How to calculate lithium-ion battery energy density

Question

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.

Answer 1

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.

Answer 2

My experience with manufacturer's specs for batteries is that they are an "or" condition. You can expect the battery to deliver 12 volts for some period of time into a light load or you can expect 6.8 amps for some period of time with an exponentially falling voltage as the internal resistance of the battery climbs. By simply assuming an average voltage of about half the rated 12 while delivering 6.8 amps for an hour gets you in the neighborhood of the Wikipedia prediction.