OK to use 85W to charge new 13" MBP?

[tempusfugit]

Hot air or anecdotal data point? I thought the OP might be interested in someone with the exact same situation who has had an issue with swapping power bricks. I can't be sure our brick swapping is what killed the 13" MB battery but if not it was some coincidence.



Theoretically, sure, but it seemed to matter to us unless we just chalk up the dead battery to a fluke.

It may be an interesting coincidence....

Lots of people have battery problems though.... Could have just been a dud in that machine.

 

[belvdr]

That doesn't make any sense at all. Power draw is power draw- there's no such thing as "too much" for a well constructed power adapter. If the parts in a particular magsafe are faulty or improperly assembled resulting in a meltdown or a fire, that's not the computer's fault.

I think we're on different wavelengths. What I'm saying is that if a computer is designed to be charged with an 85W adapter, then plugging a 60W adapter will attempt to pull too much power out of the adapter and cause it to heat up.

Sure, but by saying this you seem to imply that 85W magsafes being used with MBPs can't start fires either, which isn't true at all.

Magsafe fires are not the product of the computer, they are the result of poorly constructed adapters.

I'm saying no such thing. I'm saying if you user a smaller rated power adapter than what the system needs, you are overstressing the adapter, increasing the chance for burning/melting/fire.

 

[miles01110]

I think we're on different wavelengths. What I'm saying is that if a computer is designed to be charged with an 85W adapter, then plugging a 60W adapter will attempt to pull too much power out of the adapter and cause it to heat up.

And I'm saying it is physically impossible to "pull too much power" out of a properly constructed 60W adapter. The maximum power draw out of a 60W adapter is 60 ± maybe a few tenths of a watt.

I'm saying no such thing. I'm saying if you user a smaller rated power adapter than what the system needs, you are overstressing the adapter, increasing the chance for burning/melting/fire.

Irrelevant given what I said above.

 

[belvdr]

And I'm saying it is physically impossible to "pull too much power" out of a properly constructed 60W adapter. The maximum power draw out of a 60W adapter is 60 ± maybe a few tenths of a watt.

If you plug a 60W adapter in a system requiring 85W, then the machine will attempt to pull 85W through it. Since the power bricks don't have a replaceable fuse, they heat up. The machine just pulls whatever power it can get and the brick will attempt to deliver. It's not an intelligent system.

Irrelevant given what I said above.

It's very relevant. I've seen it in real life.

 

[miles01110]

If you plug a 60W adapter in a system requiring 85W, then the machine will attempt to pull 85W through it. Since the power bricks don't have a replaceable fuse, they heat up. The machine just pulls whatever power it can get and the brick will attempt to deliver. It's not an intelligent system.

It doesn't matter what the system "attempts" to do. No matter how hard it tries, it can't get 85W through a 60W adapter. 85W bricks heat up as well, so I don't know what you're getting at. A system requiring 85W on a 60W adapter heats up an adapter just as much as a 60W system using 60W. No difference at all.

 

[belvdr]

It doesn't matter what the system "attempts" to do. No matter how hard it tries, it can't get 85W through a 60W adapter. 85W bricks heat up as well, so I don't know what you're getting at. A system requiring 85W on a 60W adapter heats up an adapter just as much as a 60W system using 60W. No difference at all.

Of course they heat up; they all do.

Have you ever pulled 20A through a 16 guage wire? It heats up considerably (and will eventually start melting the insulation) versus pulling 2A.

The same concept applies.

Again, I've seen this in real life using an HP laptop, dock and power adapters. It is just a rectifier and a transformer in the power brick, and those are "dumb" components that will attempt to deliver whatever is asked of them.

EDIT: I should also add that Apple's power brick has two voltage settings to achieve this dual capacity. In non-Apple worlds, many devices use the same voltage. So, let's say the power brick delivers 5V @ 12A (i.e. 60W). If you plug this into a device that pulls 17A (i.e. 85W) it will heat up tremendously and describe what I'm talking about.

EDIT 2: I tried to figure out what the 60W voltages were, and Apple's site doesn't list them. I'm assuming it is 16.5V, as you listed above, which would require it to put out 5.15A for a large MBP which comes with an 85W adapter, which is well above what it is rated for (3.6A in order to achieve 60W).

 

[miles01110]

Have you ever pulled 20A through a 16 guage wire? It heats up considerably (and will eventually start melting the insulation) versus pulling 2A.

