Why my laptop only start when I charge externally or some time interna

Hi @ixlucifer

Seems like the battery is failing and needs to be replaced.

Does the laptop turn on and stay on if the charger is left connected?

If the laptop stays on, create a battery report to check the status of the battery.

Once you have the report, compare the Design Capacity value versus the Full Charge Capacity value. For a good battery they should nearly be the same.

You can work out the battery status as a percentage by using the formula Full Charge Capacity value x 100 ÷ Design Capacity value.

If the battery needs replacing search online for 933321-855 to find suppliers that suit you best.

Here's the HP EliteBook 840 G5 Battery Replacement guide that may help.