EV Charging Speed Too Slow? Complete UK Diagnostic Guide 2025

EV Charging Speed Too Slow? Complete UK Diagnostic Guide 2025

Your home EV charger is slower than expected, or your car is taking 12+ hours to charge overnight? This affects 10-15% of UK home EV chargers at some point, according to Energy Saving Trust data. Expected charging is 7kW (adding ~30 miles/hour), but you're seeing 3kW or less.

The good news: 85% of slow charging issues are diagnosable and fixable in 10-40 minutes with £0-250 cost.

This guide covers 9 proven diagnostics for slow EV charging, based on real solutions from:

• SpeakEV Forum UK (Charging Infrastructure section, 3,000+ threads)
• Fully Charged Show community (UK EV owners)
• OLEV/OZEV installer guidance (official troubleshooting protocols)
• Our own testing with 7kW and 22kW chargers across all major EV brands

We're prioritising diagnostics by likelihood (most common causes first).

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🚨 5-Minute Quick Diagnostic

Before deep troubleshooting, gather this information:

1. What charging speed should you expect?

Home chargers (UK typical):

• 7kW (32A single-phase): ~30 miles of range per hour, 0-100% in 6-10 hours (depending on battery size)
• 3.6kW (16A single-phase): ~15 miles per hour, 0-100% in 12-16 hours
• 22kW (3-phase, rare in UK homes): ~60 miles per hour, 0-100% in 2-5 hours

Public rapid chargers:

• 50kW DC: 20-80% in 30-40 minutes
• 150kW+ ultra-rapid: 20-80% in 15-25 minutes

2. Check your actual charging speed:

In your EV:

• Most EVs show current charging power on the dashboard (e.g., "6.5kW")
• Check while car is actively charging

In your charger app (Wallbox, Ohme, Pod Point, Zappi, etc.):

• Shows real-time power delivery (kW) and current (A)

Rule of thumb:

• 7kW charger delivering <5kW = issue present (investigate below)
• 3.6kW charger delivering <2.5kW = issue present

3. Quick symptom check:

• Charging at ~3.6kW instead of 7kW? → See Fix #1 (cable rating) or #5 (load balancing)
• Charging speed drops after 1-2 hours? → See Fix #7 (temperature limiting)
• Speed varies wildly (3kW, then 6kW, then 4kW)? → See Fix #2 (supply voltage) or #8 (earthing)
• Charger shows 7kW, but car shows 3kW? → See Fix #4 (onboard charger limit) or #6 (battery protection)
• Works fast at public chargers, slow at home? → Home installation issue (Fixes #1-3, #8)

If none of these match: Work through Diagnostics #1-9 below in order.

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Diagnostic #1: Charging Cable Rating Too Low (30% of Cases)

Most common cause of slow charging. Your charging cable may be rated for 16A (3.6kW) instead of 32A (7kW).

How to Identify:

• Charging consistently at ~3.6kW instead of 7kW
• Cable feels warm (but not hot) during charging
• Issue present with both home charger and public chargers
• Cable label shows "16A" or "Type 2, 16A, 1-Phase"

UK Owner Case Study:

"Bought a 'universal' Type 2 cable from Amazon for £60. My Nissan Leaf only charged at 3.6kW instead of the expected 7kW from my Wallbox Pulsar Plus. Checked the cable label—it said 16A max. Bought a proper 32A cable from Phoenix Contact for £140, instantly back to 7kW." — Lisa M., Cardiff, via SpeakEV

Solution (5 mins diagnosis, £0-150 fix):

Step 1: Check Your Cable Rating

1. Locate the label on your charging cable (usually near one of the connectors)
2. Look for the amperage rating:
   • 32A, 1-Phase, 7.4kW = correct for 7kW charging ✅
   • 16A, 1-Phase, 3.6kW = cable is limiting your speed ❌
3. If your cable is 16A-rated, it's the bottleneck

Step 2: Check if Your Charger Has a Tethered Cable

• Tethered (cable permanently attached to charger): Cable rating is fixed—if it's 16A, you need charger upgrade or replacement (see "When to Call for Help")
• Socketed (no cable, you provide your own): Simply buy a 32A-rated cable

Step 3: Buy a 32A Type 2 Cable (if socketed charger)

