Frequently, the only things that separate the two types of batteries are whose name is on the label and how much it costs. In fact, the MacBook Pro battery that Laptop Battery Express sent me was actually an Apple-branded battery with all the markings, including a serial number.
The company's supplier said it was left over from a production run made for Apple. Whether this violates agreements between the factory and Apple is for their lawyers to fight about. All I know is that I got a battery for US$30 less that appeared to be identical and gave me no problems.
Sometimes aftermarket batteries are even better than the originals. Due to added efficiencies that manufacturing plants have developed in the time since the notebook and its first battery pack were made, some replacement cells can be lighter or contain more capacity.
Two of the six batteries I looked at stand out. The MacBook Pro battery from Laptops for Less weighs 2.3 oz. less than Apple's battery but delivers about the same capacity; it ran for only five minutes less than the reseller replacement. Then there's the R50 replacement battery sold by Laptop Battery Express, which has a higher capacity but weighs 0.2 oz. less; it ran for an extra 10 minutes.
Each was significantly less expensive than the manufacturers' batteries, which sounds like a win-win to me.
Are they safe? And what about my warranty?
The aftermarket battery trade is a fact of life and will likely grow in the coming years, but that doesn't mean the notebook makers have to like it. I asked several leading laptop manufacturers if they believe aftermarket batteries are dangerous to use and if using them invalidates the notebooks' warranties. While Apple, Hewlett-Packard and Dell did not respond to my repeated queries, Lenovo and Toshiba agreed to discuss the issue.
Neither manufacturer said that aftermarket batteries are dangerous, but they stressed that their batteries were designed and manufactured expressly for their equipment. A Lenovo representative said, "We do not support the use of nongenuine Lenovo batteries in ThinkPads. ThinkPad batteries are designed and tested with ThinkPad notebooks to ensure safety, compatibility and performance."
While using an aftermarket battery doesn't invalidate a notebook's warranty in and of itself, the manufacturers probably wouldn't repair a notebook under warranty that has been damaged by a faulty aftermarket battery, according to company representatives. In other words, a faulty aftermarket battery does invalidate the warranty.