Q: I recently purchased a ZMI PowerPack 20000 from Amazon and have a question about its capacity. One of the reviews says that you use 7.2V 10000mAh lithium polymer batteries with a nominal capacity of 72Wh. It then goes on to say that most mobile power banks use LG 18650 batteries which run at 3.7V. Is this true? If so, how does that provide me more capacity than a standard 10000mAh mobile power bank?
A: It's de facto for vendors to give the mAh (milli-amp hours) rating as the nominal capacity of a lithium-ion battery. But, most consumers do not realize that mAh alone does not give the capacity of a battery. An additional piece of information is the battery voltage. The proper way to specify capacity is to state in Wh (watt-hours). We can calculate watt-hours by multiplying mAh by voltage then divided by 1000. Take our QB820 as an example, it is made of six LG 18650 battery cells, each rated for 3,350 mAh at 3.6V. So, the capacity of QB820 is 3,350 x 3.6 x 6 / 1000 = 72.36 Wh, which gives the 72Wh nominal capacity.
Battery cells can be connected in parallel or series or the combination of both. For example, two of those 18650 are connected in series (in tandem), we'll get an equivalent battery of 3,350mAh at 7.2V. So, 3,350mAh or 7.2V alone is not indicative of the capacity; they together are. Along with this line of reasoning, if we put three copies of the two-cell series in parallel, we'll get an equivalent battery of 3,350mAh x 3 at 7.2V. We multiply those numbers 3,350 x 3 x 7.2 and still end up with 72Wh.
There're six LG 18650 (with a nominal capacity of 12.06Wh) cells in the QB820. That is the basis of our claim that the QB820 is rated for 72Wh. In fact, this is done by other brands (Anker, RAVPower, etc.) as well. So, yes, 72Wh is the true nominal capacity. Regarding your last question, "how does that provide me anymore recharge capacity than a standard 10000mA mobile power bank?" the 10000mAh battery packs you can find out there are made of either 18650 (same cylindrical form factor as ours) or a slab of lithium-polymer of a different form factor and chemistry.
Case (1): The 10000mAh battery pack is made of 18650 cells. Then. we'll be looking at 3 of those 18650. Capacity is then at 3350 x 3 x 3.6 = 36.18Wh.
Case (2) The 10000mAh battery pack is made of a lithium-polymer slab. The lithium-polymer slab is typically at 3.7V. So, Capacity is then calculated as 10000 x 3.7 = 37Wh.
In either case, we can see that our QB820 offers double or roughly double the capacity of a 10000mAh battery pack.