There is a difference between voltage and amperage in that voltage is the potential difference between the positive and negative points in a circuit, in this case the terminals, potential difference is the amount of push and pull the terminals are exerting on the electrons in the system and amperage is the amount of power the battery stores. Think of it as a bucket containing water. Two buckets of the same size with different quantities of water are similar to two same weight batteries with different amperages. The bucket with more water is similar to a battery with more amperage.

Now think of the voltage of the battery as pressure produced when a water tap is turned half way on. The pressure of the water will be lesser than when the tap is fully turned on. The laptop or note book in this case is the tap. When the notebook runs a number of applications it allows more power to flow from the than when it runs simple programs that require lesser computing and processing therefore the usage of the notebook will determine how long the battery will last.

So to cut to the chase, look for the power ratings on your laptop. If, for example, your laptop says 1300mAh it means it will use 1300 milli-amperes in one hour, and if your battery stores 4000 mAh of power it should power your laptop for three hours (4000 mAh divided by 1300 mAh), it is as simple as that.