A laptop that runs on its adapter but will not charge is one of the most common repair requests, and one of the most often misdiagnosed. The cause can be the battery, the adapter, a software setting, the EC firmware or the charging circuit itself — and replacing the charger IC first is usually the wrong move. Work from the outside in.
How laptop charging works
On most modern laptops the charging circuit does three jobs at once:
- Detects the adapter and checks that it is acceptable — voltage within range and, on many brands, an adapter identification signal that reports its power rating.
- Powers the system from the adapter through the system rail.
- Charges the battery through a controlled path, managed by a charger IC that measures input and battery current through small current-sense resistors.
The embedded controller sits above it: it reads the battery's state over SMBus, decides whether charging is allowed, and tells the charger what to do. A fault anywhere in that loop produces "plugged in, not charging". The charger itself is described in The Laptop Charging IC: How It Works.
Step 1: rule out the battery
The battery is the most common cause by far.
- Check what the system reports. The operating system and BIOS show battery status, often wear level and sometimes an error. A battery reporting a very low capacity or a fault flag is likely the problem.
- Try a known-good battery of the right type.
- Inspect the connector for burnt or bent pins and corrosion.
- Measure the battery voltage at its connector. A deeply discharged battery may be locked out by its own protection circuit and refuse to accept charge until it is recovered or replaced.
If the laptop does not see the battery at all, follow Laptop Not Detecting the Battery instead.
Step 2: rule out the adapter
- Use the correct wattage. Many laptops charge slowly or not at all with a weaker adapter than specified, while still running.
- Check adapter identification. Some brands use an identification pin or chip in the adapter. If the board cannot read it — damaged centre pin, broken cable, non-original adapter — it may run but refuse to charge, or warn about an unknown adapter. The idea is explained under adapter id.
- Try a known-good original adapter. It is the quickest test.
USB-C laptops negotiate power over the connector; a cable or adapter that cannot deliver the required power gives similar symptoms. See USB-C Laptop Not Charging: PD Controller Diagnosis.
Step 3: check settings and firmware
Before opening anything, check:
- Charge limits. Many laptops have a battery care mode that stops charging at a set level, such as 60% or 80%. Vendor utilities and BIOS settings control it. A battery that "will not charge past 80%" is often working exactly as configured.
- Firmware updates. BIOS and EC updates sometimes change charging behaviour or fix known charging bugs.
- A corrupted EC. If the problem started right after an update, EC firmware is a real possibility — see EC/KBC Firmware: The Second BIOS in Your Laptop.
Step 4: board-level measurements
With the adapter connected and a known-good battery installed, use the schematic to check:
| Measure | What it tells you |
|---|---|
| Adapter input at the charger IC | Adapter power reaches the charger |
Adapter-detect signal (ACOK, ACIN, ADP_PRES) | The charger and EC know the adapter is present |
| Charger IC supply and reference pins | The charger itself is powered |
| Charge enable or inhibit signal from the EC | Whether the EC is allowing charging |
| Battery-side voltage at the charger output | Whether the charger is driving the battery path |
| Battery connector voltage | Whether charging voltage reaches the battery |
| Voltage across the battery current-sense resistor | Whether current is actually flowing into the battery |
The current-sense resistor is especially useful: a tiny voltage across it means a tiny current, and by Ohm's law you can estimate the charging current from the resistor value in the schematic.
Step 5: the battery communication line
The EC talks to the battery's internal controller over SMBus (clock and data lines at the battery connector). If that communication fails, the EC may treat the battery as missing or faulty and block charging. Check:
- the pull-up resistors and their rail on both lines;
- the series resistors and protection diodes between the connector and the EC;
- the lines with an oscilloscope for activity when the adapter is connected.
Corrosion near the battery connector often affects exactly these lines.
Estimating the charging current
The current-sense resistor in the battery path gives you a quick estimate of how much current flows into the battery. Measure the small voltage across it with a meter on its millivolt range, find its value in the schematic (often a few milliohms) and divide: current equals voltage divided by resistance. For example, 20 mV across a 10 mΩ resistor means about 2 A.
- Near zero while the battery is not full — the charger is not charging. Look at enables, battery communication and the battery path.
- A sensible current that drops to zero after a while — normal near full charge, or a charge limit at work.
- Current flowing but the percentage never rises — the battery's own gauge or cells are the suspects.
Example: the corroded sense resistor
A laptop runs from its adapter, sees its battery correctly, but never charges. Battery and adapter test fine on another machine, and there is no charge limit set.
On the board, the adapter-detect signal is present and the EC's charge enable is active, so the EC wants to charge. The charger's input is fine. But the voltage across the battery current-sense resistor is zero, and the charger's sense pins show something odd: one side of the resistor reads differently from the other far more than its milliohm value could explain.
Under the microscope, the resistor has a green crust on one end from an old spill. It has become partly open. Cleaning the area and replacing the resistor with one of the same value and power rating restores charging. The charger IC, which a quick guess would have replaced, was fine all along.
Common board-level causes
- a damaged or corroded current-sense resistor;
- a failed MOSFET in the battery path;
- an adapter-detect signal held low by a leaky part;
- broken SMBus lines to the battery;
- a damaged charger IC — real, but less common than the above.
Safety
Lithium-ion batteries can deliver large currents and catch fire if shorted. Never bridge battery connector pins with a probe, never puncture or heat a battery, and disconnect it before working on the board. When testing the charging circuit, a bench supply set to the battery's nominal voltage with a current limit can stand in for the battery on many boards — safer than experimenting with the real one.
FAQ
The laptop says "plugged in, not charging". Is the motherboard faulty?
Often not. Battery wear, a charge limit setting or a non-original adapter are more common causes. Rule them out first.
Can a laptop run without a battery but not charge one?
Yes. Running from the adapter uses the system power path; charging uses a separate battery path controlled by the charger and EC. Either can fail independently.
Why does it charge only when switched off?
Usually because the adapter cannot supply enough power for both running and charging, or because of a firmware limit. See Laptop Charges Only When Turned Off.
Should I replace the charger IC if charging fails?
Only after you have confirmed its inputs, enables and outputs point to it. Many charger ICs are replaced without fixing the fault.
Is it safe to use a non-original adapter?
It can work if voltage, power and identification are compatible. Many charging complaints trace back to cheap adapters that lack the identification the board expects.
Bottom line
Check the battery, the adapter and the charge settings before touching the board. Then measure the charging loop in order — adapter detect, charger supply, charge enable, battery path, current-sense voltage and battery communication — and the fault usually shows itself without guesswork.