A battery capable of powering a car must, by definition, hold enough energy to power that car. (Well, power enough (as energy/time) to roughly compete with a comparable petrochemical engine, the total energy capacity might be less at current tech levels, like a smaller fuel tank that needs filling more frequently). But, like all energy stores (combustible fuel, electrochemical potential, pressurised gas, wound rubber band, anvil hoisted up on a rope ad pulley arrangement) the potential is there for uncontrolled release of the energy stored.
A propane tank meant to be refilled every month or so has to hold a month or so of household energy (adjusted by efficiency metric of the 'proper' release method), while a day-charge/night-discharge battery system needs only be a day's-worth (or two, as buffer) of store, so basic 'explosiveness' of a wbole-home battery backup's intrinsic energy is less.
Though a fuel tank is usually inert(-enough) steel, which wouldn't add much to an unintentional whole-instant-release of energy (resistance, mainly, turning a bad burn into a worse explosion, give or take safety valves and well designed positions of intentional weak-points to maybe redirect in the most neutral direction possible if installed correctly) while a battery (depending upon the exact chemical make-up behind the electrochemical storage) might have more reactive material like lithium that, when compromised by a short-circuit and/or mechanical damage also holds otherwise unused 'manufacturing energy' (not discharged, to be recharged, in normal use) that might add to the the total output of energy once some non-normal irreversible failure occurs.
The inefficiencies between storing and retrieving of energy also come into play. Trivially stored and carried, hydrocarbon fluids (once cracked and fractionated and blended back as required) don't have much wastage on input to the eventual priming tank, but hot exhaust (once as much mechanical energy as possible has be extracted by the piston engine/boiler/etc) has energy that the proper use does not extract but ought to be counted in the catastrophic depletion.
Pre-storage inefficiencies (the pressurising and maybe cryogenic cooling of hydrogen, for example, whether slated for burning or reverse electrolysis) don't add to the catastrophe (unless failures of the 'charging' procedure is responsible for the catastrophe), so a battery system which heats up on charging (within design limits, properly heatsunk) shouldn't be an issue, but asociated heating upon discharge might indicate 'extra' energy that would emerge in an uncontrolled discharhe
before the full breakdown of the electrolyte that might then happen.
I've thought a lot about the (idealised) Mister Fusion power source. Fed (chemically) inert feedstock, it is technomagically fusioned to create energy enough to supply both flux-capacity energy ("jiggawatts" of it) and hovercar capabilities (probably
calculable if you have the time). Working properly, one must assume that it uses some of its own energy also to safely contain the energies involved in a feedback loop. Does it even get warm, doing this? Hot to touch, I suspect (like an esoresso machine, maybe) though not 'radioactively hot'. That's even if it's "cold fusion", as you surely can't push so much power through the base of a blender without
some heat (even with extensive room-temperature superconductors, there'd be a junction with non-superconducting contacts) howeverso arranged the internals are.
And those internals must be massively over-engineered to deal with any conceivable common-or-garden failure, to mitigate any cherenkov-like leak of radiation through a crack or weakness in the physical/field-based shielding. Treated as an alternate form of fuel-tank/battery a catastrophic failure isn't really worth considering. If H&S attitudes aren't remarkably lax (a possibility, admittedly, if brilliant but scatterbrained Doc Brown is the only one doing this precise installation of equipment) then futuretech must have solved quite a lot of the technical safety issues (maybe charged particles being the only radiation, contained with tightly defined self-reinforcing magnetic bottles, maybe some equally fantastic 'force field' internal projection that also creates the pressure surely needed)...