You may notice any of the following symptoms that may be caused by a faulty or failing laptop power supply:-
1. If the power supply has an indicator light that does not illuminate when connected to the mains then the power supply (or the mains fuse) must be faulty. The power supply in this instance should not be connected to the laptop as some motherboard faults cause the power supply light to flash or extinguish.
2. If the power supply becomes very hot when in use then it may be faulty or be of the wrong rating for the laptop. It is easy to mix up similar units between laptops. Check that the rating on the laptop ie Volts and Amps match the charger rating. The charger should show the same voltage as the laptop, however, the current should be the same or higher from the charger. Sometimes when the charger becomes very hot then it may turn off and come back on again when it has cooled down. However, this is not a safe option and it should be replaced immediately.
3. Sometimes laptop chargers produce the wrong voltage either too high or too low. Generally this can be tested by a cheap multimeter, however, a true reading can only be made under full load when connected to the laptop. But as a guide, if the voltage is more than half a volt too high or too low then the charger may be faulty. If the voltage is too low then you may find that the battery does not charge fully or only charges when the laptop is turned off, however, a faulty battery or charging circuit may show the same symptoms. If the voltage is too high then the laptop will probably work correctly but it may get hotter than usual and or the battery may overheat and fail quicker than expected.
4. Everything may seem to operate correctly, laptop running and battery charging, then for no apparent reason the battery may stop charging and or the laptop may turn off. This is often caused by the power plug that connects from the charger to the laptop failing. Sometimes the cable breaks where it joins the plug. This can occur if it has been sharply bent or is repeatedly been bent at this point. Generally wiggling the end of the cable will cause the battery to charge then not charge. Beware though this fault is often as a result of the internal laptop socket failing.
5. The laptop and charger may seem to be working ok but a high and potentially dangerous voltage may be present at the plug that connects to the laptop. Using a multimeter set to AC voltage measure between one connection (it does not matter which) of the plug that connects to the laptop (NOT the mains plug) and an earthed point, this could be any metal object connected to electrical earth. There should only be a very small or no voltage present. If a voltage is seen then the insulation inside the charger may be failing or has failed and may eventually let mains voltage enter the laptop! For safety you should immediately replace the laptop charger.
Laptop chargers are relatively cheap and should be immediately replaced if you suspect a fault is present as if ignored may cause an expensive laptop failure. If you use the manufacture's original unit the tip should exactly match the socket inside the laptop and generally they are smaller than the universal charges.