BMW App showing longer 120V charging time?

[DealMasterMike]

It seems like the charger that came with the car is charging slower than usual, or the BMW App is messed up in calculations.

Based on my screenshot it says 30% of battery was used
120V *12A = 1,440 Watts which I assume I'm getting per hour

To be full I need about 9,000-10,000 Watts, so I should be able to generate this in about 7 hours.
Why is the app showing I need to be charged for 10 hours?

Reverse math would be 10000 watts/10 hours = only 1000 watts (or less?) per hour??

Can anyone help explain this?

TIA

 

[jadnashuanh]

First off, there are losses in the conversion from 120vac to the high voltage DC needed to recharge the vehicle. Second, the conversion from 120-DC is not as efficient as from 240-DC, so, even comparing that, things would be slower. Third, your input voltage may or may not be 120vac...so, even if ideally set, you may not get the wattage you claimed. Fourth, the car does not charge at the full rate throughout the whole cycle...it slows down once it gets to the full mark. Fifth...some of that energy might end up being used for cooling the battery pack (or heating it in the winter). Sixth, the signal being sent from the EVSE might be slightly off, and the car is just responding by limiting the maximum current to what it is reporting.

So, bottom line, it's not linear, and there are losses along the way.

 

[theothertom]

Mine charges at around 3% per hour. So in your case, you need to add 30%, or about 10 hours worth of charging. This mirrors my experience.

 

[alohart]

Based on my screenshot it says 30% of battery was used
120V *12A = 1,440 Watts which I assume I'm getting per hour

Watts is a unit of power. When 1,440 watts, or 1.44 kilowatts (kW), of power have been delivered for 1 hour, 1.44 kWh of energy have been delivered.

Are you certain that your EVSE delivers 12 A of current? The occasional use cable (OUC) included with 2014 and early 2015 i3's delivered 12 A of current but all OUC's since have delivered only 10 A, or 1.2 kW of power.

To be full I need about 9,000-10,000 Watts, so I should be able to generate this in about 7 hours.
Why is the app showing I need to be charged for 10 hours?

Your 94 Ah battery pack stores a nominal usable energy of 27.2 kWh. 27.2 x 0.3 = 8.2 kWh of energy required to reach full usable capacity. The measured i3 charging efficiency at 120 V 10 A is ~88%, so your OUC would need to provide 8.2 / 0.88 = 9.2 kWh to fill your battery pack. If charging were a linear process, it would take 9.2 kWh / 1.2 kW = 7.7 hours to charge to full assuming that your OUC can charge at 10 A max. But as has been written, charging power tapers down as the battery pack charge level approaches full, so the almost 10 charging time estimate seems reasonable.

For what it's worth, I have noticed that the charging time estimates provided by our 2014 i3 tend to be a bit longer than charging actually takes, usually by 15-30 minutes.