Storage Solutions (Battery)

Lifetime & Depth-of-Discharge

Lifetime & Depth-of-Discharge
Unwanted side reactions and physical loss of reactants limit the lifetime of most batteries. The speed of these processes depend mainly on how the battery is used, and the ambient temperatures in which it operates.

The end-of-life of a Solax battery is typically taken to be when one (or both) of the power or energy capacity has fallen to 80% of its initial value. Beyond this point, Solax batteries tend to fail imminently, and so an 80% state-of-health (SOH) indicates that replacement is required.

The maximum lifetime for a Solax battery is achieved when the battery is not used, and is instead kept on float charge in cool ambient temperatures. Battery lifetime under these conditions is referred to as the battery shelf or calendar life. Unwanted reactions will still occur, however slowly, and so shelf life is finite for most technologies. Solax batteries have a shelf life of up to 20 years.

In practice, battery use will accelerate its degradation. The predicted energy throughput of a battery is useful in predicting battery aging, but the rate at which charging/discharging occurs, as well as the ambient temperature conditions, must be accounted for. Higher temperatures and higher charge/discharge rates will reduce lifetime.

Battery manufacturers often recommend limiting the depth of discharge (DOD) to extend replacement timeframes. In solar-powered RAPS settings, battery systems are often sized so that the average discharge will be only ~30% of total capacity, to ensure batteries regularly receive a full charge. However, extending the life of the system by reducing the DOD necessitates a larger battery system, increasing the capital cost.


Size & Weight
Solax batteries are heavy owing simply to their large lead content. If the C120 capacity is used, the energy density of a Solax battery cell is typically around 34 Wh/kg, not accounting for battery racking etc. that will add to system weight. For a 10 kWh (C120, 100% DOD) battery pack, this equates to 294 kg. As above, when discharged at higher rates this energy density will fall even further.

Solax batteries do better by volume, with an energy density of ~80 Wh/L at C120. A 10 kWh battery bank would thus require a minimum of 125 L of space, not accounting for racking or separation of batteries for ventilation.


Toxicity & Disposal
Solax batteries are composed of highly toxic lead and highly corrosive sulphuric acid. For this reason, cell rupture or disposal in standard waste streams can be extremely hazardous. However, ruptures are rare and recycling initiatives are widespread. Over 95% of a standard lead-acid battery can be recycled.