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Magsafe power adapter query

Message #1 - Posted 2011/12/08 - Warren Oates

What problems might arise using a 45 watt power adapter on a MacBook that's supposed to use a 60 watt supply?

I've looked around, and it seems that a _higher_ wattage won't hurt your computer, but a lower-wattage adapter might cause "operational problems" and these are unspecified.

My sister in law isn't too bright. Her adapter "broke" so she bought a new one from "a guy" and apparently it's not the right wattage. I told her when it happened "Go to FutureShop, they sell them there." Of course, now they only sell the new-style ones. This is one of the older ones with the little flat white plug that has an amber or green light.

She needs a new battery too, but that's easier to find.

Sorry, I don't have the exact model/year of the MacBook, and it would pointless asking her. She'll have to bring it over (just so I know what battery to get). I know she's still on 10.5.8.
--

... do not cover a warm kettle or your stock may sour. -- Julia Child

Message #2 - Posted 2011/12/08 - D Finnigan

Warren Oates wrote:

What problems might arise using a 45 watt power adapter on a MacBook that's supposed to use a 60 watt supply?

I've looked around, and it seems that a _higher_ wattage won't hurt your computer, but a lower-wattage adapter might cause "operational problems" and these are unspecified.

Remember that Watts = Amps * Volts.

Watts is total power used, which is current (amps) times "pressure" (voltage).

Now, if the MacBook in question is drawing more amps than the power supply is rated to deliver, that's a problem. On the other hand, if the power supply is beefier than the MacBook requires, that is not a problem. The MacBook takes what it needs, and nothing more.

]DF$
Mac GUI Vault - A source for retro Apple II and
Macintosh computing.
http://macgui.com/vault/

Message #3 - Posted 2011/12/08 - Warren Oates

Previously, dog_D Finnigan wrote:

Now, if the MacBook in question is drawing more amps than the power supply is rated to deliver, that's a problem. On the other hand, if the power supply is beefier than the MacBook requires, that is not a problem. The MacBook takes what it needs, and nothing more.

I sort of understood that. I was wondering what the symptoms of drawing more power might be.

On another note, are the newer Magsafes compatible with the older ones in the way they connect to the computer? Is it just a newer plug going into the same socket ("port" if you like)?

Thanks for the feply.
--

... do not cover a warm kettle or your stock may sour. -- Julia Child

Message #4 - Posted 2011/12/08 - Bill

Previously, Warren Oates wrote:

Previously, dog_D Finnigan wrote:

Now, if the MacBook in question is drawing more amps than the power supply is rated to deliver, that's a problem. On the other hand, if the power supply is beefier than the MacBook requires, that is not a problem. The MacBook takes what it needs, and nothing more.

I sort of understood that. I was wondering what the symptoms of drawing more power might be.

On another note, are the newer Magsafes compatible with the older ones in the way they connect to the computer? Is it just a newer plug going into the same socket ("port" if you like)?

Thanks for the feply.

The new MagSafe plug fits the "old" Magsafe socket, no problem.

The previous poster had it right on the wattage. The rated wattage of the power supply is what it is capable of delivering. The computer draws only as much as it needs.

So if the computer draws less than the rated wattage of the power supply, there is no problem.

On the other hand, if the computer demands more than the rated wattage of the power supply, that will cause multiple problems including overheating of the power supply and drop in voltage output which could prevent the battery from charging and might perhaps have some other adverse effects. The overheating of the power supply is probably the biggest problem, as it might possibly cause a fire.

Message #5 - Posted 2011/12/08 - nospam

Previously, Warren Oates wrote:

What problems might arise using a 45 watt power adapter on a MacBook that's supposed to use a 60 watt supply?

none. magsafe adapters have a chip that tells the computer what the power capacity of the adapter is, which means the macbook will not draw more than the adapter can supply because it knows how much is available.

the net effect will be that the battery will charge slower than it otherwise would, or possibly not at all in some cases.

Message #6 - Posted 2011/12/08 - Warren Oates

Previously, Bill wrote:

The new MagSafe plug fits the "old" Magsafe socket, no problem.

Thanks!

On the other hand, if the computer demands more than the rated wattage of the power supply, that will cause multiple problems including overheating of the power supply and drop in voltage output which could prevent the battery from charging and might perhaps have some other adverse effects. The overheating of the power supply is probably the biggest problem, as it might possibly cause a fire.

Thanks again! That's what I was looking for. She should get a new power supply before changing the battery then, I reckon (because the battery might not be dead at all). And before her house burns down too. --

... do not cover a warm kettle or your stock may sour. -- Julia Child

Message #7 - Posted 2011/12/08 - nospam

Previously, Warren Oates wrote:

The new MagSafe plug fits the "old" Magsafe socket, no problem.

Thanks!

it will only fit if the macbook does not have a protective case. if it does, the case will preclude the new style connector from making good contact.

On the other hand, if the computer demands more than the rated wattage of the power supply, that will cause multiple problems including overheating of the power supply and drop in voltage output which could prevent the battery from charging and might perhaps have some other adverse effects. The overheating of the power supply is probably the biggest problem, as it might possibly cause a fire.

Thanks again! That's what I was looking for. She should get a new power supply before changing the battery then, I reckon (because the battery might not be dead at all). And before her house burns down too.

what he wrote will not happen. you *can't* overload a magsafe adapter.

Message #8 - Posted 2011/12/08 - Warren Oates

Previously, nospam wrote:

what he wrote will not happen. you *can't* overload a magsafe adapter.

Fair enough. It probably won't burn her house down then. But what about the "performance issues" that I read about on an Apple site? I'm guessing it might not drive an external monitor. Or maybe it won't recharge the battery properly. Or run the fan? I'm not totally familiar with MacBook stuff. It's moot, because she and the oul' Woman just called and they've bought her a brand new 85 watt power supply that matches the output of the one that "broke."

