I have a 13" Macbook Pro that came with a 60W power adapter and a 15" Macbook Pro that came with a 85W power adapter. Can I use either adapter with either laptop? There seems to be disagreement in the Apple forums.
Apple's official word on the matter is:
Make sure the proper wattage adapter for your portable computer is used. Select the appropriate power adapter for your Apple portable computer. You can use a higher wattage power adapter, but you cannot use one with less wattage without potential operating issues. (here + discussion here).
So your 13" can use your 15" charger, but not vice versa.
I've never heard of it voiding a warranty (nor experienced it when we've used the wrong charger), but it's better to be safe than sorry.
There is a lot of misinformation in some answers. I will give the facts.
No MagSafe adapter, when plugged into a mechanically matching receptacle on a MacBook/MacBook Pro, will cause an unsafe condition. This is a given for the systems to receive safety certifications. So no, a 60W adapter won't overheat when connected to a machine that needs an 85W adapter. The MacBook it's plugged into won't operate in a "brownout". It will operate safely, but the CPU performance will be diminshed. The below explains why.
A MacBook's power management works very simply: it maintains a balance of power between the power consumed by the loads and the power available from the sources. There are two sources of power:
- power adapter,
- battery as a power source.
There are two loads:
- battery charger,
- the machine (logic, drives, memory, screen, speakers, USB devices, etc.)
Both loads are adjustable and the power management's function is to adjust them as needed.
The battery as a power source is exclusive of the battery charger: a battery may operate as a power source, or the battery charger may operate, but never both at once.
The power management must maintain the following inequality balanced, in terms of power: (power adapter + battery as a power source) >= (battery charger + the machine). The loads are prioritized: the machine has priority over the battery charger. The power management system also knows the electronic nameplate of the power supply and thus its rated power.
Thus, given an available input power, the machine load is satisfied first, and any leftover power is provided to the battery charger. If there isn't enough power left for the charger, the battery is by definition discharging unless it has no charge left. This is important. Conversely, a fully charged battery will demand a zero charger load, and that's fine.
If there isn't enough power for the machine, the load shedding kicks in and throttles the CPU (and perhaps GPU - I don't recall offhand). The CPU load shedding will, by design, always manage to balance the power. The 60W supply, even if connected to a 17 inch MBP, will satisfy all internal and external loads (USB, FW, drives, screen), except for the CPU and GPU. So the latter will be throttled to maintain the power balance. That's why the performance will be poor with an inadequate power supply.
Since the machine load takes priority and doesn't shed until there's insufficient power available, the battery will be always discharging whenever the supply can't provide sufficient power to cover the machine's needs. This means that with a 60W charger, the battery will charge only during light CPU load. If you have both cores going full-throttle, the battery will be always discharging until it reaches a zero charge state.
The rate at which the battery charges will also depend on the machine load. The battery charger can consume up to ~30W or so. With an 85W adapter, that leaves about 55W for the machine, and it's insufficient to power a full machine load. Since the machine load takes precedence, the power available to the charger will vary depending on the entirety of the machine load: CPU/GPU, drives, USB/FireWire, screen, etc. With a very high machine load, the charger is left with very little power to use, even with an 85W supply, and will take very long to charge the battery. The longest I've seen was 20+ hours with everything going full blast (full CPU+GPU load, all USB and FireWire ports delivering full rated power, all drives spinning, screen at full brightness, speakers blaring).
Finally, the supply's electronic nameplate is stored in the chip residing in the MagSafe jack. If the MagSafe jack is damaged or doesn't have the nameplate chip, the power manager does two things:
Assumes a 60W power supply.
Disables the battery charger.
You will not harm anything using the incorrect adapter. The charging circuitry in Intel Macs is very sophisticated and won't let anything bad happen.
Using the higher-wattage adapter with a low-power-requirement notebook will work. The computer will only draw as much power from the adapter as it needs. Using the low-wattage adapter on a high-draw notebook will result in the adapter powering the computer OR charging the battery, but not both. If you plug a 60W adapter in to a MacBook Pro at 50% battery charge, the battery will just stay at 50% (or either drain or charge very slowly) while the computer is on. If the computer is asleep or shut down, the battery will charge at a normal rate.
