Batteries as supercapacitors are electrical storage devices. Battery is more widely used due to its ability to store more energy for a given weight and volume: it has a better energy density. Supercapacitor on the other hand can be charged and discharged very quickly (or slowly if you do not need power): it has a very high power density.
But the differences between these two systems do not stop there. The evolution of these features with the technological improvements and declining prices mean that supercapacitors are increasingly used and will be even more in the future.
There are important differences between the technologies of batteries and supercapacitors. There is one side the lead-acid batteries, Ni-MH, LMP, Li-Po, Li-ion, etc. On the other side, there are supercapacitors with organic electrolyte, with aqueous electrolyte, with ionic liquid, hybrid supercapacitors, pseudo-supercapacitors...
To simplify things, here is a table comparing the today key battery: Lithium-ion battery that offers a very good energy density, and supercapacitors commonly used today.
|Gravimetric energy (Wh/kg)||100 - 265||4 - 10|
|Volumetric energy (Wh/L)||220 - 400||4 - 14|
|Power density (W/kg)||1,500||3,000 - 40,000|
|Voltage of a cell (V)||3.6||2.7 - 3|
|ESR (mΩ)||500||40 - 300|
|Efficiency (%)||75 - 90||98|
|Cyclability (nb recharges)||500 - 1,000||500,000 - 20,000,000|
|Life||5 - 10 years||10 - 15 years|
|Self-discharge (% per month)||2||40 - 50 (descending)|
|Charge temperature||0 to 45°C||-40 to 65°C|
|Discharge temperature||-20 to 60°C||-40 to 65°C|
|Deep discharge pb||yes||no|
|Risk of thermal runaway||yes||no|
|Risk of explosion||yes||no (1)|
|Charging 1 cell||complex||easy|
|Charging cells in series||complex||complex|
|Voltage on discharge||stable||decreasing (2)|
|Cost per kWh||200 - 1,000 €||10,000 €|
(1) With a compliant charging system and exposure temperature in the operating range (often from -40 to 65°C), there is no risk that a supercapacitor explode or catch fire, unlike a Lithium-ion battery.
(2) At a constant voltage charging, the voltage of a supercapacitor increases progressively, but the charge is more effective at constant current and voltage then rises straight. During discharge, the voltage of a supercapacitor decreases continuously. To exploit at best the energy of a supercapacitor, we use a voltage converter to obtain a stable voltage.
Battery and supercapacitor: rival or complementary?
We should not necessarily oppose battery and supercapacitor. For many applications, these storage systems enable a more efficient solution when they are used together.
Indeed, battery can be used to store a large amount of electricity while supercapacitor, positioned in buffer energy system is used for power peaks demands. This avoids too severe demands on battery witch alter its life, while gaining power.
Thus, the cars of Autolib' - a successfull electric car sharing service in Paris (FRANCE) - use supercapacitors associated with Lithium Metal Polymer batteries that have a low power density, but a longer service life than Li-ion batteries. A very appropriate choice for high demand vehicles.