According to this battery comparison sheet, lithium-ion batteries should have an energy density around 110-160 Wh/kg. Wikipedia cites it may be around 100–265 Wh/kg, although it refers to it as "specific energy" and uses "energy density" to refer to a measurement based on physical volume.
I'm thinking of buying a lithium-ion battery pack rated for 12V @ 6800mAh, giving it 81.6 Wh, and it weights 162 grams, meaning its energy density is 503.70 Wh/kg. Since that level is nearly twice the documented energy density for lithium-ion, is my math wrong? Or are the battery's specs incorrect?
This is somewhat related to this question, but I'm trying to calculate it from a real battery's specs, not the technology's theoretical limits.
Best Answer
My suspicion is that the published specs have been greatly over estimated. Also, I believe that the watt hour calculation that you have done is slightly overestimated as well.
The 12 volt lithium ion battery has a nominal voltage of 11.1 (or possibly 10.8 or even 11.4 depending on the specific lithium technology used). I base this on a nominal voltage of 3.7 volts for each of the cells, 3 cells in series to make the 12 volt battery.
So technically it is 11.1 * 6.8 = 75.48 Watt Hours. This equalls approx 466 Wh/KG.
Not a big difference but a little less.
I suspect that the battery is significantly less than 6800 mAh or possibly it weighs more.