Selecting the correct energy storage device for use with GCell, as part of an Energy Harvesting (EH) system, is an important consideration.

Due to changing ambient light levels and exposure duration, there will be variation in the amount of energy the GCell can instantaneously harvest and provide a system load.

In many products the system load requirement will exceed the energy generation, especially in wireless communication products which transmit and receive data with high peak currents.

As a result, most product applications will require an energy storage medium to store the harvested energy and act as an energy buffer to provide the required system load.

There are many trade-offs between the factors there is not one ideal type, but the most suitable technology must be selected based on the product application. Some of the factors to be considered are as follows:

Product operation type

The product can operate as either and autonomous energy system where the daily energy usage is replenished by the GCell, or as an extension of product life where the GCell provides system energy to extend the product runtime beyond the required life.

For autonomous energy systems smaller battery capacities are required with a good charging efficiency. In comparison, for life extension a larger battery capacity is preferable with low self discharge rate.

Runtime / Capacity requirement

The runtime of the product will often dictate the capacity requirements. A battery with higher energy density or larger profile will result in a larger capacity and longer runtime. The capacity has a trade-off with the ability to charge.
Larger capacities will be more difficult to charge, compared to smaller capacities particularly for indoor product applications where the charging currents will be in the micro amp (uA) range.

Self discharge rate

All energy storage mediums will have some amount of self discharge leakage current. This is caused my electrochemical processes in batteries and leakage current through the dielectric in supercapacitors.

The self discharge rate is more important for products with longer runtime / larger capacity batteries or products that may be stored in conditions with no light for longer periods. The self discharge rate needs to be calculated when determining the system energy usage.

The self discharge rates of different battery technologies and supercapacitors can vary greatly.

• Standard Nickel Metal Hydride (NiMH) typically have a self discharge rate loss of around 20-25% of the capacity per month
• Low Self Discharge Nickel Metal Hydride (LSD NiMH) 1% per month
• Lithium Ion Phosphate 1.5 – 2% per month
• Lithium Manganese around 2-5% per year
• Supercapacitors generally have a higher leakage than batteries and after high initial leakage stabilise at around 2-5uA with around 50% loss after 7 days.

Discharge current

The type of discharge current (constant drain or pulse) and amount of discharge will affect the storage selection. If low continual current draw is required, Lithium Manganese batteries are very well suited as they have a standard discharge current of 200uA with a high pulse current of 2 to 4mA (depending on the capacity).

Many Bluetooth or wireless products will have intermittent loading due to high transmit currents of around 10-30mA for a short durations. This will impact on the capacity and life of a low discharge battery type.

For larger current requirements, Super Capacitors are very well suited due to their very high instantaneous discharge capability.

For battery technologies Lithium Ion Phosphate (LiFePO4) are most suited under high pulse loads as they can discharge at a rate of 5C of the rated capacity.

Charge efficiency / Minimum charge acceptance

The charge efficiency is the efficiency at which the battery or supercapacitor can store the energy generated by the GCell. The amount of available charging current, battery chemistry and capacity of the battery all impact on the minimum charge acceptance or charge efficiency of the storage medium.

Cycle life

The cycle life of the storage medium will affect the selection decision based on the product application.

• Supercapacitors are very well suited to buffer load applications or high cycle charge and discharge applications as their life can exceed 200,000 cycles
• Thin-Film Solid State Batteries have a good cycle life typically of 100,000 cycles
• Standard battery technologies such as Lithium Ion Phosphate typically have a good cycle life of 2000 cycles.

In many indoor product applications where the storage medium is used as a buffer (GCell replenishes the system energy used) the cycle life is no so critical as products are designed for low light levels so under normal operating conditions the battery will not fully discharge and register a full discharge cycle.

Size profile

For many portable indoor products the required form factor can affect the selection of storage type.

• LSD NiMh are only produced in cylindrical format AA / AAA, so are not well suited to very thin product applications
• Lithium Manganese batteries are manufactured as coin cells
• Lithium Ion Phosphate can be manufactured as cylindrical or thin soft pack format
• FPC Lithium-Ceramic Battery technology is manufactured in a flexible form so well suited to thin flexible product applications.