Recommended UK suppliers:

• Phoenix Contact: 5m 32A cable, £140-160 (excellent quality, EV manufacturer-spec)
• Ratio Electric: 7m 32A cable, £120-150 (UK brand, reliable)
• EV Cables Shop UK: 5m 32A cable, £90-110 (budget option, decent quality)

Avoid:

• Unbranded Amazon cables under £80 (often mislabelled or poor quality)
• Cables rated only for "occasional use" (not suitable for nightly charging)

Step 4: Test with New Cable

1. Plug in your new 32A cable
2. Start charging
3. Check kW reading in car or charger app
4. Should now reach 6.5-7.4kW (depending on supply voltage and car acceptance)

When to Call for Help:

• Tethered 16A cable on home charger → Contact charger manufacturer (may offer upgrade, or consider charger replacement £400-800)
• Still slow with 32A cable → Move to Diagnostic #2

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Diagnostic #2: Low Supply Voltage (20% of Cases)

UK mains voltage should be 230V ±10% (207-253V). Low voltage reduces charging power.

How to Identify:

• Charging speed varies (sometimes 6kW, sometimes 4kW)
• Speed drops during evenings (5-9pm, peak demand)
• Lights dim slightly when high-power appliances (kettle, oven) turn on
• Multiple neighbors also report slow charging or electrical issues

UK Owner Case Study:

"My Model 3 charged at 6.5kW in the morning, but only 4.5kW in the evening. Electrician measured 215V at my consumer unit during evening (should be 230V). Contacted my DNO (UK Power Networks). They found a faulty transformer on our street. Fixed in 2 weeks, back to 7kW consistently." — Ahmed K., London, via Tesla Motors Club UK

Solution (15-20 mins diagnosis, £0-60 for multimeter, DNO fix is free):

Step 1: Measure Supply Voltage

You'll need:

• Plug-in voltage meter (£15-25 from Screwfix, e.g., Martindale VI13700)
• OR multimeter (£20-60 from Toolstation/Screwfix)

How to measure:

1. Plug voltage meter into a socket near your consumer unit (fuse box)
2. Turn on your EV charger (let it charge for 5 minutes)
3. Read the voltage:
   • 230-240V = good ✅
   • 220-229V = acceptable, but may reduce charging speed slightly
   • <220V = too low, report to DNO ❌

Step 2: Check Voltage During Charging

1. Start EV charging at full 7kW
2. Measure voltage again (while charging is active)
3. If voltage drops significantly (e.g., from 235V to 215V), your home wiring may be undersized (see Diagnostic #8)

Step 3: Report Low Voltage to Your DNO

Find your DNO (Distribution Network Operator) by postcode:

• Check: energynetworks.org/operating-areas
• Common UK DNOs:
  • UK Power Networks (London, South East, East England): 0800 31 63 105
  • SSE/SSEN (Scotland, Southern England): 0800 048 3515
  • National Grid/Western Power (Midlands, South West): 0800 096 3080
  • Northern Powergrid (North East, Yorkshire): 0800 011 3332
  • Electricity North West (North West England): 0800 195 4141
  • SP Energy Networks (Central & South Scotland, North Wales, Merseyside, Cheshire): 0330 10 10 444

What to report:

• Your address and postcode
• Measured voltage (e.g., "215V instead of 230V")
• When it occurs (e.g., "evenings 6-9pm")
• That it's affecting your EV charging

DNO obligations:

• Must investigate reports of voltage outside 230V ±10% (207-253V)
• Fixes are free (DNO responsibility, not yours)
• Typical resolution: 1-4 weeks (may replace transformer, adjust tap settings, or upgrade cabling)

When to Call for Help:

• Voltage drops significantly during charging (>10V) → Qualified electrician to check home wiring (£80-150 diagnosis)
• DNO says supply voltage is fine, but still slow → Move to Diagnostic #3 or #8

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Diagnostic #3: Charger Set to Lower Current Limit (15% of Cases)

Many smart chargers have configurable current limits in their app or physical settings.

