It was five years coming, but the battery of my ThinkPad R50 finally met its maker. On a recent trip, it conked out after powering the notebook for only half an hour -- three hours short of what I'm used to getting out it. Now it refuses to be recharged at all. Clearly, it's time for a new battery.

It's a fact of life that sooner or later, every laptop battery hits old age. Most do well for between 18 and 36 months, depending on how heavily they're used. Then they slip into a slow decline and lose the ability to take a full charge.

The reason? A typical battery pack can be recharged between 300 and 500 times before the chemicals inside start to wear out, with the result that a battery that once powered a system for three hours can now run for only an hour or less. That's when it's time to replace the battery.

There are two basic types of notebook batteries: the brand-name batteries that the manufacturer sells and the aftermarket batteries that are available from third-party resellers -- often for a significantly lower price. The trade in aftermarket batteries is growing quickly, perhaps by as much as 30 percent a year, according to Don Saxman, an analyst at BCC Research.

"This is the result of the enormous popularity of notebooks," Saxman explains. "Lots of people buy a second battery, and the longer you keep a notebook, the greater the chance that it will need a new battery."

We all want to save money, but not if it puts our notebooks at risk. Buying an aftermarket battery often goes against the advice of laptop manufacturers, and in some cases can even void the warranty. Is it worth it to save a few bucks?

I went on a mission to find out whether aftermarket replacement batteries are a good deal, safe and reliable or a dangerous fraud.

Bargain hunting

With the Lenovo replacement battery for my ThinkPad R50 selling for US$160 -- about half what the notebook itself is worth -- there was certainly room to save some money. I also went looking for a new battery for my two-year-old MacBook Pro. Both machines have worn-out batteries that power the systems for half the time or less than they could when they were new.

With the battery part numbers in hand, I did a little nosing around on the Web. I found several places that sell batteries for a wide variety of notebooks made in the past 15 years, from ones introduced earlier this year to relics like the 12-year-old Texas Instruments Extensa 600. Many of the companies also sell AC adapters and batteries for mobile phones, digital cameras and handheld devices.