With all the power juice packs available for computers and devices, you might be wondering if you could just hack together some kind of a power solution and plug it into your USB port to charge your laptop. A lot of devices can be charged via the USB port, so why can't laptops be charged as well? The short answer to this question is...
...no. Here's why: the USB ports on laptops function to dispense power to other devices and peripherals, which is why you can charge your iPad, iPod, and other devices by simply plugging them in. This means that any battery pack or power solution meant for your laptop needs to be plugged into the power port. Macs have a patented MagSafe connector which complicates matters further, as the chargers tend to be more expensive than for PCs.
It's one of the reasons why hacking batteries can get quite complicated. Naturally, all manufacturers have their own type of charger, so whenever you start messing around with one, you should get the right model for your specific laptop, as some notebooks will different voltage and amperage. You wouldn't want to blow your laptop by plugging in the wrong kind of power.
Just check out how complicated it is to plug in a Hypermac external battery to a MacBook. You'll need a MagSafe Airline adapter to plug into Hypermac's Auto/Airline adapter. Some external batteries work for a number of brands. This one works for IBM, Sony, Acer, Toshiba, HP, Hitachi, Fujitsy, Asus, and Sharp laptops. It's also got a USB port for devices that need to be charged this way.
So if you're planning on using your old laptop batteries to cobble together an external battery of some kind for your laptop, make sure you've got the right type of ports and connectors specific to your own laptop.