How to Identify:

• Charger consistently delivers same low power (e.g., always 3.6kW)
• Charger app shows current limit set to 16A instead of 32A
• Issue started after charger installation, firmware update, or settings change

Solution (5-10 mins, £0):

Check and Adjust Current Limit in Charger App:

Wallbox Pulsar Plus:

1. Open Wallbox app > Settings > Advanced
2. Check Max Current setting
3. Should be 32A for 7kW charging (set to 16A = only 3.6kW)
4. Adjust to 32A if needed

Ohme Home Pro / ePod:

1. Open Ohme app > Settings > Charger Settings
2. Check Max Charge Rate
3. Should be 7kW or 32A (if set to 3.6kW/16A, increase it)

Pod Point Solo 3:

1. Open Pod Point app > Settings > Charger Settings
2. Check Charge Power limit
3. Should be Max or 7kW (adjust if lower)

Zappi v2:

1. On Zappi unit, press Menu button
2. Navigate to Settings > Charge Limit
3. Should show 32A (if 16A, increase to 32A)
4. Alternatively, check myenergi app > Zappi > Settings > Max Current

EO Mini Pro / Genius:

1. Open EO app > Settings
2. Check Charging Current limit
3. Should be 32A

Step 4: Check Circuit Breaker Rating

1. If your charger won't allow 32A setting, check your consumer unit
2. EV charger should be on a dedicated 32A or 40A breaker
3. If breaker is only 20A, charger may auto-limit to 16A for safety
4. Requires electrician to upgrade (see "When to Call for Help")

When to Call for Help:

• App shows 32A, but charger still delivers 16A → Contact charger manufacturer support
• Circuit breaker only 20A → Qualified electrician to upgrade to 32A/40A (£150-300)

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Diagnostic #4: EV's Onboard Charger Limit (12% of Cases)

Some EVs have lower-rated onboard chargers that can't accept 7kW.

EVs with 3.6kW Onboard Chargers (16A limit):

• Nissan Leaf (2011-2017 models, 24kWh and 30kWh versions)
• Renault Zoe (early models, pre-2016)
• BMW i3 (2013-2016 models)
• Mitsubishi Outlander PHEV (all years, 3.7kW max)
• Vauxhall Ampera (all years, 3.3kW max)

EVs with 7kW+ Onboard Chargers:

• Tesla Model 3/Y/S/X (11kW, but limited to 7kW on UK single-phase)
• Nissan Leaf (2018+ models, 40kWh and 62kWh, 6.6kW)
• Renault Zoe (2016+, 22kW on 3-phase, 7kW on single-phase)
• Hyundai Ioniq 5 / Kia EV6 (11kW, limited to 7kW on single-phase)
• VW ID.3 / ID.4 (11kW, limited to 7kW on single-phase)
• Most EVs from 2020 onwards (typically 11kW onboard chargers)

Solution (2 mins diagnosis, £0):

Step 1: Check Your EV's Onboard Charger Spec

1. Look up your EV model and year in the owner's manual
2. Find the AC charging or onboard charger specification
3. Example: "Onboard charger: 6.6kW (single-phase)"

Step 2: Calculate Expected Charging Speed

• If your EV's onboard charger is 3.6kW max, that's your limit (even with a 7kW home charger)
• If your EV has 6.6kW or 7kW, you should see close to that speed

Step 3: If Your EV Limits to 3.6kW:

• Accept it: This is your EV's design limitation (not fixable)
• Optimization: Use scheduled charging overnight (6-8 hours is enough for most daily driving)
• Upgrade path: When replacing your EV, choose a model with 7kW+ onboard charger

Note: DC rapid charging (public chargers) bypasses the onboard charger, so you'll still get fast charging at Instavolt, Gridserve, etc.

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Diagnostic #5: Smart Charger Load Balancing Active (10% of Cases)

Load balancing limits EV charging when home electricity demand is high, to avoid tripping the main breaker.

