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Discussion Starter · #1 ·
Ok, bit of a newbie question, i've had a bit of a Google and a look on the forum and apologies if I have missed this answer .... So, lovely i3, 4000 miles from new two in months (all part of the plan) all going swimmingly (brilliantly in fact, smitten!). The weather is getting colder so we are getting into REx territory when completing the daily commute and I have the car set to off-peak charging.

When I first plug the car in I always get the initial "clunk" of the contractor in my Rolec, on then after a few seconds off again (now the car is waiting for the built-in off-peak timer). But, for the last two nights the car has gone on charging for a few (quite a few yesterday) minutes to bring the overall charge state of the battery to 10% or 12% ... Is this normal?

It's not a real concern, I am just wondering if there is a minimum charge configuration I have missed reading up on where the car will always charge-up regardless of on the off-peak timer or not??

As always, in anticipation for the answer to be provided .... Thanks in advance.

Cheers - Mark
 

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Yes this is completely normal. If the charge level is low when you plug in it charges to as you say around 10% so the battery doesn't sit for long periods of time at a low level. Theoretically your next departure time could be in 6 days time (if you have just one day selected in the charging scheduler) and so I guess as a fail safe, the software just charges it to the safe level whenever you have off peak charging set.
 
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Oooo. Can I hijack this with a quick question? Looking at the BMW website for charging the I3 33kwh battery 100% from empty takes 4hrs 50 mins on the 32a I wallbox pro. So this means around 36kwh for a full charge.

Is this about the same for an empty i3 rex to 100% or is it 6% less, around 34 kwh ?

Cheers.

M
 

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Discussion Starter · #4 ·
Thanks to Alexander for his input, sounds sensible to me.

With regards to the new point, is not the usable battery (27.2 Kw) the 100% or the full 33Kw 100%?
 

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Fender boots hi just out of curiosity what is your daily commute that is now getting u into rex territory?


Sent from my iPhone using Tapatalk
 

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Discussion Starter · #6 ·
I always drive on Eco-Pro, fully pre-condition (time of departure + pre-condition), 96 mile round-trip. Cold(ish) weather just starting and still getting used to the heating/driving of the lovely i3 (only 65% efficient on the drive home tonight), having said that, I only used 69% in total battery today, so it's not going too badly.

Yesterday was the first time I was down to single figures (6%) at home last night and the few min of charging was a complete surprise.

It's all a bit variable at the moment, but I will get there ..... Is there any hint's and tips on the heating for cold weather? Leave it alone, use the heated seats (sparingly as they are way too hot even on the first setting for more than a couple of minutes!). Mess around with the air-con recirc.?
 

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Heated seats are WAY more efficient than cabin heating but heated seats don't keep your hands and feet warm...! On Eco Pro it reduces the intensity of all heating so I find setting 1 in eco pro for the seats is just about right for more gentle bum warming! I never touch the recirculation button unless I am behind a smoky diesel and smell their fumes.

Main thing that helps is preconditioning the car on at least a 7kW charge point. Then it activates the battery preparation (heating/cooling) and optimises that for the start of the journey which has an actual effect on range along with pre heating the cabin so it then can be set to low from the word go and still be perfectly comfortable.

No major tricks other than that for me. Speed is king in terms of efficiency and therefore range...
 

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Thanks to Alexander for his input, sounds sensible to me.

With regards to the new point, is not the usable battery (27.2 Kw) the 100% or the full 33Kw 100%?
Yes, 33 is the overall battery capacity, not useable. Also the charging tapers off after about 90% so as not to over charge any cells. So the calculation of 7.4kW X 4 hours 50mins is not correct. For much of the last hour or so it is not charging at anywhere near full speed.

It will get to 90-95% quickly and then the last few percent take ages relatively.

Lastly, the i3 REx will charge quicker generally from 'empty' because the car will always kick in the REx with about 6% charge remaining so as to always have a buffer for overtaking etc. Therefore even at empty you have around 1.5kWh in the battery unless you drive fast to deplete the battery faster than it can charge from the REx... generally this is only when you gun it off roundabouts constantly or are sitting at 75mph+.
 

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Also the charging tapers off after about 90% so as not to over charge any cells.
This isn't quite true, the system is set to charge as fast as possible until 100% (there is enough overhead beyond 100% to guard against over charging), the 94aH will very rarely if ever taper its charge. (This is what the UK tech's told us)
 

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Main thing that helps is preconditioning the car on at least a 7kW charge point. Then it activates the battery preparation (heating/cooling) and optimises that for the start of the journey which has an actual effect on range along with pre heating the cabin so it then can be set to low from the word go and still be perfectly comfortable.
Battery preconditioning will always activate so long as you are connected to a power source and the departure time is set to at least 3hours in the future. There are situations when using 13a OUC where the power required to heat the cabin cannot be replaced as quick so you don't get 100% charge. In the USA the OUC provides too little power at 120v in any situation, so its disabled.
 

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This isn't quite true, the system is set to charge as fast as possible until 100% (there is enough overhead beyond 100% to guard against over charging), the 94aH will very rarely if ever taper its charge. (This is what the UK tech's told us)
Really? Huh that's interesting then. Maybe the estimate for when it's full is off on my car then. Frequently I will be home for a bit and leave it charging, it will say it's 45 mins from being full but when I get it it's sat at 98% SOC or something close to full. Perhaps it would actually be full 30 mins before it thinks but is pessimistic on the dash.

I've never actually measured the taper or not, just my gut feeling which may be more software related...
 

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I always assumed that it tapered as part of cell balancing. But tech said it will charge as fast as it can until 100%, then the car goes to sleep, after about 1hr it wakes up the battery pack and uses small (really small) resistors to start draining the cells to balance them, it can take upwards of 10weeks to balance the pack as we are talking about a mA drain at a time.

This is one of the reasons why charging to 100% is encouraged, if you never charge to 100% and leave it long enough to "sleep" the pack will never balance.
 

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I might fit a power meter to my 11kW charger to see if there is any actual taper.

Very interesting (relatively ) About the cell balancing
 

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Discussion Starter · #15 ·
Just took a screen grab from my power monitoring software (see attached) from my charge this morning. Charging started at 02:40am (3.493 Kw) and the main charging stopped at 05:40am (3.553 Kw). The 1st drop-off peak was at 06:00 (2.025 Kw) and 06:15 (1.4 Kw). Pre-condition started at 06:50 (3.59 Kw).
 

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Are you charging on 'reduced' charge speed or from a 16amp charging station? How comes its just 3.6kW?
 
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