The 9-cell Think Pad Battery 57++ extends the run time of the W541, T440p, T540p, W540, L440 & L540 systems. On average, the battery lasts for an incredible 100 watt hours. Each one is rechargeable & uses over-discharge protection to ensure power flows securely. Advanced Lithium-Ion (Li-Ion) technology is used to combine efficiency & performance, making this battery ideal for replacing ageing systems or for use as a convenient spare. Each one is safeguarded with a chip which authenticates with Lenovo systems. * Many factors cause battery life to differ. These include: screen brightness, battery conditioning, features, applications, power management & other customer preferences.