8 Reasons Charging Speeds May Differ

As one of the country’s premier installers of EV infrastructure, we’re often asked by clients why vehicle charging speeds tend to differ so dramatically. The answer is… there is no one answer. In fact, reasons can be diverse and complex. Below, we summarize several factors that may be affecting your charging speeds. The more you understand about these factors, the better prepared you’ll be to maximize your rate of charging while protecting your vehicle’s longevity.

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Charger Specifications

The type of charger being used is critical. Level 3 (DCFC) will charge much faster than Level 2, which in turn is much faster than Level 1. EVCS only installs Level 2 and 3 chargers. A charger’s power rating (measured in kilowatts) increases with each level, and just like greater pressure through a ventilation system blows more air out at faster speeds, so will a 100kW charger power up a battery faster than a 50kW one. Moreover, vehicle batteries only accept direct current, so chargers that only use DC power will be faster because they don’t require the excess time to convert AC power to DC.

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Vehicle Battery Capacity

Just as the power rating on a charger might limit speed, the vehicle’s battery may also have limitations that render the charger’s maximum capability moot. Your EV battery has both voltage and current limits that determine power (kW) intake. If your vehicle’s voltage limit is 380V, for instance, that will cap your charging speed even if the charger offers up to 500V.

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Grid Capacity

A robust and effective electrical grid is instrumental to fast, reliable charging speeds. Factors like dependable energy sourcing, peak load management, demand response and real-time performance monitoring by both utility companies and network operators like EVCS can affect charging speeds.

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Connector Type

Different connectors like CHAdeMO, CCS, J1772 and Type 2 Tesla offer varying designs that range from three to seven pins and single- versus three-phase power, all of which can alter maximum driving range per hour of charging, independent of the charger or vehicle battery capacity.

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State of Charge

Batteries charge faster the emptier they are because it’s then easier for charged ions to find space to fill. However, vehicle manufacturers install a BMS (battery management system) to preserve battery health because lithium-ion batteries can’t handle high voltages for extended periods. This system will regulate how fast your battery charges at different stages, slowing it down when the battery is at 80% since it requires the same amount of time to go from 0% to 80% as it does to go from 80% to 100%. This sometimes results in the EV’s charging display being slightly off from the actual charging state.

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Hot or Cold Car Batteries

Batteries charge best at optimal temperatures, which is anywhere from 50°F to 95°F. The EV’s thermal management system (part of the BMS) will slow the charging speed if it’s extremely hot outside because fast-charging an already hot battery could cause overheating, which will in turn damage the battery. Likewise, charging slows in overly cold weather because cool air diminishes conductivity, making the transfer of electricity to the battery less efficient.

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Battery Age and Condition

Over time, battery capacity and efficiency will diminish. Call it basic wear and tear. This results from many things, including battery age, manufacturing quality, total miles driven, amount of fast charging done, and the extent of general care and maintenance over the battery’s lifespan.

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Additional Loads

Using any part of your EV that consumes electricity – for instance, the radio, phone charger, heating/AC unit, lights or navigation – while charging diverts energy away from the battery, thereby slowing the overall rate of charging. Also, the BMS uses energy to regulate charging based on many factors and can affect charging speed depending on how much power it must siphon to do its job.

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For more information on how lithium-ion batteries work, check out this article from the DOE: https://www.energy.gov/science/doe-explainsbatteries

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