The short answer is: “Whichever type fits your laptop.” Laptop batteries vary according to the model and type of laptop computer for which they’ve been designed. All laptop batteries do, however, share some common characteristics that differentiate them from regular household batteries: • Laptop batteries are rechargeable • They are composed of multiple internal cells • Within each laptop battery is a small printed circuit board • Shape and location of terminals differ from model to model. As is the case with all batteries, laptop batteries create an electrochemical reaction, forcing a stream of electrons from one position to another. Also like other batteries, laptop batteries have positive and negative terminals that receive and send electrons. Rechargeable batteries are more complex, however, than standard batteries. This is because the electrochemical reaction that provides power must be reversible. Care has to be taken when recharging them to ensure they perform at their best. Frank McLarnon, Lawrence Berkley lab’s staff scientist explains it this way to Scientific American.com: "When a battery is discharged, an electrochemical oxidation reaction proceeds at the negative electrode, and an electrochemical reduction reaction occurs at the positive electrode. When one attempts to recharge a battery by reversing the direction of electric current flow, the opposite takes place: a reduction reaction proceeds at the negative electrode, and an oxidation reaction takes place at the positive electrode. In the case of the rechargeable battery, the electrochemical oxidation-reduction reactions are reversible at both electrodes. In the case of the non-rechargeable battery, when one attempts to recharge the battery by reversing the direction of electron current flow, at least one of the electrochemical oxidation-reduction reactions is not reversible. When the battery is charged, the overall reduction reaction that proceeds at the negative electrode may not be the true reverse of the oxidation reaction that proceeded when the battery was discharged. If the laptop battery process isn't exact, then unwanted build up can take place on either terminal over time, which can cause a dangerous short-circuit.” It is so important to ensure that your laptop battery or notebook battery is the one designed for your particular model and type. Only buy Dell laptop batteries for a Dell laptop computer, Gateway aren’t compatible with Compaq laptop batteries and Toshiba laptop batteries won’t sit well in a Sony. It is also wise to remember that old adage: “You get what you pay for.” Some companies offer used “cheap laptop batteries” for sale, saying that they have refurbished each battery and restored most of its useful life. The fallacy of this is that, although possible to refurbish a second-hand laptop battery, the cost to do so would be more than the price of a new battery. So, “Buyer beware,” and if you must buy discount laptop batteries, buy new discounted batteries. Laptop computers are a boon to business, large and small. As a wise person is just about to say: “Look after your laptop and it will look after you.”