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BMW i3 REx 2014
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297 Posts
In theory, just keeping it plugged in might be enough - the recovery method when the 12V battery is discharged was described in the now removed on-line BMW workshop manual as:
  1. open car door with manual key,
  2. open charge door with manual pull
  3. plug car in
  4. wait for 12V battery to 'fill' enough for relays to bring online the traction battery which then keeps 12V charging.
Not tested by me though.
 

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BMW i3 REx 2014
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297 Posts
Three things to consider:
  1. flashing the lights via the app engages some relays, so this may charge the 12V battery - too cold to wait to hear the relays switch off
  2. the car will tell you if the battery is failing and so prone to premature discharge
  3. I think rusty brake disks are the main problem, so a weekly drive is a must - a few miles can be enough - kill 2 birds, etc.
 

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BMW i3 REx 2014
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297 Posts
The car is coming up to four years old in March, that seems to be from what I have read when the battery should be changed.
When I bought my car 2 years ago, I asked for the BMW diagnostics on the batteries. I got a printout saying the 12V battery 'performance' was at 98% and was fitted at 0 km. This means my battery is now 6 years old - still seems OK .......
 

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BMW i3 REx 2014
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297 Posts
More 12V battery info. It seems to be available via the MyBMW (connected drive) portal. Request a data archive from the BMW CarData webpage. I got a snapshot of the data from 16/3/2021. It includes the 'Low-voltage battery' status, which on my 6+ year old battery is still 85%. Nothing on the traction battery capacity - my kapa is now 16.2.

It also shows 'Confirmed fault memory', I have had an EME (this was actually changed) and an SME (previous owner?) fault.
 
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