Just to add, I always drain the flea power from any laptop or tower before I begin to work on it.
Residual current can remain in the capacitors on the board or the power supply.
The function of the power buttons and power supplies behave differently on notebooks (which have no ATX Form Factor Standard) than on PCs (which must conform to ATX Form Factor Standards). Holding down the power button on notebooks does serve other purposes on many notebooks. But again, because there are no industry standards for notebooks, every notebook maker can and typically does have their own proprietary way of doing things. For example, on many notebooks, holding down the power button is a step to recalibrate the notebook's battery monitoring feature so it will properly report the remaining charge. Of course, PCs don't run off batteries.
Unlike notebooks, PCs have the ATX Form Factor standard that "ALL" ATX (in it various sizes) motherboards, cases, and PSUs
must comply with to ensure all ATX compliant devices will physically and electrically fit together and work - that is, be totally compatible and work in non-proprietary ways. This is exactly why there is a thriving self-build industry for PCs while such an industry for notebooks is virtually non-existent. This is also why notebooks are more expensive, cost more to repair, have limited upgrade options, and typically the only upgrades (if any) are available only from the notebook maker.
Without the ATX Form Factor Standard, you could not buy a Gigabyte motherboard, ASUS graphics card, Samsung SSD, Seagate hard drive, Crucial RAM, EVGA PSU and put them all in an Antec case and expect them to work.
And while it is absolutely true that some capacitors will hold a charge (for a little while) once power is removed, it is the large capacitors in PSUs that contain the destructive charges. But engineers and circuit designers are fully aware of this and where necessary (such as inside power supplies where deadly voltages exist), "bleeder" (parasitic) resistors are included in the circuits for the very purpose of quickly bleeding off those voltages. Of course, that assumes the PSU has not been physically damaged but regardless, pressing and holding the power button on a PC does not bleed any residual voltages off because that is not an ATX Form Factor requirement for that circuit.
If it was an ATX requirement (1) it would be published in the
ATX Form Factor Specifications and (2) it would be published as a safety step when replacing the CMOS battery or doing other maintenance in every ATX motherboard manual, and most likely every ATX case manual too. And neither is the case.
So I am sorry, but pressing down the power button
on a PC with the intent of bleeding off residual voltages does nothing but make your finger tired.
And there seems to be some confusion over "electrostatic" voltages. There would never be any "electrostatic" voltages in a motherboard. Electrostatic voltages (static electricity) is caused by "free" electrons moving from one object to another
as they pass close by or rub against each other - like when you walk across carpet or rub a balloon in your hair. That type of physical movement does not occur on a motherboard or inside a computer case.
The very real concern about ESD (electrostatic discharge) is the static in your body being discharged through a ESD sensitive device like CPUs, memory modules and other high-density ICs. And just squirming in your chair can build up enough charge to destroy sensitive devices. For this reason, it is essential to put (and keep) your body and the computer "at the same potential". Why? Because voltage always seeks the shortest path to ground. But if there is no "difference in potentials" there can be no current flow - that is, no path. And since the PSU, motherboard and everything else in the computer are in direct physical and electrical contact with the bare metal of the case interior, touching it (or connecting to it via a wristband) discharges any static in your body, thus preventing any static discharges. And staying in contact via a wrist strap or frequent touching prevents build-up of those destructive potentials. Note that static charges are in the many 1000s of volts - typically 20,000 to 30,000 volts.
So, in a PSU, residual voltages are drained off by high ohmic bleeder resistors. There are no high, unsafe voltages on motherboards (12V max). If you are working inside a computer (notebook or PC) the power supply should be disconnected anyway. If working on live electronics, it should grounded to "Earth" ground (and don't depend on a $.25 power cord through the wall outlet to provide a proper "Earth" ground either).
Lastly, notebook power supplies are sealed with no way to become exposed to deadly AC voltages. And only qualified and trained electronic technicians should open a PC PSU.
So by all means, unplug the PSU from the wall. This ensures all standby voltages are removed. Touch bare metal of the case interior with your finger or a good wrist band properly clipped to bare metal of the case interior. Then pry out the battery.
Note in some cases, this is a two-handed job. One to hold the spring clip open and the other to pry out the battery. A
wood cuticle pusher works great for this. They are non-conductive and strong, but should you slip, being wood, are not likely to dig a Grand Canyon sized gorge across your motherboard. And never touch a new battery with your bare fingers. Skin oils can promote corrosion and attract dust. I put a clean cotton sock over my hand when inserting these batteries.
Sorry for the lengthy reply.
If you read my post, you would note I already gave it. I explained how to prepare the computer and to ensure any static in his body was discharged. Then I provided a link showing what I use to pry batteries out, wooden cuticle pushers. I recommend you try them out too. Note you can also use them to hold fans stationary when blasting out dust with compressed air so you don't over spin the bearings. Then I explained how to replace the battery without getting skin oils it.
