You may eventually find yourself needing a big battery, say because your project has twice as many LEDs or you want it to last twice as long. A lot of people think "well I know that if you parallel two batteries that doubles the capacity so I'll do that!"

For example, we could take two 1300mAh batteries, that are 3.7V nominal and connect them together in parallel…

Not only should you not do this with alkaline batteries, but its especially dangerous with lithium batteries. One battery can discharge into another, damaging it or causing a fire!

If you ever need a larger battery, you should upgrade to a larger battery! Like this one which is twice the size and has twice the capacity.

You may be asking then, How come adafruit sells battery packs that are paralleled?

These two packs are clearly two and three batteries. Since they are 3.7V they must be in parallel. Isnt this unsafe?

The answer is that these batteries are assembled by a company that is experienced and certified to test and assemble battery packs. The individual batteries are tested and sorted by machine so that each pack has matching batteries with the same capacity and internal resistance. Individuals do not have this equipment, which is why you should not try to make your own packs.

So the next question is whether you can connect batteries in series to get higher voltages?