Lithium polymer batteries are supposedly less volatile than Lithium-ion batteries when overcharged or overloaded, which means less support circuitry should be required. Smart and swappable Some readers have asked what makes a battery smart. A smart battery has built-in circuitry that lets it communicate with the laptop to monitor battery performance, temperature, output voltage, and so on. The result is battery life that lasts maybe 10 percent to 15 percent longer and offers more accurate feedback on how much runtime actually remains. Besides battery type, look for the number of batteries that a notebook can simultaneously accommodate.
Some pricier notebooks have modular bays that can accept a second battery.
Of course, a second battery adds to the total weight, but if runtime is more important, it's a worthwhile trade-off. You should also find out whether the notebook must shut down completely before you can swap batteries. Some newer designs let you swap batteries while in sleep mode, thus saving time. It hooks up via your notebook's external power connector and fits easily into a notebook carrying case. I've had a chance to test one of the notebooks which readers told me could play an entire DVD movie.