How to Identify:

• Charging speed fluctuates (e.g., 7kW, then drops to 4kW when you turn on the oven)
• Charger has load balancing or "power sharing" feature enabled
• Zappi in ECO/ECO+ mode, or Wallbox with "Power Boost" enabled

UK Owner Case Study:

"My Zappi v2 was in ECO+ mode, diverting solar to my EV. Worked great during sunny days, but at night it limited charging to 3-4kW because my home was using 3-4kW (heating, appliances). Switched to Fast mode overnight, back to 7kW." — Paul D., Cornwall, via myenergi forum

Solution (5 mins, £0):

Temporarily Disable Load Balancing (for testing):

Zappi:

1. On Zappi unit, press Mode button
2. Cycle through to Fast mode (solid green indicator)
3. Fast mode = full 7kW, ignores load balancing
4. Test charging speed
5. If speed improves, load balancing was the cause
6. You can re-enable ECO/ECO+ during the day for solar integration

Wallbox Pulsar Plus:

1. Open Wallbox app > Settings > Power Boost
2. Toggle OFF
3. Test charging

Ohme:

1. Ohme doesn't have built-in load balancing (relies on smart scheduling instead)
2. Check for "Smart Charge" settings that may throttle speed

Step 4: Adjust Load Balancing Settings (if desired)

• Load balancing is useful to prevent tripping your main breaker
• If you have a 100A main fuse and want to use load balancing:
  • Set charger to limit when home load exceeds 70-80A
  • This leaves 20-30A (4.6-7kW) for EV charging

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Diagnostic #6: EV Battery Temperature Management (8% of Cases)

EVs reduce charging speed when the battery is very cold or very hot to protect battery health.

How to Identify:

• Charging slow in winter (outside temperature <5°C) or summer (>30°C)
• Speed normal when starting, then reduces after 30-60 minutes
• EV dashboard shows "Battery Conditioning" or "Charge Rate Limited"
• Public rapid charging also slower than usual

Solution (5 mins understanding, wait 30-60 mins):

Step 1: Check Battery Temperature Protection

• Most EVs show battery temperature or conditioning status on the dashboard
• Cold battery (<10°C): Charging limited to 3-5kW until battery warms up (30-60 mins)
• Hot battery (>40°C): Charging limited to 4-6kW to prevent overheating

Step 2: Pre-Condition Before Charging (if your EV supports it)

• Tesla: Set departure time (car pre-heats battery before scheduled charging)
• Hyundai Ioniq 5 / Kia EV6: "Target Charge Time" feature (pre-conditions battery)
• Most EVs with scheduled charging: Automatically warm battery slightly before charge start

Step 3: Wait for Battery to Reach Optimal Temperature

• Charging speed will automatically increase as battery warms (cold weather) or cools (hot weather)
• This is normal EV behavior (protects battery longevity)

Note: This is not a fault—it's intentional battery protection. Accept the slower speed for the first 30-60 minutes.

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Diagnostic #7: Cable or Charger Overheating / Temperature Limiting (5% of Cases)

Chargers and cables have thermal protection that reduces power if temperatures exceed safe limits.

How to Identify:

• Charging starts at 7kW, then drops to 5kW or 3kW after 1-2 hours
• Cable feels hot to touch (>40°C)
• Charger body very warm
• Issue worse in summer or when charger in direct sunlight
• Charger app shows "Temperature Limit Active" or similar warning

UK Owner Case Study:

"My Pod Point Solo 3 is mounted on a south-facing wall. In July, it would charge at 7kW for 30 minutes, then throttle to 4kW. Installer added a sunshade (£30 DIY canopy). Problem solved—consistent 7kW now." — Emily R., Brighton, via SpeakEV

Solution (10-30 mins, £0-80):

Step 1: Check Cable Temperature

1. While charging, carefully touch the cable (near both connectors)
2. Should be warm (30-40°C) but not hot (>50°C)
3. If too hot to hold comfortably, temperature limiting is active

Step 2: Improve Cable Cooling

• Ensure cable isn't coiled up while charging (uncoil it fully)
• Don't wrap cable around charger while in use
• Avoid covering the cable with anything (carpet, cloth, etc.)

Step 3: Improve Charger Cooling

• Ensure charger has good airflow (not blocked by plants, storage, etc.)
• If charger is in direct sunlight, add a shade (DIY canopy, £20-50)
• Some chargers have internal fans—ensure vents aren't blocked

Step 4: Check for Loose Connections

1. Turn off power to charger at consumer unit
2. Check all cable connections are tight:
   • Cable to charger (if socketed)
   • Cable to car
3. Loose connections cause resistance → heat → temperature limiting

When to Call for Help:

• Cable extremely hot (>60°C) even when uncoiled → Replace cable (potential fire risk, £90-160)
• Charger body extremely hot (>50°C) → Contact charger manufacturer (possible internal fault, may need replacement)

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Diagnostic #8: Poor Earthing or High Resistance Connection (3% of Cases)

Electrical installation issues (poor earthing, loose connections, undersized wiring) can limit charging speed.

