When it comes to charging your electric vehicle, speed isn’t the only factor to consider. Many EV owners are curious about whether adjusting their charging amperage can impact efficiency and potentially save money. Let’s delve into the details of EV charger efficiency, particularly focusing on 3.6kW and 6.6kW onboard chargers, to understand if charging slower is actually better.
The discussion often starts with the idea that slowing down your charge rate might lead to greater efficiency. This concept stems from observations and research into the efficiency curves of EV chargers. For instance, studies on older Nissan Leafs with 3.6kW chargers have indicated that these chargers operate most efficiently at a charging rate lower than their maximum capacity.
Specifically, for a 3.6kW charger, the peak efficiency is often found around the 2kW charging rate. While the charger can handle up to 16 amps at 240V, efficiency tests reveal that it performs better at around 12 amps. This means that while charging at the full 16 amps will get your battery replenished faster, it might come at a slight cost in terms of energy wastage.
Data from research on Leaf models with 3.6kW chargers shows a difference in efficiency. At the maximum charge rate for the 3.6kW charger, efficiency was measured around 88%. However, when charging at a reduced rate that maximized efficiency, the efficiency climbed to approximately 89%. This suggests a marginal gain in efficiency by dialing back the amperage.
In contrast, the behavior of 6.6kW chargers is different. These more powerful chargers tend to become more efficient as the charging amperage increases. For 6.6kW chargers, maximizing the charging rate generally leads to better overall efficiency. Research indicates that the peak efficiency for a 6.6kW charger can exceed 90%, and this is typically achieved at or near its maximum charging amperage.
However, there’s another crucial element to consider: overhead waste. While reducing amperage might improve the charger’s internal efficiency slightly for 3.6kW units, it also extends the total charging time. During the entire charging process, the EV’s ancillary systems, such as battery management, cooling, and other electronics, consume power. This is the “overhead waste.”
If the charging process takes longer due to a reduced amperage, these ancillary systems will operate for a more extended period, potentially increasing the overall energy consumption. For 3.6kW chargers, the efficiency gain from reduced amperage might be offset, or even negated, by the increased overhead waste from the longer charging duration.
Calculations suggest that for a 3.6kW charger, the difference in efficiency between charging at full speed (around 16A) and a more efficient rate (around 12A) might be quite small – possibly around 2%. When factoring in a relatively constant overhead waste, this small efficiency gain might be practically negligible in terms of overall energy savings.
The “sweet spot” for charging, therefore, involves balancing charger efficiency with the impact of overhead waste. For 3.6kW chargers, charging too slowly (significantly below 12 amps at 240V or 10 amps at 208V) could lead to the overhead waste becoming a dominant factor, negating any efficiency gains from the charger itself. Between a moderately reduced amperage (like 12A) and the maximum rate (16A), the difference in overall efficiency might be minimal.
On the other hand, for 6.6kW chargers, the advice is clearer: charge as fast as possible. These chargers are more efficient at higher amperages, and reducing charging speed would not only increase charging time but also likely reduce overall efficiency due to the overhead waste accumulating over a longer period.
Considering seasonal variations, the optimal charging strategy might subtly change. In summer, battery cooling systems might work harder, potentially increasing overhead waste. In winter, battery heating might also add to overhead. However, the core principle remains: for 3.6kW chargers, a slight reduction to around 12A might be marginally beneficial, but for 6.6kW chargers, faster is generally better for efficiency.
In conclusion, while 3.6kW chargers show slightly improved internal efficiency at reduced amperages, the real-world impact on overall energy consumption is likely minimal due to the trade-off with charging time and overhead waste. For 6.6kW chargers, maximizing the charging rate is generally the most efficient approach. EV owners should consider these nuances to make informed decisions about their charging habits and optimize for both efficiency and convenience.