I did use a 13'/65W power adapter on my 15'er for a long time.
What happened with me was that the battery got significantly bloated (visible from the outer aluminim shell and only after ~150 loadcycles) while still delivering proper batterylife...
Please take into consideration that this might be completely unrelated, probably. Plus my battery was replaced at no cost in an Apple store (Macbookpro late 2008, 1st gen unibody) while I'm still not sure if Applecare also covers the battery on 3yrs.
Do NOT use a lower-than-spec'd power-supply with your portable. It will power on, but will be running in a brown-out condition, causing shortened life due to excessive heat.
We found this out the hard way with an after market power-supply on an iBook. It was just under the needed power output, causing the unit to always run hot, and eventually shutting down because of too much heat. After several months it got so it would run for 10-20 minutes then turned off, and eventually quit entirely. We tried getting it repaired, but it was too far gone.
This is a great question, and I kind of wish I'd thought to ask it before I started doing exactly what you're proposing. I suppose I assumed that the OS would warn me if using a lower/higher output adapter was somehow unsafe for my computer. In other words... keep in mind that what follows is only from my personal experience, and is far from scientific evidence one way or another.
Before picking up 2 15" mid-2010 MBPs in June, my wife and I were sharing a 2006 15" Core Duo MBP. The 2006 uses the 65W adapter, the 2010s use the 85W. We quickly noticed that the socket on the new ones was identical to the old. I figured that with Apple's attention to detail, it would be safe to try charging my 2010 with the old adapter.
Since it worked, we hooked up the 65W in the living room and put our 85Ws in our respective offices. Since June, both of our 2010s have been charged using both adapters interchangeably. I would say that mine (the 2.66GHz i7 model) has used it about 1/3 of the time. My wife's (the 2.4GHz i5 model) has probably used the 65W at or above 50% of the time.
Have I noticed any issues or degraded performance? Not one bit. In fact, I'm a bit surprised at how healthy the battery is on mine considering how often I use it without plugging it in. You can see my almost-current battery specs here. I thought that with as many cycles as I've put it through, my battery would be getting pretty close to 80% already. Also, I haven't noticed any difference in temperature when using a specific adapter.*
Bottom line... the only difference I've noticed is that the computers take longer to charge when using the 65W instead of the 85W, but even that difference has been minor.
*However, now that I'm thinking about it, I'll probably look a little closer at this.
After posting this, I took a look at my '65W' adapter. Apparently, it's not a 65W adapter after all, but an 85W with the older connector (the fat head connector). I always assumed it was 65W because of the age of our 2006 MBP... I guess the guy we bought it from used had replaced the adapter. So basically my answer is completely invalid (other than the stuff about there being no difference between the adapters) and should be disregarded :)
I'll leave this up for a day or two so that hopefully I don't lead anyone astray... On a side note, it's interesting that I've noticed the computer taking longer to charge on the older adapter. Oh, the power of the placebo effect...
I have been using the 60w adapter from my old 13"macbook pro(which was stolen a while ago) for my new 15" macbook pro for about a month because I assumed that all the cables or adapters are the same from Apple(since it is the case for the usb cable) and I did not realize until one day it turned off itself. Most of the time, my macbook pro was connected to the power. Now I have switched to the 85w adapter and everything works fine, thank god. my question is: will this cause any issue or damage to my battery or hard drive or other parts of my macbook pro?
Is it a good idea? Nope. Does it work? Yes. The wattage rating is matched up with your particular laptop's needs. If you use a lower rated power supply, in many circumstances it will work ok, it may just charge a little bit slower. If your laptop is powered off, it'll charge decently fast too.
However if you then boot into something less power efficient like linux or windows, you'll notice that despite being hooked up to a power supply, you are slowly losing battery life because there's simply not enough wattage to drive the whole thing and a lot of the power saving features aren't properly implemented. This also can be an issue if you have modified the hardware in your laptop, or if you try to do heavy gaming (anything that gets disks and fans spinning). If this is your situation, you can still use the lower power supply, but you're going to eventually hit a point where the transformer will overheat (really bad), or you will just run out of power.
So in a perfect world, you should always have the adapter that apple intended you to have (or a higher rated one). But if you have to use a lower rated one for a little while it won't be the end of the world.