Lead Acid

Introduction

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.

PbS working principle

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.

Discharging

Lead-acid batteries display a large decrease in total capacity at 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.

PbS Capacity v discharge rate

Figure 2. Lead-acid battery capacity versus discharge rate

The high dependency of storage capacity on discharge rate makes the technology less suited to applications where continuous high rates are required.

Charging

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 discharge, sulphate ions from the electrolyte react with the lead electrodes to form amorphous lead sulphate. Upon recharging, these compounds dissolve with the sulphate ions returning to the electrolyte. “Sulfation” refers to the irreversible formation of crystalline lead sulphate  that can occur if the battery is left discharged too long, if the battery is not fully recharged, or if the electrolyte level is too low and the electrodes are exposed to air. Sulfation reduces battery capacity and increases internal resistance (causing voltage to sag more heavilty when the battery is loaded).

Sulfation occurs naturally during lead-acid battery use, and a periodic over-charge is therefore required to “blast” off sulfate crystals. This over-charge is referred to as an “equalisation charge”, and is typically carried out at higher voltage than used for regular charging. The over-charge helps to remove lingering sulfation, equalise individual battery cell voltages in a string, and, in flooded lead-acid cells, mix 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 the applied over-voltage leads to gassing (see below) 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.

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 scope for reducing charge times is limited.

Efficiency

Battery efficiency is most easily described as the ratio of energy out to energy in. 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 higher voltages (ie. absorption and equalisation charging voltages).

Heat generation and gassing are responsible for round-trip losses of ~15-25% in lead-acid battery operation, giving an overall round-trip efficiency of ~75-85%.

Lifetime & Depth-of-Discharge

Parasitic reactions and physical loss of active materials limit the lifetime of most batteries. The speed of these processes are effected by how the battery is used, and the ambient temperatures in which it operates.

For a lead-acid battery, end-of-life is typically taken to be when one (or both) of the power or energy capacity has fallen to 80% of its initial value. Lead-acid batteries tend to fail shortly after this point, 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, the rate of which will depend mostly on the ambient temperature. For this reason, shelf life is finite for most technologies. Lead-acid batteries can have shelf lives of up to 20 years.

In practice, battery use accelerates degradation, meaning the shelf life is rarely the limiting factor. Degradation is directly related to battery throughput and temperature, which itself is dependent on ambient temperature and discharge rates. In the case of lead-acid batteries, manufacturers often recommend that the depth of discharge (DOD) is limited to extend replacement timeframes. Of course the result is that less capacity is available for the same capital outlay (in the short term at least).

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.