Lead-acid batteries are the dominant technology for stationary storage applications. They have been in use for over 100 years, and are ubiquitous and well-understood. Their composition and working principles are depicted in Figure 1.
Figure 1. Lead-acid battery working principle (source: Georgia State University Department of Physics and Astronomy)
The nominal voltage of a lead-acid cell is taken to be 2 V. The voltage of a lead-acid battery cell fluctuates within a range of ~0.5 V (1.75 – 2.25 V) depending on its state of charge (SOC) and temperature. Thus, battery voltage and temperature are most often used to determine SOC, where a higher voltage indicates a higher SOC. Cells are connected in series to increase voltage, typically to 12 V for automotive starter batteries, and 24/48 V for UPS or RAPS systems.
Lead-acid batteries are considered to have high internal resistance, meaning the lower voltage limit is hit quickly when the battery attempts to discharge at high rates. In conjunction with the large losses to heat generation, they display a large decrease in total capacity with increasing discharge rates (Figure 2). In this graph, the length of discharge is the time taken for the battery to run ‘flat’ (as defined by the lower voltage limit), and thus a long discharge implies a low discharge rate.
Figure 2. Lead-acid battery capacity versus discharge rate
The tendency for lead acid battery voltage to plummet at high discharge rates makes them more suited to low-rate applications.
Lead-acid batteries are most commonly charged in three distinct phases:
- Constant current phase: bulk charging is carried out with the battery charger supplying constant current to the battery until it reaches its maximum voltage
- Constant voltage phase: absorption charging follows, where the battery charger applies a constant voltage while the current into the battery decays to zero.
- Float phase: trickle charging is required when the battery is not immediately discharged following the completion of the absorption phase. This is carried out at a constant voltage lower than that of the absorption phase, and opposes any self-discharge that might otherwise occur (typically ~5% per month).
During discharging, sulphate ions from the electrolyte react with the lead electrodes to form lead sulphate. Upon recharging, these compounds dissolve with the sulphate ions returning to the electrolyte. Sulfation refers to the irreversible formation of lead sulphate crystals that can occur if the battery is left discharged too long (or if electrolyte level is too low and electrodes are exposed to air). When left discharged, the amorphous crystals formed during discharge stabilise to become crystalline lead sulphate that does not dissolve during recharge. The result is lost battery capacity, and increased internal resistance.
Generally speaking, the faster the bulk charging phase the longer the absorption phase. As lead-acid batteries have to be fully charged regularly to avoid sulfation, the result is that there is not usually an opportunity to reduce charging time without risking long-term damage to the battery.
Sulfation occurs naturally during lead-acid battery use, and a periodic over-charge is required to “blast” off sulphate crystals. This over-charge is referred to as an equalisation charge, and is carried out at higher voltage than would otherwise be recommended. The over-charge helps to remove lingering sulfation, equalise individual battery cell voltages in a string, and, in flooded lead-acid cells, mixes the electrolyte in order to reduce stratification of the aqueous mixture. However, the equalisation charge cannot be conducted too frequently or for too long, as over-voltage conditions lead to gassing (evaporation) of the water component of the electrolyte. In flooded lead-acid cells, this gassing can be reversed with battery watering, but for sealed technologies (eg. gel or AGM), excessive gassing may cause irreversible electrolyte loss.
Electrical (Coulometric) Efficiency
Battery efficiency is most easily described as the ratio of energy out to energy in. The more efficient a battery, the lower the losses will be during charging/discharging. Heat generation is a significant loss mechanism for most batteries, but lead-acid batteries also lose electrical energy to gassing. Gassing is the decomposition of water in the aqueous electrolyte into hydrogen and oxygen gas. This process, more commonly known as electrolysis, increases at the higher voltages associated with the absorption charging phase.
Heat generation and gassing are responsible for ~20% electrical loss in lead-acid battery operation, giving an overall round-trip efficiency of ~80%.
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 lead-acid 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, lead-acid batteries tend to fail imminently, and so an 80% state-of-health (SOH) indicates that replacement is required.
The maximum lifetime for a lead-acid 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. Lead-acid 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
Lead-acid batteries are heavy owing simply to their large lead content. If the C120 capacity is used, the energy density of a lead-acid 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.
Lead-acid 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
Lead-acid 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.