Q: When unplugging the power chord, how is the switch made from AC power to battery power ?
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Jan 8, 2016 8:17 AMby ColinMcCollins,Ok, but then, when the power cord is plugged AND the battery is there, how does the laptop know from which source it should draw power ?
And when it actually draws power from one source (say the power cord), how is the second source (here the battery) prevented from delivering additional (and unwanted) power ?
Finally, when the power source disappears (power cord is unplugged), how does it know so quickly (quick enough for the laptop not to turn off) that it should now draw power from the battery ?
Also, I was under the impression than (at least on macs), when the battery reaches 96-97%, it stops being charged, and is sort of "disengaged" from the powering circuit. So that must mean that the powering is now done pass-through, right ? And that there must be a mechanisms to instantly "re-engage" the battery when the power chord is unplugged ? Correct me if I'm wrong, of course
I did have the intuition that this is provided by the way the wiring is done but I still don't quite fully understand how this works...
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Jan 8, 2016 8:43 AMby Lanny,Ok, but then, when the power cord is plugged AND the battery is there, how does the laptop know from which source it should draw power ?
It always draws from the battery.
And when it actually draws power from one source (say the power cord), how is the second source (here the battery) prevented from delivering additional (and unwanted) power ?
It always draws from the battery. It's the battery that draws the power from the charger.
And that there must be a mechanisms to instantly "re-engage" the battery when the power chord is unplugged ? Correct me if I'm wrong, of course
No, because it's never disengaged.
The AC adapter is a power supply that makes a DC voltage that is used to charge the battery. It also contains circuitry that controls the way the battery charges, but the Mac, or any other battery operated device, always draws it power directly from the battery.
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Jan 8, 2016 8:47 AMby ColinMcCollins,Ok so you mean pass-through charging never happens on a laptop ? Doesn't this unnecessarily wear the battery down ?
And that the "on macs, the battery doesn't resume charging until it is below 96% so your battery lasts longer " thing is only true in that it prevents the battery from doing multiple charging and uncharging cycles between 96% and 100%, BUT the battery is still worn down a little because it is constantly proving power even when the laptop is plugged in ?
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Jan 9, 2016 11:49 AMby K Shaffer,There is another circuit that helps modulate the power between charging the battery
and running the computer when charging; the DC-in Board or MagSafe Board in the
computer. This is one of the items that would be tested by a professional diagnostic
if there still were issues after replacement of a battery and the charger.
New MacBook 12-inch Retina model does not use the MagSafe power unit so some
of the known troubleshooting ideas do not apply; the Apple Store genius can help.
With older model portable computers where the battery is readily removable by user,
the troubleshooting can be less tedious, if there is a known-good replacement of both
battery and charging adapter. Of course, most people may not have them on hand.
• Apple Portables: Troubleshooting MagSafe adapters - Apple Support
• About Mac notebook batteries - Apple Support
The computer can run for awhile without the battery installed, however in many cases
the computer will be restricted to run at less than optimal processor capacity w/o battery.
There are several iFixit.com guides for Mac computers, and portables, worth a look online.
Usually, if you have an issue with an older Mac, you'd seek an authorized Apple service
provider who may still be able to repair (and find replacement parts) for those old models.
An Apple Store (retail) with Genius bar may be helpful for basic troubleshooting of issues
but would be unable to repair them due to the vintage status or lack of replacement parts.
They tend to have diminishing stock of parts as model versions age, & new ones arrive.
Good luck & happy computing!
