MacBook Power Requirements
Posted by stephen on Thursday, 17th August, 2006 @ 13:33
You might have seen that when Apple launched the MacBook Pro back at the start of this year, the power supply was somewhat larger than what we've come to expect from Apple notebooks. Indeed, the power supply is rated at 85W, a good rise on the 45-55W or so that came with the G4 series of laptops.
When the MacBook was released several months later I was relieved to find that not only were the keyboard and screen much better than I'd imagined they might be (it still looks a little Sinclair Spectrum to me), but the power brick was the same size as the one provided with my old PowerBook. And the reason being, this little brick is rated at 60W.
Which raised the question at the time... would we be able to use the PSUs interchangeably as we've always done before with Mac notebooks? Richard's got the MacBook Pro and I've got the MacBook, so it's often convenient when we go to the same meetings to share a power supply.
Apple's answer... yes. The only caveat being that if the MacBook Pro is under load, the MacBook's PSU might only be able to power it and there won't be enough energy to charge the battery at the same time. I'd assumed this was likely due to the larger screen, separate GPU and associated power requirements. Good news.
On to today, and I've got some H.264 video encoding to do. I decided to use a nice, high quality setting and 2-pass encoding to make a decent resulting file. I had one going on the iMac and was running short on time, so decided to set another going on the MacBook. When this thing gets busy it gets very hot! So, having read one of the biggest causes of Li-ion battery charge capacity loss is heat, I decided to remove the battery while it was encoding the video.
And to my surprise the fan never kicked in once during the encode, which I thought was a little strange. So I looked at the CPU speed and it had been speed-stepped down to 1GHz (it's the 1.83GHz model). I didn't really think much more of it.
Then I decided I actually wanted to use the laptop and thought it best to put the battery back in, otherwise accidentally knocking out the power lead would produce a rather annoying result! And within a minute the fan kicked in and up to a massive speed.
At first I assumed that the battery charging circuitry might perhaps produce an amount of heat that needs to be dissipated, which combined with the high CPU load made the fan necessary. Then I checked out the CPU speed, and it was running at 1.83GHz (and 73 degree Celsius, over 10 degrees hotter than before).
So it would seem that the MacBook power supply alone can't provide enough juice to run the machine at full speed. Which leaves me wondering (as I tend to destroy notebook batteries within 18 months), when this battery is on it's last legs and gives me 10 mins of use between charging, will my MacBook be able to manage full speed, even when plugged in?
