A hybrid battery is a fraction of the size of an electric car's pack, yet it frequently outlasts the vehicle. The difference comes from being designed for a completely different duty.
Power and energy are separate requirements
Energy capacity determines how far a vehicle can travel on stored charge. Power capability determines how quickly that charge can be delivered or absorbed.
An electric car needs large capacity because range is the point, and its power requirement follows naturally from having so many cells connected together.
A hybrid needs almost no capacity but very high power, because it must absorb a heavy braking event or assist a full-throttle acceleration within seconds.
Cell design follows the requirement
Cells optimised for energy use thick electrodes that hold more active material, which stores more charge but slows how quickly ions can move in and out.
Cells optimised for power use thin electrodes with large surface area, accepting less capacity in exchange for very low internal resistance.
A hybrid pack is therefore built from power cells and is physically small, which is how it fits under a seat or behind a rear bench without dominating the vehicle.
The charge window is deliberately narrow
Lithium cells degrade fastest when held at very high or very low states of charge, and cycling across the full range accelerates the process further.
A hybrid controller keeps the pack near the middle of its range, using only a narrow band and never charging or discharging it fully.
This sacrifices most of the nominal capacity in exchange for very slow degradation, which is why these packs tolerate an enormous number of shallow cycles.
Cycle counts differ by orders of magnitude
An electric car might complete one deep cycle every few days, so its pack sees a few hundred over many years of ordinary use.
A hybrid pack is charged and discharged many times on a single urban journey, accumulating a vast number of very shallow cycles instead.
Because degradation depends more on depth than on count, the hybrid pack survives that treatment while a pack designed for range would not.
Why thermal management differs too
Hybrid packs are often cooled by cabin air drawn through a duct, which is adequate because the total energy moved is small even though the power is high.
Electric packs generally require liquid cooling, since rapid charging and sustained high-speed driving generate heat over long periods rather than in bursts.
The simpler system is another reason hybrid packs are inexpensive relative to their reputation, and blocked cooling intakes are a more common failure than cell degradation.