Lead-acid Battery
Lead-acid batteries are secondary (rechargeable) batteries that consist of a housing, two lead plates or groups of plates, one of them serving as a positive electrode and the other as a negative electrode, and a filling of 37% sulfuric acid (H2SO4) as electrolyte. The battery contains liquid electrolyte in an unsealed container, requiring it to be kept upright and the area well ventilated to ensure safe dispersal of the hydrogen gas it produces during overcharging. Lead acid batteries typically have coulombic efficiencies of 85% and energy efficiencies in the order of 70%.
The lead–acid battery is relatively heavy for the amount of electrical energy it can supply. Its low manufacturing cost and its high surge current levels make it common where its capacity (over approximately 10 Ah) is more important than weight and handling issues. The disadvantage of this battery chemistry is that it is very sensitive to deep cycling compared to other battery systems, and due to the high density of lead, the specific energy of the batteries is quite low.
Characteristics of Lead-acid Batteries
To compare and understand the capability of each battery, some important parameters are characteristic of each battery, also within a type of battery. These parameters are a reference when a battery is needed, and specific qualities are required since batteries are used in all types of devices and for infinite purposes.
Cell Voltage
The voltage of electric batteries is created by the potential difference of the materials that compose the positive and negative electrodes in the electrochemical reaction.
A common voltage for automobile batteries is 12 volts (DC). But this battery consists of six 2V lead cells.
Cut-off Voltage
The cut-off voltage is the minimum allowable voltage. It is this voltage that generally defines the “empty” state of the battery.
The truth is that any lead acid battery, be it a Gel Cell, AGM or flooded batteries, should be cut off at 11.6 volts.
Capacity
The coulometric capacity is the total Amp-hours available when the battery is discharged at a certain discharge current from 100% SOC to the cut-off voltage.
A common voltage for automobile batteries is 12 volts (DC). But this battery consists of six 2V lead cells. An average automotive battery might have a capacity of about 70 Ah, specified at a current of 3.5 A.
C-rate of Battery
C-rate is used to express how fast a battery is discharged or charged relative to its maximum capacity. It has units h−1. A 1C rate means that the discharge current will discharge the entire battery in 1 hour.
Self-discharge
Batteries gradually self-discharge even if not connected and delivering current. This is due to non-current-producing “side” chemical reactions that occur within the cell even when no load is applied.
A lead acid battery left in storage at moderate temperatures has an estimated self-discharge rate of 5% per month. This rate increases as temperatures rise and the risk of sulfation increases.
Degradation
Some degradation of rechargeable batteries occurs on each charge–discharge cycle. Degradation usually occurs because electrolyte migrates away from the electrodes or because active material detaches from the electrodes.
Deep cycle batteries can provide 200 to more than 3000 discharge/charge cycles. Starting batteries are not designed for sustained discharge and will last for only 50-60 use cycles.
Depth of Discharge
Depth of discharge is a measure of how much energy has been withdrawn from a battery and is expressed as a percentage of full capacity. For example, a 100 Ah battery from which 40 Ah has been withdrawn has undergone a 40% depth of discharge (DOD).
