4. Wise Shopping Decisions

Eventually, you're going to wind up buying a replacement battery. Often by the time you're ready to switch batteries, higher capacities are available. The simple rule of thumb for comparing any two batteries is:

Multiply the (mAh) capacity by the (V) voltage and divide by 1000 to get Watt-Hours.

Let's give it a try:
The first thing you do is read the mAh and Voltage off your used battery. Then multiply them together and divide by 1000. If your battery has Ah then they already divided it by 1000. For example, if you have a 10.8V @ 4400mAh battery: you get 10.8 x 4400 / 1000 = 47.52 or roughly 48 Watt Hours (Wh). This gives you a single performance "energy" number on any two similar batteries so you do some smart comparisons. For example, if your current battery has a rating of 48Wh and you see the information in the picture below in the store, this tells you that the run time is likely to be twice that of your 48Wh battery since 95Wh is twice as good as your current battery.

Often the difference between lower and higher capacity batteries is the number of cells. More cells mean a larger battery. In the picture below the notebook computer is jacked up an inch or so by the 12 cell battery.