See "Shipstone" in R.A. Heinlein's novel Friday.
See "Molecular Distortion Battery" in Niven's Known Space novels.
Both give it a name, and ignore the details.
H. Beam Piper refers to isotope powered nuclear batteries. The batteries are armoured in collapsium. In effect a single layer of nuclei. About the same weight penalty as a foot of lead. The best reference is in the novel, "The Cosmic Computer"
More near future examples:
A large amount of fuss and feathers some years back about eestor was making a capacitor capable of storing about 50 kWh in little more than a cubic foot. Turned out to be either vapour ware, or quality control issues, but nothing so far has come of it. It used barium titanate, with small impurities for a purported dialectric constant in the range of about 20,000 charged to 3500v. (Capacity is proportional to dialectic and plate area; energy proportional to capacity and the square of voltage.)
Superconductor loops approach energy densities comparable to gasoline. All you would need is some high temp superconductor material. There is no theoretical base I know of that it's impossible, and graphene makes it look possible. Superconductors also superconduct heat. So superconducting armour will be resistant to focused laser/plasma fire.
VERY strong materials (graphene, carbon nano tubes) potentially allow the storage of energy in flywheels. These have angular momentum issues for a combat application. Switzerland has used flywheel powered busses.
Solid carbon fuel cells look interesting. These in theory can convert 80% of the energy in carbon into energy. Currently both fussy and bulky, but the power density may be able to be increased using nano-particle carbon. The battery would have to be kept hot (800C) to work. Has potential at ship sized installations.
That said: Lasers have more waste heat at the source than they deliver to the target. As a weapon they work only by virtue of concentrating that energy is a very small spot. Lasers in air are inefficient. At high power, they ionize the air, then that blocks the beam. Maintaining a good focus at long distances is difficult even with a stationary target, and a rock stable platform.
There's a lot to be said for using a bullet. Chemical energy storage is dense. Most of the energy is delivered to the bullet. Much of the waste energy is in the propellent gasses. (There is still enough energy that barrels and breach get HOT. The Maxim machine gun is water cooled -- and boils the water.) The downside is that it's not as fast.