A new "Power Saving Feature" in the Unibody MacBook and MacBook Pros was inadvertently discovered by Gearlog while they were doing a routine benchmark.
Gearlog was benchmarking was to test third party ram modules for use in the new MacBook Pros, but discovered that the notebook’s processing power significantly decreased after the battery was removed during the test. It was confirmed that Apple does indeed scale back the CPU processing speed while operating on AC power without a battery installed.
According to this support document, Apple essentially down-clocks the CPU to prevent the system from shutting down if it happens to demand greater power than the AC adapter alone can provide: "If the battery is removed from a MacBook or MacBook Pro, the computer will automatically reduce the processor speed. This prevents the computer from shutting down if it demands more power than the A/C adaptor alone can provide."
Collaboration from other users can be seen on the Gearlog comments; however, since this support document was last updated in August 2008, I decided to do some independent testing on my own MacBook Pro. I wanted to see if this problem existed in late model MacBook Pros or were they limited to only the new Unibody MacBook Pros. Using the same Cinebench R10 benchmarking software, I conducted the full tests three times each with the WiFi and Bluetooth radio disabled and carrying forward the best scores achieved each time.
Gearlog’s own results show a decrease of 36 percent in processor speeds without a battery attached on their Unibody MacBook Pro. Our own tests show a decrease of 50 percent when rendering with one CPU, a decrease of 52 percent in multiprocessor rendering, while the GPU suffered a 40 percent decrease in performance.
What does Apple have to say about this? The official statement from its support document is that “It is strongly recommended that you do not use your MacBook or MacBook Pro while the battery is removed.”
The only benefit in operating a notebook without its battery is to conserve charge cycles and thereby extending the battery life. Apple recommends that with notebook batteries, occasional power flow is necessary for its longevity. Standard maintenance is not to constantly keep the battery charged but rather allow it to discharge partially before recharging it again. If your use is infrequent, it is recommended that the battery be completely discharged and recharged once per month.
With such drastic performance decreases on a previous generation Macbook Pro, it doesn’t seem logical to remove the battery when a brand new battery can be purchased for $129 from Apple or possibly even less from other resellers. I would rather have a full powered notebook and a few months less with my battery than the alternative.