Craig Arnold and colleagues study energy storage systems, particularly for use with renewable energy. This past year the researchers have been studying the amount of energy stored in commercially available batteries in response to variable rates of charge and discharge. Energy harvesting systems like wind and solar generators or regenerative braking in cars deliver power in a very non-constant manner. Therefore, when one tries to store this energy, one has to consider the effects of variable rates on the efficiency of charge storage in batteries. Traditional measures of battery efficiency are based on constant charge and constant discharge, but to truly meet the demands of alternative energy and electric vehicles, the effects of variations in charging and discharging must be understood.

Battery discharge efficiency is known to decrease with increasing power, i.e., the faster one tries to pull energy out of a battery, the less energy the system can provide. This relation is known as the Ragone relation between power and energy density. The Arnold Group has modeled a similar but critically important trade-off – between charging power and battery storage efficiency. The team found good agreement with experimental measurements in lithium-ion batteries in their simulations, in contrast to traditional resistance-only models that fail to reproduce the dramatic fall-off in storage efficiency at high powers (Figure 4).

Efficiency loss at large charging rates is found to result from internal losses as well as premature arrival at voltage limits which are a practical safety feature in all battery systems. The latter effect was found to have a greater impact on battery charging as compared to discharge, which is a significant interpretation of how the way a battery is used affects its storage capacity. These results represent a first step toward determining the projected efficiency of a complete storage system operating over a wide range of charging powers and will help improve the design of such systems.

The group has further initiated a set of experiments to determine the effect of variable wind turbine input on the lifespan of four different battery chemistries. The results of this aging experiment will allow for improved design of battery storage systems for small-scale wind energy storage and offer insight into the effects of variability on battery degradation.

Figure 4. Battery storage efficiencies calculated from simulations and experiments. Battery storage efficiency is lower when charging versus discharging (blue versus red shapes). Simulations using traditional techniques (labeled “resistance only models”) do not accurately represent the amount of energy stored in the batteries, but the Arnold Group’s simulations match the measured data well.