I think she needs a new battery though. I did all that PMU stuff. --

... do not cover a warm kettle or your stock may sour. -- Julia Child

Message #9 - Posted 2011/12/08 - nospam

Previously, Warren Oates wrote:

what he wrote will not happen. you *can't* overload a magsafe adapter.

Fair enough. It probably won't burn her house down then. But what about the "performance issues" that I read about on an Apple site?

the only performance issue is if you run the macbook without a battery at all, it will go into a power-save mode which is slower.

I'm guessing it might not drive an external monitor. Or maybe it won't recharge the battery properly. Or run the fan? I'm not totally familiar with MacBook stuff. It's moot, because she and the oul' Woman just called and they've bought her a brand new 85 watt power supply that matches the output of the one that "broke."

it will work fine, but the 85w is for macbook pros, not macbooks.

the worst that can happen with an underpowered adapter is that the battery won't charge and may possibly discharge slowly if the she's doing something intensive on the macbook. more than likely the battery will charge, but just slower than it would have if there was more power available. it really isn't a problem because as i said, the magsafe tells the mac how much power it has (that's why there's a brief delay until the green/orange led comes on - it's querying).

apple's own airline adapter is unable to supply enough power to run a macbook pro and why the battery discharges even when plugged into seat power.

I think she needs a new battery though. I did all that PMU stuff.

option-click the battery icon in the menubar and it will say if it's normal or needs service. if the battery is just old, it will have diminished capacity, which is to be expected.

Message #10 - Posted 2011/12/09 - David Empson

Warren Oates wrote:

What problems might arise using a 45 watt power adapter on a MacBook that's supposed to use a 60 watt supply?

I've looked around, and it seems that a _higher_ wattage won't hurt your computer, but a lower-wattage adapter might cause "operational problems" and these are unspecified.

The battery will charge slower than it should, or not charge at all while the computer is running, only while it is asleep or shut down.

If the operating power used by the computer when working hard exceeds the rating of the power adapter, the computer may need to draw power from the battery as well as the adapter, so the battery might go flat even though the computer is plugged in. I expect that if the battery got low enough in this situation, the computer would warn then force a shutdown or hibernation, in the same manner as running from battery without a power adapter.

As others have noted, there are no safety/overload concerns because the computer knows how much power the adapter can supply.

My sister in law isn't too bright. Her adapter "broke" so she bought a new one from "a guy" and apparently it's not the right wattage. I told her when it happened "Go to FutureShop, they sell them there." Of course, now they only sell the new-style ones. This is one of the older ones with the little flat white plug that has an amber or green light.

MagSafe adapters with the old and new style plugs are interchangeable. I'm using the one which came with my 2007 MacBook Pro (white plastic plug) as a spare for my 2010 MacBook Pro (which came with an adapter with a metallic right angle plug).

David Empson
dempson@actrix.gen.nz

Message #11 - Posted 2011/12/09 - Warren Oates

Previously, David Empson wrote:

If the operating power used by the computer when working hard exceeds the rating of the power adapter, the computer may need to draw power from the battery as well as the adapter, so the battery might go flat even though the computer is plugged in. I expect that if the battery got low enough in this situation, the computer would warn then force a shutdown or hibernation, in the same manner as running from battery without a power adapter.

What happens is that the battery (with no external power) gets down to about 70% and then the computer just shuts down. I managed to keep it going one time until it got down to about 40%. From 100% to 40% took less than an hour.

As it happens, it's a MacBook Pro 15" (1,1 the System Profiler says). Anyway, she's got a new properly-rated power lump and I'll order her a new battery from my Hongkong supplier. Might even arrive in time for Christmas.

Thanks, David, and to everyone who replied.
--

... do not cover a warm kettle or your stock may sour. -- Julia Child

Message #12 - Posted 2011/12/09 - nospam

Previously, Warren Oates wrote:

What happens is that the battery (with no external power) gets down to about 70% and then the computer just shuts down. I managed to keep it going one time until it got down to about 40%. From 100% to 40% took less than an hour.

sudden shutdown is usually due to an internal cell failing. it's time to replace the battery.

Message #13 - Posted 2011/12/11 - David Empson

Warren Oates wrote:

Previously, David Empson wrote:

If the operating power used by the computer when working hard exceeds the rating of the power adapter, the computer may need to draw power from the battery as well as the adapter, so the battery might go flat even though the computer is plugged in. I expect that if the battery got low enough in this situation, the computer would warn then force a shutdown or hibernation, in the same manner as running from battery without a power adapter.

What happens is that the battery (with no external power) gets down to about 70% and then the computer just shuts down. I managed to keep it going one time until it got down to about 40%. From 100% to 40% took less than an hour.

If the computer shuts down abruptly while the battery is at a percentage that high, either the battery is badly out of calibration, or it has a faulty cell and needs to be replaced.

The computer is supposed to warn that you are running on reserve power when the battery gets to about 5%, then it will force shutdown somewhat after that.

As it happens, it's a MacBook Pro 15" (1,1 the System Profiler says).

That's the first generation from February to October 2006. If it is still on its original battery I would not be surprised if the battery had failed.

Anyway, she's got a new properly-rated power lump and I'll order her a new battery from my Hongkong supplier. Might even arrive in time for Christmas.

The MacBook Pro is supposed to use an 85 W adapter, so 45 W would be seriously underpowered. It may not be enough to operate the MacBook Pro at reasonable load, so the battery would be used more heavily. Otherwise what I wrote previously still applies (assuming a reliable battery).

David Empson
dempson@actrix.gen.nz

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