I will store mine at that charge level.

That recommendation might be made to prevent the 12 v. battery from being discharged completely by 12 v. loads that are always on. To prevent that, I will disconnect my 12 v. battery.

Fair enough.

Do you have friends about? I think the best option for the 12v would be to leave it on an intelligent charger. That way, it would be in tip top condition on your return and you're good to go once it's re-installed.

..the owner's manual it says the 12v battery must not be charged

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So don't do it. You can't disconnect the HV battery, and it will keep the 12V battery "trickle" charged forever.
Because even 0.1 Amp for 5 months, 24/7, would use only 4.3kWh of the HV battery to sustain the 12V current. To put 0.1A in perspective, that would flatten an ICE's 60Ah battery in about three weeks, and I don't hear people worrying about that happening unless they left the lights on while they were on holiday!
So I suggest you depart with the car fully charged, and let the car look after itself and the 12V battery. When you get back, I doubt if more than 25% of the HV battery will have been used.

FrancisJeffries said:

You can't disconnect the HV battery, and it will keep the 12V battery "trickle" charged forever.

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The HV battery pack charges the 12 v. battery only when the car is in the "Ready" state. So it would not charge the 12 v. battery while a car is in storage, yet the 12 v. battery would be continuously discharged by the remote receiver, the Comfort Access electronics, the clock, etc.

FrancisJeffries said:

So I suggest you depart with the car fully charged, and let the car look after itself and the 12V battery. When you get back, I doubt if more than 25% of the HV battery will have been used.

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If I leave the HV battery pack fully charged, it would still be close to fully charged after 5 months because there would be no HV loads to discharge it, and a Li ion battery cell has a very slow self-discharge rate. Even though "fully charged" is not actually maximum charge due to the buffer that exists to prevent overcharging, leaving a Li ion battery cell near maximum charge for extended periods isn't good for the battery cell's longevity.

I will follow the approach I used successfully with my previous EV which is to store it with the HV battery pack 40-50% charged and the 12 v. battery disconnected. Absorbent glass mat (AGM) 12 v. batteries like that in the i3 do not self-discharge as fast as conventional flooded-cell 12 v. batteries, so it should not be dead after 5 months. The car that I store for 6-7 months/year in Sweden also has an AGM 12 v. battery. Its manufacturer recommends that it not be trickle-charged when not in use because it will maintain its charge very well when not being used. I have never had a problem with its 12 v. battery after storing it for the past 7 years, so I expect my i3's 12 v. battery to be fine after 5 months.

I will plug in my i3 immediately when I return which will charge both the HV and 12 v. batteries. What I don't know is whether information that is stored when one parks an i3 will still be available after the 12 v. battery has been disconnected for 5 months. The car I store in Sweden loses its battery pack charge level information, so something similar might occur with my i3.

Since the 12vdc battery powers all of the electronics in the vehicle...everything will probably end up being reset to factory defaults when you reconnect things. Probably not a major issue, especially if you have saved whatever you can to a profile in the fob.

Well, we are on holidays, and a bit of a rush to get going. Completely forgot to plug in the i3 to the charger before we left!

Checked the app today because we are in range now (outback trip) and still communicating, so the 12v battery is holding up. It will be a total of 6 weeks when we return.

While I had online access, I unlocked the i3, so if the battery dies we can hopefully plug it into the charger.

As soon as we get home, will hook up the charger and hopefully the 12v battery will be ok. The main battery is sitting at 90%

Anyone else had this issue?

I33t said:

As soon as we get home, will hook up the charger and hopefully the 12v battery will be ok. The main battery is sitting at 90%

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I know that the 12 V battery will be charged while the Li-ion battery pack is being charged, but I don't know what the 12 V charging rate is or whether it would continue being charged after the Li-ion battery pack is fully charged and the car has signaled the EVSE to quit charging. So your 12 V battery might not be fully charged after your Li-ion battery pack has been charged. I suppose over time while driving, your 12 V battery will become fully charged.

Arrived back on Friday. The i3 was fine, the 12v battery had a low charge icon on the display, but clearly it was not dead.

Used the onboard charge cord that came with the i3, set it up on the display for minimum charge rate and plugged it in. Took a few hours to bring the main battery up to 100% Left it connected for 3 days. Went for a city trip today and everything is normal. So the 12v battery can last for 6 weeks without charge if it's in good condition and minimum power draw.