The use of batteries in the EverGEN systems (including charging, discharging, voltage and temperature) are all monitored and controlled via the energy management system (EMS). Carmanah EverGEN batteries are rated for 4,000 cycles to 20% depth of discharge at 25° C (77° F).

Battery lifespan is a function of battery characteristics, battery use and the ambient temperature in which the battery operates.

Battery Characteristics

For solar LED lighting applications the most important criteria for a battery is the number of charge-discharge cycles it can deliver. The number of cycles is more important than initial battery capacity when considering how often batteries will have to be replaced.

Initial battery capacity in Amp Hours (Ah) is often used when specifying batteries; however (as seen in the graph below), batteries with high initial Ah capacity can actually have a shorter cycle life. A preferred method of specifying batteries is by the cycle-rating, which is the number of cycles the battery is rated for at a given level of discharge.

Battery Use

As batteries charge and discharge, a small amount of damage is done to them. The amount of damage depends on the type of battery and how deep the discharge is (i.e. how far the batteries are depleted). The graph below shows the relationship between depth of discharge and the number of discharge-recharge cycles a battery can provide before it falls below 80% of its rated capacity. For example, if only 20% of the battery capacity is used each cycle, the battery can withstand 4,000 cycles. However, if 80% of the battery capacity is used each cycle, the battery will only withstand 500 cycles.

The number of cycles a battery can provide is a function of the depth of discharge of each cycle and the ambient temperature in which the batteries operate.

The depth of discharge a battery experiences is determined by the way the solar LED lighting system is configured. Factors include the lumen output of the system, the location of the system, and the operating profile the system is running. It is critical that system modelling take battery depth of discharge into account to preserve battery health and lifespan and to ensure system performance over the long term.

Ambient Temperature

Temperature changes accelerate battery aging making thermal management of the batteries an important consideration in solar LED lighting design.

Charge acceptance of batteries varies with temperature, making them prone to over and under-charging when temperatures reach high or low extremes. Over and undercharging affects the life span of the battery. The negative impact of temperature extremes can be mitigated by controlling the ambient temperature of the batteries.

Heat

As temperature increases, batteries accept charge more rapidly at a given voltage and the voltage at which they indicate full charge will decrease. If the battery charger does not account for temperature variations, it may believe that the battery never reaches full charge. This can create thermal runaway with batteries becoming hot, accepting more charge, creating more heat, which allows them to continue accepting charge. In this way, a hot battery can become overcharged if accepting charge at a voltage intended for room-temperature charging. This situation significantly reduces battery life span.

Temperature must be taken into consideration in the design of the system, including battery placement and battery charging technology.

Learn more about temperature compensation and battery charging in the energy management system section.

Cold

Cold conditions result in reduced battery capacity. Batteries in lower states of charge are more likely to freeze in lower temperatures. System modelling tools must account for temperature extremes to avoid deep discharging batteries in cold conditions. This preserves system health and ensures reliable system operation in cold temperature extremes.