How to find the laptop battery specifications
There are different types and sizes of laptop batteries. To determine what laptop battery you have, remove the battery from the laptop and look at the top or bottom of the battery.
As you can see in the picture, this Dell battery shows that it is an Li-ion battery, has a model number of Li-ion II, rating of 10.8V, 4050mAH, part number (labeled as "P/N"), and a charging current of 2.5A.
Many laptop manufacturers will provide details on their website for which batteries their laptops use, including model numbers and part numbers. Check your laptop manufacturer's website to get battery details for your laptop model. The website may also allow you to purchase a new battery or provide a link to where you can purchase a new battery.
If your battery is not labeled or you're unable to read the information on the battery, consider using a third-party utility that's capable of giving you detailed battery information. For example, BatteryCare is a free software utility capable of doing this.