That one line about this battery weighing just as much as the empty battery at the village picked my interest.
So, both batteries weigh the same, and they weigh thrice as much as the dinosaurs. Per chapter 12, dinos are estimated to be 3m tall; seems close enough to an achillobator to me (5 meters long, so maaaybe 3m tall if upright). Those are estimated to be 250kg, putting the batteries in the 750kg range.
Square-cube law (mecha fans know this one) means the new battery, being "half the size" should have 1/8th the volume. All things equal (aka, no charge) that should mean 1/8th the mass. Where's the remaining 656.25kg coming from?
Components that don't miniaturize well, containment layers that remain the same thickness... The remainder has to be due to charge, but we don't know what the proportions are here.
I could make some wild assumptions: pretend "dry" mass is indeed 1/8th, ancient magitech civilizations knew the secret to 1:1 mass-energy conversion, cows are spherical, and neither I nor our dear Dioota fudged any numbers. So we can just plug in e = mc^2 and claim these things hold 58.9 exajoules of charge (aka 58.9 * 10^18 J). Five times the global nuclear arsenal, no biggie.
These seem too numerous for the planet to have survived this long if they're known to explode, so I'm probably way off.
Hopefully we'll get some better numbers soon.