Battery technologies and their discharge times
Battery technologies for energy storage devices can be differentiated on the basis of energy density,
charge and discharge (round trip) efficiency, life span, and eco-friendliness of the devices.
Energy density is defined as the amount of energy that can be stored in a single system per unit volume
or per unit weight. Lithium secondary batteries store 150–250 watt-hours per kilogram (kg) and can
store 1.5–2 times more energy than Na–S batteries, two to three times more than redox flow batteries,
and about five times more than lead storage batteries.
Charge and discharge efficiency is a performance scale that can be used to assess battery efficiency.
Lithium secondary batteries have the highest charge and discharge efficiency, at 95%, while lead
storage batteries are at about 60%–70%, and redox flow batteries, at about 70%–75%.
| Discharge Time | Energy-to-Power Ratio | Technologies |
| Short (seconds to minutes) | Less than 1 (e.g., a capacity of less than
1 kWh for a system with a power of 1 kW) |
DLCs, SMES, FES |
| Medium (minutes to hours) | Between 1 and 10 (e.g., between 1 kWh and 10 kWh for a 1 kW system) | FES, EES such as PbA, Li-ion, and Na–S batteries
EES technologies have relatively similar technical features. They have advantages over other technologies in the kW–MW and kWh– MWh range. |
| Long (days to months) | Considerably greater than 10 | Redox flow batteries are situated between storage systems with medium discharge times and those with long discharge times. But their rather low energy density limits the energy-to- power ratio to values between about 5 and 30.
DLCs and FES have high power density but low energy density. Li-ion batteries have both high energy density and high power density. This explains the broad range of applications where these batteries are now deployed. Na–S and Na–NiCl2 batteries have higher energy densities than mature battery types such as PbA and Ni–Cd, but they have lower power density than Ni–MH and Li-ion batteries. Metal–air cells have the highest potential in terms of energy density. |
