Leave plugged in when away for a month?

[jcallum]

Anybody know if it is recommended to leave the i3 plugged in while you are away for a month? I am wondering if this would serve to keep the 12v battery topped up?

 

[EvanstonI3]

The car will automatically take care of the 12v whether it is plugged in or not. If the 12v fails (and it can), there is not really anything you can do to prevent it.

 

[jcallum]

I know about 12v failures. I am on my third 12v battery with a 2018.

 

[alohart]

My 12V system logger has shown our 2019 i3 automatically charging its 12V battery both when its charge level decreased due to not driving enough and when its 12V battery was failing, never with EVSE plugged in although that shouldn't matter. I don't think any of us knows under what conditions automatic 12V battery charging occurs. In my case, it seemed to happen when the 12V battery's voltage decreased below 12.5V (~67% charge level). Others have reported 12.0V (~25% charge level). Maintaining an AGM battery at a fairly low charge level could reduce its life expectancy, and being undocumented, I don't trust automatic 12V charging to function as I expect. Because I know doing so will work, I have always disconnected the 12V battery negative cable before being away for significant periods (8 times for 3 to 9 months each). I then fully charge the 12V battery with a battery charger before departing. When I return, I fully charge the 12V battery before reconnecting it. The 12V battery in the 2014 i3 that we bought new failed after 7.5 years, so disconnecting it while fully charged for months at a time didn't seem to shorten its life.

If your battery is in good condition, it should not discharge enough to cause problems over 1 month even without plugging in an EVSE. 12V battery charging normally occurs only when the HV system is on as when active HV battery pack charging is occuring. However, once the HV battery pack is full, a plugged in EVSE does nothing and only automatic 12V battery charging would charge the 12V battery.

 

[jcallum]

Ok. Thanks. I normally leave it unplugged with the HV battery over 80%. Haven't had any issues recently. This time I think I will leave it plugged in for a change. Someone told me once that BMW recommends leaving it plugged in when you go away for expended periods. Batteries are an enigma to me.

 

[alohart]

I normally leave it unplugged with the HV battery over 80%. Haven't had any issues recently. This time I think I will leave it plugged in for a change. Someone told me once that BMW recommends leaving it plugged in when you go away for expended periods.

The i3 Owner's Manual recommends leaving an EVSE plugged into a stored i3 for up to 3 months and, I believe, recommends asking a BMW Service Advisor how to store an i3 for longer periods. I don't understand what this would do because after the HV battery pack is fully charged, an i3 signals the EVSE to turn off the charging power. Then the EVSE just sits there in standby mode waiting for an i3 to signal it to begin charging. The HV battery pack's self-discharge rate is so low, ~1%/month, that charging the HV battery pack isn't necessary when an i3 is stored for up to 3 months, so why leave an EVSE plugged in? Much of an i3's behaviors aren't documented publicly, so maybe there's a good reason.

When we were living part-time in Sweden, our i3 would be stored for periods of 6 to 9 months each time, so I asked our local BMW Service Advisor what to do. He recommended storing at a charge level near 50% with the 12V battery disconnected which is what I've always done with no resulting problems.

We suffer more power outages and surges than most locations due to our isolated power grid (no interconnections). These can damage electrical equipment. If an EVSE is actively charging an i3 during a power surge, that surge would be passed through to the on-board charger. If it's damaged, an expensive repair would be necessary. Our charging circuit includes and on-off switch that I always turn off when not charging so that any power surge would not be passed to our EVSE which could damage it.

 

[madasa fish]

We always disconnect - or switch off - all electrical devices - except alarms - when we leave the house for more than 2 days.
There is a small but perceptible risk of failure leading to fire in most appliances so not worth the risk.