Let's take a brief survey of the overall bike-light battery landscape, but first off, lets define a few terms and clear up a few misconceptions.

Although, there used to be a few Nickel-Metal-Hydride (NiMH) packs out there (Cygolight for one), they have all but gone by the wayside, in the wake of more efficient and lighter lithium-ion technology. The self-discharge characteristics of the NiMH were really miserable, so I was glad to see them go.

When it comes to li-ion batteries, we need to start by defining a "CELL."
A single lithium-ion cell has a NOMINAL charge of 3.7 V, and a MAXIMUM full charge of 4.2 Volts. Bike light packs are put together by stacking these individual cells together in a series and/or parallel arrangement. There is a distinct LACK of a common standard when referring to the "voltage" of any particular pack. Some will use the nominal, some will use the max, while others will use something in-between. It can be very confusing.

We haven't mentioned "capacity" yet, but you've probably seen numbers like "4400mAH" or "2200mAH" with regard to certain packs. These numbers give you the amount of "energy" in the pack in units of mill-amp-hours. There is a lot to know about how to interpret the capacity under certain conditions, but in essence, it gives you the knowledge of how long the pack can run while delivering a certain amount of power. Most bike light loads (the drain put on the battery) are dynamic in nature, which means the power being drawn from the battery does not remain constant during a discharge cycle, but for purposes of illustration, let's say you had an electrical load on the battery that was drawing a continuous current of 1 AMP. In this case we could calculate the run time of a 4400maH battery as:

4.4 Amp-hour/1Amp = 4.4 hours

With few exceptions, almost all bike lights that use an external pack are using "7.4V" packs, and if you refer back to the NOMINAL li-ion cell charge, you'll see that this is exactly TWO-cells added together (3.7 x 2). Electrically this requires two cells to be connected in SERIES. When you stack cells in SERIES, the voltage of the cells adds, but the capacity does NOT. In other words, a couple of individual 3.7V, 2200mAH cells in a 2-cell series configuration is now a 7.4V, 2200mAH pack. Take the 2-cell Gemini pack for example:
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This is as small as you can get for a 7.4V pack, using the 18650 li-ion cell (same as what is used in PC laptops).
Some folks will refer to the 2-Series (2S for short) as 8.4V (using the maximum cell charge), but as long as you know that you have a 2S li-ion pack, and you are aware of the information here, all ambiguity goes away.

If you want to start increasing the capacity of your 2S pack, then you have to start putting banks of 2S stacks in parallel. In this case, the voltage of the pack stays the same, but the capacity is additive (per parallel bank). So, take 2 of our 2S stacks and put them in parallel and now you have a 2S-2P pack at 7.4V and 4400mAH. Take the 2S2P Magicshine pack for example:
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Some folks may incorrectly refer to this as a "4-cell" pack. However, the term "4-cell" would imply 4 x 3.7 = 14.8V! True, the pack has 4 physical "cells," but they are in the 2S2P configuration. If we continue on up in capacity, we would have a 6600mAH, 2S3P pack, which would look something like this:
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Jump to PART 3, to learn specifically about the DesignShine system and how it "plays" with the many battery options that are readily available now, off the shelf.