Slow EV charging at home is almost always explained by one of four things: your car’s onboard charger is capped below the wallbox output, load management is reducing power to protect your home’s circuit, the battery temperature is limiting the charge rate, or a setting in the charger app is restricting speed. This guide walks through each cause and tells you exactly what to check.
Key Takeaways
- Your car’s maximum AC charging rate is fixed by its onboard charger — a 7.4 kW wallbox cannot push more power than the car will accept.
- Smart chargers with load management automatically reduce charging speed when other high-draw appliances are running — this is normal and intentional behaviour.
- Cold batteries charge slowly by design; the battery management system limits current to prevent lithium plating damage until the cells warm up.
- Charging slows significantly above 80% state of charge — this is not a fault, it is battery protection built into every lithium-ion EV.
- If your charger was previously faster and has suddenly slowed, check the app for error states, and inspect the cable and connections.
Understanding Why EV Charging Speeds Vary
Before troubleshooting, it helps to know how AC charging speed is determined. Three limits apply simultaneously, and the lowest one wins:
- The charger’s maximum output — for a standard 7 kW home charger, this is 32A at single phase
- The car’s onboard charger (OBC) rating — the maximum rate the car will accept on AC; fixed hardware, cannot be changed by the user
- The battery management system (BMS) — dynamically adjusts the rate based on temperature, state of charge, and cell health
If any one of these is below the others, that is your actual charging speed. A 22 kW charger connected to a car with a 7.4 kW onboard charger will always charge at 7.4 kW regardless of the charger’s rating.
Common Causes of Slow AC Charging
1. Vehicle Onboard Charger Limit
This is the most frequently misunderstood cause of slow charging. Many popular EVs — particularly plug-in hybrids and entry-level battery EVs — have onboard chargers rated at 3.7 kW or 7.4 kW. If your car has a 3.7 kW limit, it will never charge faster than that from any home charger, no matter how powerful.
Check your vehicle’s specification sheet for “maximum AC charging rate” or “onboard charger capacity.” This figure, expressed in kW, is your absolute ceiling at home.
2. Load Management Reducing Power
Many smart chargers — including Ohme, Zappi, Hypervolt, and others — include load management (sometimes called dynamic load management or power sharing). When your household electricity use gets close to the limit set for your supply, the charger automatically reduces its output to prevent the main fuse from blowing.
If you charge during peak household usage times — early evening when the oven, dishwasher, and other appliances are running — the charger may be running at 10A or 16A rather than 32A. This is not a fault; it is a safety feature working correctly.
To check: open your charger app during a slow session and look for a “current limit” or “load management active” indicator. If load management is engaged, either wait for other appliances to finish or manually override the limit in the app (only do this if your supply capacity allows).
3. Cold Battery Temperature
Lithium-ion battery cells slow their charging rate significantly when cold. Below approximately 5°C, the battery management system restricts charge current to prevent lithium plating on the anode — a form of permanent damage that cannot be repaired. Below 0°C, most BMS systems will halt fast charging entirely and allow only a trickle until the pack warms up.
You will see this most often on winter mornings when the car has been parked outside overnight. The charger connects and shows a low charge rate — sometimes as little as 3A — until the pack temperature rises.
The fix: use your car’s pre-conditioning feature (available via most manufacturer apps) while the car is still plugged in. This heats the battery using grid power rather than battery power, accelerating the warm-up and reducing range impact. Set it to run 20 to 30 minutes before you need to leave.
4. Charging Above 80% State of Charge
AC charging above 80% slows noticeably on most EVs. This is standard lithium-ion behaviour: the BMS reduces charge current as the cells approach full capacity to prevent overcharging, heat stress, and long-term capacity degradation. On some vehicles the slowdown begins at 90%; on others you will notice it from 80% onwards.
If your car is nearly full and charging slowly, this is working as designed. Setting a regular charge limit of 80% for daily use (most car apps and smart chargers allow this) means you will always charge at the faster constant-current rate.
5. App or Charger Settings Capping the Rate
Some charger apps allow you to set a maximum charge current — useful for reducing noise or heat during overnight sessions. Check your charger app settings for:
- Maximum current (A) or power (kW) setting
- Eco mode or green mode (often limits to surplus solar or a low fixed rate)
- Scheduled charge time that has not started yet
Also check the vehicle app if your car has one — many EVs allow you to set a maximum charge rate or charge limit from the infotainment or manufacturer app.
6. Cable or Connection Issue
A damaged or poorly seated cable creates resistance in the circuit, which causes the EVSE to reduce current automatically. Inspect the connector pins at both the charger and vehicle ends for corrosion, bent pins, or debris. A cable rated below the charger’s output (for example, a 16A Type 2 cable on a 32A charger) will also limit speed.
7. Charger Firmware or Software Issue
Occasionally, a firmware update introduces a bug that caps the charge rate below the correct level. Check your charger app for any available updates, and look in the update history for any notes about charging speed changes. If the charger was previously faster before a recent update, contact the manufacturer’s support team.
Diagnosis Checklist
Work through this in order until you find the cause:
- Look up your car’s maximum AC charging rate in the owner’s manual or manufacturer website
- Open the charger app and check the current session rate — compare it with the car’s rated limit
- Check whether load management is active in the app
- Note the outside temperature and battery temperature if shown
- Check the app for any current cap settings or eco mode
- Inspect the cable and connectors for damage or debris
- Check for recent firmware updates on the charger
If the charger rate matches the car’s rated limit, charging is working correctly — the car is the limiting factor, not the charger.
When to Call an Installer
Contact your charger manufacturer or installer if:
- The charge rate has dropped significantly compared with previous sessions with no change in settings
- The charger app shows an error code alongside a slow rate
- The cable or connector is visibly damaged
- You have worked through the checklist above and cannot identify the cause
For more detail on what to check when your EV will not start charging at all, the EV charger troubleshooting hub covers the full diagnosis process for both partial and complete charging failures.
Frequently Asked Questions
Why is my 7 kW charger only charging at 3.7 kW?
The most likely explanation is that your car’s onboard charger is rated at 3.7 kW, which means it will never charge faster than that regardless of how powerful your wallbox is. Check your vehicle specification for “maximum AC charging rate.” This is a hardware limit in the car and cannot be changed. Some EVs offer a higher-rate onboard charger as a factory or dealer option on certain trim levels.
Does charging speed drop overnight when I’m asleep?
It can, for two reasons. First, if your smart charger is on a time-of-use tariff with a set charging window, it may only run for part of the night. Second, if your car has reached its programmed charge limit (for example, 80%), it will stop automatically. It will not charge again until the session is restarted or a new schedule runs. Check the charger app in the morning to see the session log.
Is it bad to charge slowly all the time?
Slow charging — below the car’s rated maximum — is not harmful. In fact, lower charge rates produce less heat in the cells and are generally gentler on long-term battery health. The only scenario where slow charging is a concern is if it is caused by a wiring or connection fault, which can generate heat in the circuit rather than in the battery. If you suspect a wiring fault, have it inspected.