The same concept applies.

Yes, I know that wires have resistance. Ok?

Again, I've seen this in real life using an HP laptop, dock and power adapters. It is just a rectifier and a transformer in the power brick, and those are "dumb" components that will attempt to deliver whatever is asked of them, up to their maximum rating.

Fixed that for you.

In summary:

-I realize that an 85W system will attempt to pull 85W through a power adapter.
-If said power adapter cannot supply 85W when asked by the system, it will supply the maximum it is able to.
-An 85W system pulling 60W is no different from a 60W system pulling 60W.
-Generated heat, the by-product of the power supply, will be the same for all practical purposes between an 85W system pulling 60W and is no different from a 60W system pulling 60W.
-Whether or not the 60W draw melts the power adapter has nothing to do with the system attached to it. It is faulty design or construction.
-Therefore, you're not any better off plugging a faulty 85W Magsafe into your machine than you are plugging a faulty 60W adapter into your machine, no matter what the power rating. It is perfectly fine to power your machine off of a 60W or 85W Magsafe, because if you have a faulty unit you're screwed either way.

 

[belvdr]

Yes, I know that wires have resistance. Ok?

Guess you missed the point. You can probably put 30-40A through a 12 gauge wire, but it is unsafe to do, so they rate it at 20A. Same concept... Onward we go...

Fixed that for you.

Shame it's not correct. The rating is the maximum the manufacturer wants you to pull through it in order for the product to last and be safe to handle. It's not the electrical maximum. There's a huge difference between peak ratings and continuous. The rating they give is the continuous rating of the device, not the peak.

In summary:

-I realize that an 85W system will attempt to pull 85W through a power adapter.
-If said power adapter cannot supply 85W when asked by the system, it will supply the maximum it is able to.

Right there is the issue. It is the maximum it is able to. 60W isn't the maximum the brick can put out. It is the maximum power that is safe to draw through the brick while keeping temps in spec. It is not the very maximum the brick can handle. Otherwise, the brick would be piping hot, maybe burn someone or their house down, and wouldn't last long (especially in a house fire it caused).

-An 85W system pulling 60W is no different from a 60W system pulling 60W.
-Generated heat, the by-product of the power supply, will be the same for all practical purposes between an 85W system pulling 60W and is no different from a 60W system pulling 60W.
-Whether or not the 60W draw melts the power adapter has nothing to do with the system attached to it. It is faulty design or construction.

You keep thinking that the electrical limit equals the rating, and it is not. The brick can actually output more than it is rated, but they need to keep the thing safe to use, so they rate it lower for continuous use.

-Therefore, you're not any better off plugging a faulty 85W Magsafe into your machine than you are plugging a faulty 60W adapter into your machine, no matter what the power rating. It is perfectly fine to power your machine off of a 60W or 85W Magsafe, because if you have a faulty unit you're screwed either way.

This is irrelevant and not sure why faulty adapters is being brought up. If the adapter is faulty, who knows what to expect.

I keep saying I've seen this in real life, but I guess you don't want to accept that.

 

[no-one]

From what i know, the computer recognizes the power bricks wattage from the voltage it gets. Therefore, if the computer is pluged to a 16,5v power supply, it should not attempt to consume more than 60W of power, period.

Nothing to do with the adapter, it's the power management unit of the laptop that is in control.

Beside the fact that i read this information some weeks ago on a website designing external battery packs for the MBP (no reference, sorry), it's a logical explenation of the presence of two different voltages for the power bricks.

And finally, this does not negate the chances of overheating being greater when plugging a 60W adapter to a 85W-needing-MBP, because it sure has more chances to draw the full 60W and being closer to heat dissipation limits of the power brick.

 

[belvdr]

From what i know, the computer recognizes the power bricks wattage from the voltage it gets. Therefore, if the computer is pluged to a 16,5v power supply, it should not attempt to consume more than 60W of power, period.

Nothing to do with the adapter, it's the power management unit of the laptop that is in control.

Beside the fact that i read this information some weeks ago on a website designing external battery packs for the MBP (no reference, sorry), it's a logical explenation of the presence of two different voltages for the power bricks.

And finally, this does not negate the chances of overheating being greater when plugging a 60W adapter to a 85W-needing-MBP, because it sure has more chances to draw the full 60W and being closer to heat dissipation limits of the power brick.

