Real physicist chiming in - yes, E=mc^2 applies, even with electron binding energies rather than nuclear binding energies. It's just much smaller, as the correct calculations already done in this thread show. If we take 85 kWh as having 2 significant figures, than 3.4 micrograms of additional mass is the correct answer. However, when compared with a car that has a curb mass of 2.1 trillion micrograms...
Well, have to admit I originally asked my question to provoke thought that (I thought) would lead to the conclusion the fully charged S would not weigh more than the nearly empty one. I really thought that all the e=mc^2 stuff was fun for giving me college flashbacks, but not at all relevant to the original question. My basis was basically that of potential energy not being something that would weigh more. I wasn't thinking that a rock at the top of a hill would weigh more than a rock at the bottom of a hill (again, ignoring gravity change based on distance, etc.), or a compressed spring would weigh more than an uncompressed spring. Basically just didn't think e=mc^2 applied here. My thinking was empty S = weight x, charging it with e amount of energy, it now has x+e, then drained we're back to x, but didn't think of the e itself having a weight stored as potential energy in the chemical bonds of the battery. Battery being a sealed system helped along that belief. After weeding through a bunch of pages from a google search of this topic, I guess I was wrong, though to be honest I still didn't see any explanation of exactly *how* a compressed spring would weigh more that made sense to me.
On the topic in general, I vaguely remembered some examples from my physics days about various scenarios, but they initially didn't seem like they applied, nor did I think that the energy itself, despite me knowing it was affected by gravity, would have a weight itself in this context (despite that being the same thing, right?). Reading of a box of mirrors with light in it weighing more than a box of mirrors without light was, um, enlightening…
I'd still like to understand the actual change in weight from a before/after chemical standpoint (or same explanation for a capacitor), but if I can buy that a box of light has weight (and I kind of have to, once I thought about it), then it seems to me that the battery *has* to weigh that little bit more, even though it has all the same bits of "stuff" in it, in terms of protons, neutrons, electrons (but arranged in either higher energy states or lower ones, and those states should - must - be different weights).
By the way, sense of scale here. From wikipedia when searching for things that weigh a microgram, the 3.4 micrograms we are talking about would be ~10 very fine grains of sand, or about 1/20 of an eyebrow hair.
