The 1eV per atom is really a rule of thumb, but easy to understand: At that value you are getting dangerously close to the ionization energy 13.6 eV for hydrogen, 5.4 eV for lithium), so you can no longer have a chemical battery. Note also that this is average eV for all atoms in the battery, so those actually providing electricity will be carrying atound 1.5 - 2 eV.

See e.g. here: http://www.ohio.edu/people/piccard/radnotes/radioactive.html