A laptop that charges perfectly when switched off but holds its level, or even drains, while running is usually telling you something about power budget rather than a broken part. The adapter can supply only so much; the laptop's own load comes first, and charging gets whatever is left. Sometimes nothing is left. But the same symptom also comes from an adapter the laptop does not fully recognise, a battery that is too warm, a deliberate setting, or a fault in the charger's current measurement. This guide shows how to tell which.
How the power budget works
The charger IC limits the total current drawn from the adapter to protect it. When the laptop is running:
- The system takes what it needs.
- Whatever remains, up to the charge limit, goes to the battery.
- If the system needs everything — or more — charging stops, and the battery may even help supply the system.
When the laptop is off, the system needs almost nothing, so all available power goes to charging. That is why charging-only-when-off is a natural result of an adapter that is small for the laptop's load. The charger's role is explained in The Laptop Charging IC: How It Works.
Cause 1: the adapter is too small
- A lower-wattage adapter than the original, often borrowed from another model.
- A USB-C charger with less power than the laptop's original.
- A worn adapter delivering less than its rating.
Compare the adapter's wattage with the original. With the laptop running a normal workload, measure what it draws — on USB-C with a power meter, on barrel adapters at the input. Laptop Charging Time Calculator shows how much power is left for charging for a given adapter and load.
Cause 2: the adapter is not recognised
Many laptops identify the adapter through an ID pin or the USB-C negotiation. If the ID is missing or wrong, the laptop assumes a small adapter and limits its input current — often to a level that covers the system but not charging. The ID concept is described under adapter id. Signs include warnings about an unknown or underpowered adapter, and charging that works with the original adapter but not with an apparently identical one.
Cause 3: battery temperature
Lithium batteries should not be charged when too warm. The charger and EC read the battery's thermistor and pause or slow charging above a threshold. A laptop running a heavy load warms the battery, and charging stops; switched off, the battery cools and charging resumes. Check battery temperature readings in software, and check the cooling.
Cause 4: settings
- Charge thresholds — a setting to stop charging at, for example, 80 % or to start only below a level.
- Battery care or conservation modes — keeping the battery at a moderate level while plugged in.
- Performance modes — some gaming laptops prioritise system power and reduce charging under load.
Check the manufacturer's power utility and the firmware settings.
Cause 5: a charger section fault
When the adapter is right, recognised and the battery cool, but charging still stops under even moderate load, the board may be limiting current too early:
- Input current sense resistor — if it has drifted high or is the wrong value, the charger measures more current than really flows and limits the input too soon. Sense resistors are covered in Current Sense Resistors in Laptop Charging Circuits.
- Charger configuration — the EC writes the input current limit and charge settings; a firmware problem can set them too low.
- Adapter detection circuit — a corrupted divider can make the charger treat a good adapter as weak.
The broader charging diagnosis is in Laptop Not Charging: Charging Circuit Diagnosis.
USB-C laptops
On USB-C laptops the available power is set by the negotiated contract. A charger offering a lower profile than the laptop wants — or a cable that limits current — reduces the budget, and charging stops under load. A USB-C power meter shows the negotiated voltage and current directly. If the contract is lower than expected, try a charger and cable that support the laptop's full rating before suspecting the board.
What to tell the owner
Many owners see "plugged in, not charging" and assume a fault. Explain the power budget in plain terms: the laptop powers itself first and charges with whatever remains. Recommend the correct adapter for heavy use, mention any battery care settings that limit charging on purpose, and point out cooling problems if the battery runs warm. Most of these cases are solved without opening the laptop.
Measuring what is happening
With the laptop running a steady load and the battery partly discharged:
- Measure the input current — across the input sense resistor (millivolts ÷ resistance) or with a USB-C meter.
- Measure the charge current — across the battery-path sense resistor.
- Compare the input current with the adapter's rating.
| Finding | Meaning |
|---|---|
| Input current near the adapter's limit, charge current zero | Adapter too small for the load |
| Input current well below the adapter's rating, charge zero | Input limit set too low — ID, settings, sense resistor or firmware |
| Charge current pulses on and off | Temperature or protection behaviour |
When the battery drains while plugged in
If the battery level actually falls while the laptop runs on its adapter, the system is drawing more than the adapter supplies and the battery makes up the difference. That is a stronger version of the same budget problem. It is common on gaming laptops with undersized adapters, and on laptops whose adapter is not recognised and is limited to a low input current. The same measurements apply; the input current will sit at its limit.
Example: a gaming laptop on a borrowed adapter
A gaming laptop charges only when off. Its adapter has the right connector and voltage but is rated well below the original. Under a moderate game load, the input current sits right at the adapter's maximum and the battery-path sense resistor shows no charge current at all. With the original adapter, the laptop charges while running. Nothing on the board was faulty.
Example: a correctly rated adapter that still limits
A business laptop with its original adapter charges only when off. The input current under load sits at roughly half the adapter's rating, and the firmware reports an unknown adapter. The adapter's ID centre pin is bent and not making contact. A new adapter — or a repaired plug — restores full input current, and the laptop charges while running.
FAQ
Is charging only when off a fault?
Not necessarily. It is normal with an adapter that is small for the laptop's load.
Why does my laptop say the adapter is underpowered?
The adapter's power is below what the laptop expects, or the laptop cannot read its ID.
Can heat stop charging?
Yes. Batteries are not charged above a temperature threshold.
How do I check whether charging is limited by settings?
Look at battery care, conservation or threshold settings in the manufacturer's software and firmware.
Can a board fault cause this?
Yes — a drifted input sense resistor, a faulty adapter-detection circuit or EC firmware can limit input current too early.
Bottom line
Charging only when off usually means the adapter cannot cover the running load, the laptop does not recognise the adapter, the battery is too warm, or a setting is limiting charge. Measure input and charge currents under load and compare with the adapter's rating; if the input is limited well below the rating with a correct adapter, check the adapter ID path, the input sense resistor and the charger's configuration.