Batteries fail, as certainly as death and taxes. Rechargeable batteries at least offer the possibility of repeating the cycle, so are in this sense more like recurrent taxes than death. But, the story cannot repeat indefinitely. One cheerful thought after the other, yes? But wait, there’s more. Add to their inevitable demise an overall lackluster performance in battery storage technology, and we have ourselves the makings of a blog post on the failure of batteries to live up to their promises.
To set the stage, the specific energy of gasoline—measured in kWh per kg, for instance—is about 400 times higher than that of a lead-acid battery, and about 200 times better than the Polymer Lithium ion Battery in the Chevrolet Volt. We should not expect batteries to rival the energy density delivered by our beloved fossil fuels.
The chief measure of a battery is how much energy it can store. But it makes sense to adjust this concept to the size or mass of a battery. Obviously, a more massive and voluminous battery can pack in more energy. So for a given mass, we want to know how much energy a battery can store, called specific energy.
At low power demand, sipping rather than gulping, lead-acid batteries tend to hold about 30–40 Wh per kilogram, one Watt-hour is equivalent to 3600 J, or 0.001 kWh of energy. Ni-MH batteries score 45–60 Wh/kg, and Lithium-ion gets about 120–180 Wh/kg. Part of the reason for Li-ion’s better performance is that lithium itself is lightweight. By volume lead-acid has about 40% the capacity of Li-ion. Gasoline, at 36.6 kWh/gal, has a specific energy of 13,800 Wh/kg.
As power demand increases, the battery flags, and will not offer as much total energy. Obviously, the 18650 battery discharges faster under heavier power demand, but the effect is exacerbated by less actual energy available. This is best shown on a Ragone plot, in which specific energy is plotted against specific power.