The power circuitry is regulated, meaning it isn't auto-switching. It expects a certain voltage. That's why auto-switching is done at the wall outlet, and the rectifier and transformer provide the same level of output voltage for a semi-variable input.

The MBP that ships with an 85W adapter requires 18.5V and the 60W only supplies 16.5V. Since:

Power (in Watts) = Volts (V) * Current (A)

Then since Volts dropped, Amps are increased to keep the same power using a 60W adapter. Like I said above, the rating of the brick is a continuous rating. It is not the electrical maximum. See my post 2 or 3 above. It has the calculations.

 

[miles01110]

Shame it's not correct. The rating is the maximum the manufacturer wants you to pull through it in order for the product to last and be safe to handle. It's not the electrical maximum. There's a huge difference between peak ratings and continuous. The rating they give is the continuous rating of the device, not the peak.

But the peak rating is not sustained over an appreciable period of time (usually when first powering up), thereby negating your earlier "heat" argument.

Right there is the issue. It is the maximum it is able to. 60W isn't the maximum the brick can put out. It is the maximum power that is safe to draw through the brick while keeping temps in spec. It is not the very maximum the brick can handle. Otherwise, the brick would be piping hot, maybe burn someone or their house down, and wouldn't last long (especially in a house fire it caused).

(I assume you mean "isn't" in the second sentence?) I disagree. It's quite common- I'd almost say the norm- to report the maximum draw on things like transformers (like power adapters).

If the adapter is faulty, who knows what to expect.

Exactly. A 60W Magsafe should be fine powering an 85W MBP. Fires and meltdowns occur from faulty units, not sustained overdraw.

 

[Smoothie]

Gee, I wonder who's going to have the last word on this?

 

[belvdr]

But the peak rating is not sustained over an appreciable period of time (usually when first powering up), thereby negating your earlier "heat" argument.

Normally, yes. But if you attach a 60W brick to a device requiring an 85W, the higher amount suddenly becomes continuous.

(I assume you mean "isn't" in the second sentence?)

Yeah, it's been a long day, and I'm ready to eat and head home.

I disagree. It's quite common- I'd almost say the norm- to report the maximum draw on things like transformers (like power adapters).

Have you ever felt a power brick so hot the plastic beings to melt? In our case, it wasn't a faulty brick, it was because the dock used a 90W brick and someone plugged in a 60W brick.

Even the large transformers that power our data centers and labs get warm, but not extremely hot. Think about why it is not safe to allow someone to pull the electrically maximum power through a transformer. It generates more heat than is safe.

Also, you admitted above there is a peak and continuous rating. Peak ratings are dangerous if done continuously; that's why they're considered peak. They are short-lived. That's why the rating on the devices is a continuous rating. Take a look at car amplifiers, home theatre setups, and the like. All are rated continuous, but usually give the peak value.

Exactly. A 60W Magsafe should be fine powering an 85W MBP. Fires and meltdowns occur from faulty units, not sustained overdraw.

So you admit it is possible to overdraw, then. After seeing a brick smoking and melting, you cannot convince me of that it is not possible. The 60W adapter still powered another laptop fine after that, but we tossed it to be sure. (Seeing that mess meant I had to experiment).

 

[belvdr]

Gee, I wonder who's going to have the last word on this?

Not you!

Miles,

For the record, I'm in no way intending to be disrespectful.

 

[cababah]

But the peak rating is not sustained over an appreciable period of time (usually when first powering up), thereby negating your earlier "heat" argument.



(I assume you mean "isn't" in the second sentence?) I disagree. It's quite common- I'd almost say the norm- to report the maximum draw on things like transformers (like power adapters).



Exactly. A 60W Magsafe should be fine powering an 85W MBP. Fires and meltdowns occur from faulty units, not sustained overdraw.

The power circuitry is regulated, meaning it isn't auto-switching. It expects a certain voltage. That's why auto-switching is done at the wall outlet, and the rectifier and transformer provide the same level of output voltage for a semi-variable input.

The MBP that ships with an 85W adapter requires 18.5V and the 60W only supplies 16.5V. Since:

Power (in Watts) = Volts (V) * Current (A)

Then since Volts dropped, Amps are increased to keep the same power using a 60W adapter. Like I said above, the rating of the brick is a continuous rating. It is not the electrical maximum. See my post 2 or 3 above. It has the calculations.

