A laptop that runs perfectly on its charger but shuts off — or will not start — as soon as the charger is removed is a common complaint. Most of the time the battery is simply worn out. But the same symptom comes from a battery the laptop cannot see, a switch on the board that never connects the battery, or a damaged connector. A few measurements tell these apart before you buy a new battery that does not fix anything.
How the laptop switches between charger and battery
On most modern laptops the charger IC manages both sources:
| Situation | What powers the system | Board parts involved |
|---|---|---|
| Charger connected | The charger IC, from the adapter | Input MOSFETs, charger IC, system rail |
| Charger removed | The battery, through the battery switch | Battery MOSFET, charger IC control |
| Heavy load with charger connected | Adapter plus battery on some designs | Charger IC, battery MOSFET |
The battery connects to the system rail through one or more MOSFETs controlled by the charger IC. When the adapter disappears, the charger IC must turn that switch on immediately — and the battery must be able to supply the full load. The structure of this circuit is described in the glossary under power path, and the controller under charger ic.
Step 1: what does the laptop say about the battery?
Check the battery status in the operating system and in the BIOS:
| Battery status | Likely cause |
|---|---|
| No battery detected | Detection fault — pack, connector, SMBus, EC |
| Detected, 0% or not charging | Locked-out or deeply discharged pack, charging fault |
| Detected, normal percentage, health very low | Worn pack |
| Detected, normal percentage and health | Battery path on the board, or pack failing under load |
If the battery is not detected, follow Laptop Not Detecting the Battery first. A laptop that cannot see its battery cannot use it.
Step 2: the worn battery
Lithium-ion packs lose capacity and gain internal resistance as they age. A worn pack may:
- report a reasonable percentage, yet collapse the moment it has to supply the full load;
- run for a minute and then shut down abruptly;
- fail only when the processor or GPU work hard.
A battery report from the operating system shows design capacity against current full-charge capacity, and some laptops' BIOS diagnostics show battery health. A full-charge capacity far below the design capacity, or a high cycle count, makes a worn pack the prime suspect. A known-good battery confirms it.
Step 3: measure the battery rail on the board
If the pack is detected and seems healthy, measure the board side:
- With the battery connected and the charger connected, measure the battery rail at the connector or its capacitors — it should be at the pack voltage.
- Measure the system rail — it should be at its normal level.
- Unplug the charger while measuring the system rail, if the laptop stays on long enough, or measure with the laptop off and the charger removed.
What the results mean:
- Battery rail present, system rail collapses when the charger is removed — the battery switch is not turning on, or the pack voltage sags too far under load.
- Battery rail zero with charger removed — the pack's own protection has switched its output off, or the pack is not connected.
- Both fine with the laptop off, but it will not start on battery — the start-up current is too much for the pack, or the EC does not allow battery-only start.
Step 4: the battery MOSFET
The battery switch is usually one or two MOSFETs between the battery rail and the system rail, driven by the charger IC. With the charger removed and the battery connected:
- Measure the voltage on both sides of the switch. A large difference means it is not switched on.
- Measure its gate drive. The required gate voltage depends on the MOSFET type and design — compare with the charger IC's datasheet and the schematic.
- With power removed, check the MOSFET between drain and source in diode mode for a short or open. Methods are covered in How to Test a Transistor with a Multimeter: BJT and MOSFET.
If the gate drive never changes, the charger IC is not commanding the switch. That points at the charger IC itself, its configuration from the EC, or a missing input signal it needs.
Step 5: the charger IC and EC
The charger IC receives charge and operating settings from the EC over SMBus. Some designs also have modes in which the battery is deliberately disconnected — for example a calibration or "learn" mode, or a ship mode. Firmware or EC problems can leave the board in such a state.
- Update or reflash the BIOS if the EC firmware is part of it.
- Check BIOS settings related to battery, such as storage or ship modes, battery health limits and "battery disconnect" options on some business laptops.
- Measure the charger IC's supply and the SMBus lines to the EC.
How the charger IC works is explained in The Laptop Charging IC: How It Works.
Step 6: the connector
A battery connector with a damaged power pin can pass the small currents needed for detection and charging but fail under full load. Inspect the pins for burn marks, pitting and corrosion. Measure the battery rail at the connector and at the board side of the connector while the laptop runs on battery — a significant voltage difference between the two means a bad contact.
Example: dies under load without the charger
A laptop runs on battery at idle but shuts down as soon as a demanding program starts, unless the charger is connected. The battery is detected, reports full charge, and the operating system's battery report shows a capacity close to the design value. With the charger removed, the battery rail measures normally at idle. Under load, the voltage at the board side of the battery connector drops far more than at the pack's own contacts. Under magnification, two of the connector's power pins are darkened and pitted. A new connector — and a cleaned contact area on the pack — keep the voltage steady under load, and the laptop no longer shuts down.
Quick reference
| Finding | Likely cause |
|---|---|
| Battery not detected | Detection chain — follow the detection guide |
| Health far below design capacity | Worn pack |
| Battery rail fine, system rail collapses | Battery MOSFET or its drive |
| Battery rail zero without charger | Pack protection or pack not connected |
| Voltage drop across the connector under load | Damaged connector pins |
| Everything fine, still no battery operation | EC firmware or BIOS battery mode |
FAQ
Is it always the battery?
Most often, yes — a worn battery is the commonest cause. But test with a known-good battery before buying, because board faults produce the same symptom.
Can I use the laptop without a battery permanently?
Many laptops run from the charger alone, but some reduce performance without a battery, and you lose protection against power cuts.
Why does the laptop shut off instantly when I unplug the charger?
Either the battery cannot supply the load, or the board does not switch the battery in quickly enough. Measure the battery and system rails to tell which.
Can a BIOS setting stop the laptop using its battery?
Some business laptops have battery storage or disconnect modes. Check the BIOS before opening the laptop.
Why does it work on battery at idle but not under load?
The pack, its connector or the battery switch cannot deliver the higher current. Measure voltages under load to find where the drop occurs.
Bottom line
Check what the laptop reports about the battery, then test with a known-good pack. If the battery is healthy, measure the battery and system rails, the battery MOSFET and its gate drive, the connector under load, and the charger IC's control from the EC. A worn pack is the usual answer, but the board can produce exactly the same symptom. To check the pack itself, see How to Test a Laptop Battery with a Multimeter Safely.