That's not necessarily true.

The number of cells makes not one bit of difference as long as the combination does not exceed the voltage rating of the ESC.

Assuming that the original poster is using LiPo cells, well, the generally accepted setup for smaller planes is 2S or 3S (two series or three series) for 7.8 or 11.1 (nominal) volts. But, the number of cells in series/series parallel/parallel makes no difference as I said, as long as the voltage rating of the ESC is not exceeded.

With that said, increasing the size of the battery pack has some drawbacks, primary one is weight.

When figuring out the total weight for the watts/pound factor, adding a new, larger battery pack means more weight, so the weight factor, watts per pound has to be reconsidered. That's 16 ounces per pound, or 28.35 grams per ounce.

50 watts per pound for an easy flyer
75 watts per pound for a sport flyer
100+ watts per pound for a high performance

Watts = voltage X current

So, before changing over to a larger battery, make sure your ESC is rated for a three cell pack, and then consider how the added weight will effect the performance of the plane. And, of course, re-set the Center of Gravity.

As far as an audible indicator of low battery, well, you won't really need one becasue you can "feel" the performance draining off as the battery drains. Once you reach a certain point, it will begin to get really sluggish.. time to land and re-charge or change battery packs.

CGr.