[Paper Review] Reliability of Open Public Electric Vehicle Direct Current Fast Chargers

David Rempel. Carleen Cullen, Mary Matteson Bryan+1 more|arXiv (Cornell University)|Mar 30, 2022

Electric Vehicles and Infrastructure4 citations

TL;DR

This study evaluates the real-world reliability of 657 CCS connectors across 181 open public DC fast chargers in the Greater Bay Area, finding only 72.5% were functional during on-site assessments. The discrepancy with EVSP-reported 95–98% uptime highlights a critical need for standardized definitions and third-party verification of reliability metrics in public EV charging infrastructure.

ABSTRACT

In order to achieve a rapid transition to electric vehicle driving, a highly reliable and easy to use charging infrastructure is critical to building confidence as consumers shift from using familiar gas vehicles to unfamiliar electric vehicles (EV). This study evaluated the functionality of the charging system for 657 EVSE (electric vehicle service equipment) CCS connectors (combined charging system) on all 181 open, public DCFC (direct current fast chargers) charging stations in the Greater Bay Area. An EVSE was evaluated as functional if it charged an EV for 2 minutes or was charging an EV at the time the station was evaluated. Overall, 72.5% of the 657 EVSEs were functional. The cable was too short to reach the EV inlet for 4.9% of the EVSEs. Causes of 22.7% of EVSEs that were non-functioning were unresponsive or unavailable screens, payment system failures, charge initiation failures, network failures, or broken connectors. A random evaluation of 10% of the EVSEs, approximately 8 days after the first evaluation, demonstrated no overall change in functionality. This level of functionality appears to conflict with the 95 to 98% uptime reported by the EV service providers (EVSPs) who operate the EV charging stations. The findings suggest a need for shared, precise definitions of and calculations for reliability, uptime, downtime, and excluded time, as applied to open public DCFCs, with verification by third-party evaluation.

Motivation & Objective

• To assess the actual functional reliability of open public direct current fast chargers (DCFCs) in the Greater Bay Area.
• To identify common failure modes affecting EVSE (electric vehicle supply equipment) functionality.
• To compare real-world performance with reported uptime from EV service providers (EVSPs).
• To highlight inconsistencies in reliability definitions and reporting practices across the EV charging industry.
• To advocate for third-party verification and standardized metrics for uptime, downtime, and excluded time.

Proposed method

• Conducted on-site evaluations of all 181 open public DCFC stations in the Greater Bay Area.
• Tested each EVSE connector for functionality by attempting to charge an EV for at least 2 minutes.
• Classified non-functional EVSEs based on failure categories: unresponsive screens, payment issues, charge initiation failures, network outages, or broken connectors.
• Measured cable reachability to determine if physical limitations prevented charging.
• Re-evaluated 10% of EVSEs approximately 8 days later to assess temporal stability of functionality.
• Compared observed functionality rates with EVSP-reported uptime figures to identify discrepancies.

Experimental results

Research questions

• RQ1 What is the actual functional reliability of open public DC fast chargers in the Greater Bay Area based on direct field testing?
• RQ2 What are the primary technical and operational causes of EVSE non-functionality in public DCFC stations?
• RQ3 How does real-world functionality compare to the 95–98% uptime reported by EV service providers?
• RQ4 Does EVSE functionality remain stable over short-term intervals, as indicated by repeat assessments?
• RQ5 To what extent do inconsistent definitions of uptime and excluded time contribute to reporting discrepancies?

Key findings

• 72.5% of the 657 EVSE connectors evaluated were functional, meaning they successfully charged an EV for at least 2 minutes or were actively charging during inspection.
• 4.9% of EVSEs were non-functional due to cable length limitations, preventing connection to the EV inlet.
• 22.7% of non-functional EVSEs were caused by system-level failures, including unresponsive screens, payment system malfunctions, charge initiation issues, network outages, or damaged connectors.
• A follow-up evaluation of 10% of EVSEs after approximately 8 days showed no significant change in functionality, indicating stable performance over short timeframes.
• The observed 72.5% functionality rate significantly contradicts the 95–98% uptime reported by EV service providers, suggesting reporting inconsistencies.
• The study concludes that shared, precise definitions of reliability metrics—including uptime, downtime, and excluded time—are urgently needed, with verification through independent third-party assessments.