NB: that's probably backwards. Batteries contain a
lot of energy, they just don't release it particularly quickly.
Most explosives have relatively low energy density, however the energy they have is released far faster than with conventional fuels. By unit mass, TNT (or other comparable explosives such as C4, RDX, etc.) have about 1/10th the energy as liquid petroleum fuels (petrol, diesel, kerosene).
Though again most battery technologies also have fairly low energy densities. But those are probably roughly comparable with most mainstream explosives.
TNT has an energy density of 4.184 MJ/kg.
A LiON battery: 0.36–0.875 MJ/kg.
Motorola pagers (a widely used type) seem to typically take a 3.5V 500mAh battery, which if I'm doing my conversions correctly (mAh * V * 3.6) works out to about 23 kilojoule. That would be the energy equivalent of ~5g TNT. A light charge, but one you wouldn't want going off on your hip.
(Note: I've corrected an off-by-an-order-of-1,000 error above, earlier read 23 MJ / 180g TNT. As I said, I'm not entirely certain of my calculations, which are using the Wikipedia energy densities noted and GNU Units.)
https://en.wikipedia.org/wiki/Energy_density>
Again, batteries won't explode as footage of the presumed Israeli attack on Hezbolla members shows. But they do contain appreciable energy. It would more likely burn rapidly at worst case.