Hello everyone,
I've had my Smart #5 Premium for about two weeks now, and I'm a bit concerned about the AC/DC conversion efficiency during home charging.
Background: I've done some research and I understand that when charging at home via AC, the On-Board Charger (OBC) must convert AC to DC. This process involves losses that typically increase as the charging current decreases, since you are moving away from the OBC's optimal operating point (usually its maximum rating). Most data online suggests an average efficiency loss of around 10%, or a range of 10-20%. Essentially, to put 100 kWh into the battery, you'd expect to draw between 110-120 kWh from the grid.
Before the #5, I had a Smart #1, which I charged using the 2 kW portable charger. I never paid much attention to efficiency then, because those chargers are notoriously inefficient and prone to overheating. However, just before picking up the #5, I installed a 7.4 kW Wallbox and have used it exclusively.
The Issue:
I've performed three charges in the last 10 days, and I've noticed something is off. Because of my domestic electricity contract limits, I charge at 16A (approx. 3.6 kW). I've tried setting this limit both via the car's software and the wallbox; the results are identical.
Test 1: Charged for 8 hours (11 PM to 7 AM, set in the car) outdoors in my courtyard. Temp was above 10°C (mild Italian climate), no conditioning ON during charge. The car gained 22% (from 64% to 86%). According to the Wallbox stats, 30 kWh were consumed.
Test 2: Same conditions, 8 hours. The car gained 21% (from 68% to 89%). Again, 30 kWh consumed.
I checked my photovoltaic system monitoring, which confirmed a constant draw throughout the charging period. There are no indications on data online that the wallbox is prone to malfunctioning or miscounting.
The Math:
Since the Smart #5 has a 100 kWh battery (approx. 94 kWh usable), the math is straightforward. Gaining 21% of the battery should require roughly 21 kWh. Pulling 30 kWh to achieve this represents a 30% loss. This is significantly higher than any figure I've seen reported elsewhere.
Yesterday, I ran a final test from 87% to 100% (I don't like charging to full, but I needed to test). As you can see in the attached screenshots, the car's software immediately overestimated the time required. To add 13% at 3.6 kWh, it should theoretically take about 3.5 hours, yet the car estimated (and took) 5 hours. This suggests the inefficiency is constant and starts immediately, rather than being caused by components overheating over time. In this session, 19 kWh were consumed to deliver a theoretical 13 kWh.
I realize that charging at 16A is low and penalties efficiency, but I am fairly certain my previous Smart #1 maintained at least 80% efficiency under similar conditions.
My questions for the community:
Is there a flaw in my reasoning or calculations?
Has anyone else experienced similar efficiency figures with the Smart #5?
Are these values "normal" for this specific OBC, or could my on-board charger be defective?
Thanks to anyone who took the time to read this long post. I'd appreciate any insights!
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