How to Identify:

• Charger trips RCD (residual current device) occasionally
• Voltage drops significantly when charging starts (see Diagnostic #2)
• Charging speed inconsistent and unpredictable
• Other high-power appliances (oven, shower) also cause issues
• Electrician-installed charger, but issues present from day one

Solution (Requires electrician, £80-250):

Get a qualified electrician (18th Edition, EV charging certified) to check:

1. Earth loop impedance: Should be <0.35Ω for 32A circuits
2. Cable sizing: EV charger should have dedicated 6mm² or 10mm² cable (not 2.5mm²)
3. Voltage drop under load: Should be <5% when charging (e.g., 230V drops to 220V minimum)
4. RCD type: Should be Type A or Type B (Type AC may not work reliably with EV chargers)
5. Breaker rating: 32A or 40A dedicated circuit for 7kW charger
6. All connections tight: Consumer unit, charger terminals, earth bonding

Cost: £80-150 for diagnosis, £100-200 for repairs (wiring upgrades, RCD replacement, etc.)

When to Call for Help:

• If you suspect installation issues, call a qualified electrician immediately
• If charger was recently installed, contact the installer (may be warranty issue, £0 fix)

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Diagnostic #9: Onboard Charger Degradation / Fault (2% of Cases)

Rarely, the EV's onboard charger module can fail or degrade, reducing charging speed.

How to Identify:

• Charging speed gradually decreased over months/years (e.g., was 7kW, now only 5kW)
• Multiple different chargers and cables tested, all slow
• DC rapid charging still works at full speed (50kW+)
• EV dashboard shows no error messages

Solution (Requires dealer diagnosis, £80-150 diagnosis, £800-1,500 repair):

Step 1: Test at Multiple Chargers

• Try at least 3 different chargers (home, workplace, public AC chargers)
• If all are slow, issue is in your EV (not the chargers)

Step 2: Test DC Rapid Charging

• Visit a public DC rapid charger (50kW+)
• If DC charging is fast, but AC charging is slow, it confirms onboard AC charger fault

Step 3: Book EV Dealer Diagnostic

• Contact your EV manufacturer's service centre
• Describe the symptoms (slow AC charging, normal DC charging)
• Diagnostic cost: £80-150
• Onboard charger replacement cost: £800-1,500 (often warranty-covered if EV <8 years old)

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When to Call a Professional

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Quick Reference: Expected Charging Speeds

Note: Actual speeds depend on your EV's onboard charger capacity, battery size, and temperature.

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Prevention Tips: Maintain Fast Home Charging

1. Use quality cables: Invest in 32A-rated cables from reputable brands (Phoenix Contact, Ratio)
2. Keep connections clean: Inspect cable connectors monthly for dirt/corrosion
3. Monitor voltage: If you notice dimming lights, measure voltage and report to DNO
4. Annual charger check: Have your installer or electrician inspect your charger annually (£80-120)
5. Avoid extreme temperatures: If possible, charge in a garage or shaded area (reduces temperature limiting)
6. Update charger firmware: Check your charger app for updates (can fix power limiting bugs)

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Related Articles

• Complete Home EV Charger Installation Guide UK 2025
• MG ZS EV Won't Charge at Home? UK Troubleshooting 2025
• Tesla + Third-Party Charger Problems? UK Fix Guide 2025
• Ohme Not Connecting to Octopus Intelligent? UK Fix Guide 2025

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Still stuck? Join SpeakEV Forum UK (Charging Infrastructure section) for peer advice, or contact:

• Your charger manufacturer support (Wallbox, Ohme, Pod Point, myenergi, etc.)
• Qualified EV electrician via OZEV-approved installer list: ozev.campaign.gov.uk

Found this guide helpful? Share it with other EV owners in your local EV community or on UK EV Facebook groups.