Guess you missed the point. You can probably put 30-40A through a 12 gauge wire, but it is unsafe to do, so they rate it at 20A. Same concept... Onward we go...



Shame it's not correct. The rating is the maximum the manufacturer wants you to pull through it in order for the product to last and be safe to handle. It's not the electrical maximum. There's a huge difference between peak ratings and continuous. The rating they give is the continuous rating of the device, not the peak.



Right there is the issue. It is the maximum it is able to. 60W isn't the maximum the brick can put out. It is the maximum power that is safe to draw through the brick while keeping temps in spec. It is not the very maximum the brick can handle. Otherwise, the brick would be piping hot, maybe burn someone or their house down, and wouldn't last long (especially in a house fire it caused).




You keep thinking that the electrical limit equals the rating, and it is not. The brick can actually output more than it is rated, but they need to keep the thing safe to use, so they rate it lower for continuous use.



This is irrelevant and not sure why faulty adapters is being brought up. If the adapter is faulty, who knows what to expect.

I keep saying I've seen this in real life, but I guess you don't want to accept that.

Yes, I know that wires have resistance. Ok?



Fixed that for you.

In summary:

-I realize that an 85W system will attempt to pull 85W through a power adapter.
-If said power adapter cannot supply 85W when asked by the system, it will supply the maximum it is able to.
-An 85W system pulling 60W is no different from a 60W system pulling 60W.
-Generated heat, the by-product of the power supply, will be the same for all practical purposes between an 85W system pulling 60W and is no different from a 60W system pulling 60W.
-Whether or not the 60W draw melts the power adapter has nothing to do with the system attached to it. It is faulty design or construction.
-Therefore, you're not any better off plugging a faulty 85W Magsafe into your machine than you are plugging a faulty 60W adapter into your machine, no matter what the power rating. It is perfectly fine to power your machine off of a 60W or 85W Magsafe, because if you have a faulty unit you're screwed either way.

Of course they heat up; they all do.

Have you ever pulled 20A through a 16 guage wire? It heats up considerably (and will eventually start melting the insulation) versus pulling 2A.

The same concept applies.

Again, I've seen this in real life using an HP laptop, dock and power adapters. It is just a rectifier and a transformer in the power brick, and those are "dumb" components that will attempt to deliver whatever is asked of them.

EDIT: I should also add that Apple's power brick has two voltage settings to achieve this dual capacity. In non-Apple worlds, many devices use the same voltage. So, let's say the power brick delivers 5V @ 12A (i.e. 60W). If you plug this into a device that pulls 17A (i.e. 85W) it will heat up tremendously and describe what I'm talking about.

EDIT 2: I tried to figure out what the 60W voltages were, and Apple's site doesn't list them. I'm assuming it is 16.5V, as you listed above, which would require it to put out 5.15A for a large MBP which comes with an 85W adapter, which is well above what it is rated for (3.6A in order to achieve 60W).

It doesn't matter what the system "attempts" to do. No matter how hard it tries, it can't get 85W through a 60W adapter. 85W bricks heat up as well, so I don't know what you're getting at. A system requiring 85W on a 60W adapter heats up an adapter just as much as a 60W system using 60W. No difference at all.

If you plug a 60W adapter in a system requiring 85W, then the machine will attempt to pull 85W through it. Since the power bricks don't have a replaceable fuse, they heat up. The machine just pulls whatever power it can get and the brick will attempt to deliver. It's not an intelligent system.



It's very relevant. I've seen it in real life.

And I'm saying it is physically impossible to "pull too much power" out of a properly constructed 60W adapter. The maximum power draw out of a 60W adapter is 60 ± maybe a few tenths of a watt.



Irrelevant given what I said above.

I think we're on different wavelengths. What I'm saying is that if a computer is designed to be charged with an 85W adapter, then plugging a 60W adapter will attempt to pull too much power out of the adapter and cause it to heat up.



I'm saying no such thing. I'm saying if you user a smaller rated power adapter than what the system needs, you are overstressing the adapter, increasing the chance for burning/melting/fire.

 

[belvdr]

Hey, I'm enjoying it. Pass the popcorn.

EDIT: I'm heading home and getting food. Have a good night, all.

 

[redshovel]

Will using a 85w on a 13" MBP charge it faster than a 60w?

Bordering on 'special needs' that question. If it did charge faster on 85w apple would just ship it with one!

