WiremanWireman · EV charging slower than expected: troubleshooting for electricians · 2023 NEC ·
EV charging slower than expected
Level 1 vs Level 2, the charger setting, the car’s limits, 208 V vs 240 V and battery temperature.
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How it works
Charging speed is set by the lowest of three limits: what the circuit and charger allow, what the car’s onboard charger can take, and what the battery will accept right now. The charger only advertises a maximum current; the car decides how much to draw. Level 1 (a 120 V outlet) gives about 1.4 kW at 12 A, while Level 2 at 240 V and 32 A gives about 7.7 kW, so the same car can charge more than five times faster on Level 2.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- Is it charging on a 120 V receptacle or a 240 V charger?
- 120 V receptacle (Level 1)Normal: Level 1 is slow
- 240 V or 208 V charger (Level 2)Test: Measure the charging currentExpect: Close to the charger’s setting (for example about 32 A on a 40 A circuit, or 40 A on a 50 A circuit).
- Matches the charger’s setting, still slowWhat maximum current is the charger set to, and what size is the breaker?
- Set lower than the circuit allowsCharger set below the circuit’s capacity
- Set to match the circuit (80% of the breaker)Test: Measure voltage while chargingExpect: About 240 V (or 208 V on a 208Y/120 service), dropping no more than a few percent under load.
- About 208 VNormal for a 208 V service
- Drops more than about 5% under loadExcess voltage drop or a loose connection
- Normal 240 V, little dropThe car’s onboard charger is the limit
- Well below the settingIs the battery cold, nearly full, or has the car set a reduced current?
- Cold, or above about 80%Normal: the battery is limiting the rate
- Car is set to reduced ampsCar is set to draw less current
- None of theseThermal derating at the charger or plug
- Matches the charger’s setting, still slowWhat maximum current is the charger set to, and what size is the breaker?