 

[knownikko]

If you plug a 60W adapter in a system requiring 85W, then the machine will attempt to pull 85W through it. Since the power bricks don't have a replaceable fuse, they heat up. The machine just pulls whatever power it can get and the brick will attempt to deliver. It's not an intelligent system.

Machines don't "pull" anything from a power supply.

Power supplies supply power to the computer. Not the other way around.

 

[knownikko]

Of course they heat up; they all do.

Have you ever pulled 20A through a 16 guage wire? It heats up considerably (and will eventually start melting the insulation) versus pulling 2A.

The same concept applies.

No, it doesn't. Nothing "pulls" power through a wire.

If you have a 20 amp power source supplying a 20 amp device over wire rated for 2 amps, then yes, you'll have problems.

If you have a 2 amp power source supplying a 20 amp device over wire rated for 2 amps, you'll certainly have problems, but it won't be with the wire.

 

[knownikko]

FWIW, data I've seen on this seems to indicate that of course, using the 85W adapter on a 60W machine is not a problem as they are rated for both outputs.

I've also read that a 60W adapter on an 85W machine will work fine to either power the machine OR charge the battery, but not both (ie the machine will be running off wall power when connected to the adapter, but NOT charging the battery. Putting the system to sleep/shutdown and leaving the adapter connected will result in the battery getting charged). I can't actually confirm that, but would make sense.

 

[belvdr]

Machines don't "pull" anything from a power supply.

Power supplies supply power to the computer. Not the other way around.

Yeah, and the load draws current. The load in this case is the machine.

No, it doesn't. Nothing "pulls" power through a wire.

If you have a 20 amp power source supplying a 20 amp device over wire rated for 2 amps, then yes, you'll have problems.

If you have a 2 amp power source supplying a 20 amp device over wire rated for 2 amps, you'll certainly have problems, but it won't be with the wire.

How can the problem not be in the wire? The wire has too much resistance to carry that current. So the problem is with the wire. Same in the power brick. If the components are only rated to handle 3A and you put a load on it that draws 15A, you will generate more heat.

FWIW, data I've seen on this seems to indicate that of course, using the 85W adapter on a 60W machine is not a problem as they are rated for both outputs.

I've also read that a 60W adapter on an 85W machine will work fine to either power the machine OR charge the battery, but not both (ie the machine will be running off wall power when connected to the adapter, but NOT charging the battery. Putting the system to sleep/shutdown and leaving the adapter connected will result in the battery getting charged). I can't actually confirm that, but would make sense.

I have read that too, and I'm wondering if Apple has done something slick with their power setup.

 

[no-one]

I have read that too, and I'm wondering if Apple has done something slick with their power setup.

Yeah, it's called a power management unit (you know, the famous SMC ?), that detects the rated power of the adaptator it is plugged to based on the voltage.

 

[jpma]

i've been using in multiple ways
13'' with 85w
15'' with 60w

there shouldn't be any problem beside how fast or slow the charge will become.

lets look at the cinema display, the power charger must be beyond 60w in order to charge 15 or 17'' in a normal speed.

there's nothing to worry about, at least for me

 

[cluthz]

I've used both MB and MBP adapters on my MBP alot.
The only difference is that the machine takes a lot longer to charge on a MB adapter.

 

[magbarn]

FWIW, data I've seen on this seems to indicate that of course, using the 85W adapter on a 60W machine is not a problem as they are rated for both outputs.

I've also read that a 60W adapter on an 85W machine will work fine to either power the machine OR charge the battery, but not both (ie the machine will be running off wall power when connected to the adapter, but NOT charging the battery. Putting the system to sleep/shutdown and leaving the adapter connected will result in the battery getting charged). I can't actually confirm that, but would make sense.

This is true. There's a chip or circuit on the magsafe plug that tells the macbook/pro what kind of adapter is being used. I've modified both a 85w and a 60w (broken power supplies bought off ebay for parts) to work with a kensington adapter so I can charge my laptops while in the SUV. Although using the same 90watt kensington, my MBP behaves differently if I plug in the 60w magsafe vs. the 85w even though the power supply is able to put out 90w total. Heck, even Dell has 3 different levels of their power supply: 65w, 90w, 135w. If you plug any of those 3 levels into any of their laptops, the laptop BIOS can tell which wattage is plugged in. If cheapy dell can do that, I'm sure apple can. In other words: don't worry what you plug into your MBP