12V Lead Acid Battery Discharge Indicator

The circuit was designed to produce an indication before a 12 V lead acid battery would reach the discharged state

• LM723 – a positive NPN standard voltage regulator mainly designed for series regulator applications which can be utilized for both foldback and linear current limiting due to its very low standby current drain circuit
• Trimmer – a miniature variable component used to make fine adjustments to capacitance, inductance or resistance (potentiometers)
• BC547 – NPN small signal transistors designed for general purpose switching and amplification due to its low voltage, low current and three different gain selections

Lead acid batteries are comprised of a Sulphuric acid solution electrolyte, a sponge metallic lead anode and a lead dioxide cathode. Because of the chemical combinations, this heavy metal element is toxic and disposing it improperly would be hazardous to the environment. Mechanically, the lead acid battery is made of a series of identical cells where sets of positive and negative plates comprise each cell. A typical cell is built with more plates in order obtain the needed current output. The positive plates are attached together as well as the negative plates. There are always one or more negative plates than positives because the arrangement of the positive plate is always in between the negative plates. These plates are immersed in an electrolyte of dilute sulphuric acid and distilled water. Sulphuric acid is a very reactive substance of oxygen atoms, sulphur, and hydrogen. The sulphuric acid has the ability to distribute itself very evenly throughout the electrolyte in the battery due to its instability. This will always to an even reaction between all the plates by producing current and voltage. Both plates are turned into lead sulphate by the chemical reaction between the lead dioxide at the positive plates, the spongy lead at the negative plates, and parts of the electrolyte.
The result of all the reaction is a potential difference between the two plates where the positive plate gives up electrons and the negative plate gains them in equal numbers. However, there is a limitation in the length of time for the reaction to produce the cell voltage. The voltage will remain constant without the presence of connection between the two plates. The chemical reaction will be able to continue the electrons to flow through the circuit from the negative plate to the positive in the event that a load is placed between the positive and negative plates. The current produced by the cell is represented by the flow of electrons. The battery will fail to produce any current when the supply of electrons becomes depleted. The cause of this depletion may pertain to the electrolyte being turned mostly into water or the active material on the negative plate has been used up. The efficiency of the battery system depends on the heating levels of the chemical process wherein greater heating means the battery is quickly exhausted.
To protect the batteries for the fear of being destroyed, the circuit provides the detection of discharge. Since the lead acid battery has a voltage of 12 V, it is not allowed to be rated below 10.8 V. otherwise, the LED would light to indicate the voltage drop. Lead acid batteries should never run flat and the maximum recommended discharge is 75% of the total. This signifies that the minimum charge remaining on the battery should be around 25% before recharging. Lead acid batteries should always be regularly charged even on its idle state once it has been filled with electrolyte. Due to its own resistance, the battery is self discharging when not in use. This could lead to a flat discharge even without putting the battery into service. The self discharge rate is a measure of how much the batteries discharge by them and is managed by the metallurgy of the lead used inside and the construction of the battery. The depth of discharge is a measure of how deeply the battery is discharged. The deeper the batteries are discharged, the shorter is their life cycle.
On order to control the discharge of the battery, a stable voltage in a circuit is required to check the voltage. This can done using the IC1 LM723 which is a monolithic integrated circuit programmable voltage regulator, assembled in 14-lead dual in-line ceramic and plastic package. It can provide internal current limiting and an external NPN or PNP pass element may be used when the output current exceeds 150 mA. The specifications are made for remote shutdown and adjustable current limiting. The poles of the battery are connected to the terminal entry of the circuit. The stability voltage around 7.15 V is handled by the pin 6 of the IC as it receives an input voltage larger than 9.5 V. the stability voltage is provided to the pin 5 while pin 4 obtains a part of the input voltage which is checked by the trimmer. The trimmer is meant to be set correctly when installed. IC1 operates as a voltage comparator which compares the value of voltage between pin 4 and pin 5. When the voltage in pin 4 is larger than the voltage in pin 5, the output in pin 9 becomes low. But when the output in pin 9 becomes high, the LED will turn on as a result of the conduction of Q1. The regulation of the circuit requires an external power supply which can be regulated with a value of 10.8 V. the trimmer is also adjusted so that when the LED turns ON, the voltage from the supply will be improved.
The rate of discharge is also affected by storage. A battery should never be stored directly on the ground especially not on the concrete. Using wooden pallets is the best storage method since it will not allow damp paths, will not conduct, and will provide good air circulation. A refreshening charge once every two months or more is recommended by most manufacturers during storage. Thin electrode plates allow a lot of energy to be discharged quickly for a short period of time like in car batteries. Thick electrodes in lead acid batteries can tolerate discharges better than thin but at the expense of producing heavier batteries.
Lead acid batteries are being used in several industries because they are tried and tested, robust, tolerant to abuse and overcharging, low internal impedance, can deliver high currents, wide range of capacities and sizes, supplied worldwide, indefinite shelf life if stored without electrolyte, reliable, and low cost. They are used in equipments like the electrical motors in conventional submarines and nuclear submarines where large lead acid batteries are used; in power failure cases which use lead acid batteries for emergency lighting; in uninterruptable power supplies used for small computer systems, marine applications, electric scooters, and electrified bicycles where backup power supplies for alarm are utilizing gel batteries; in supplying heater voltage in the early radio receivers constructed out of vacuum tubes; in providing large backup power supplies for computer centers and telephone, off-grid household electric power systems, and grid energy storage where wet cell batteries are employed which is designed for deep discharge; in battery electric vehicles like golf carts which use propulsion batteries; in forklifts where batteries are used as counterweights; and in motor vehicles for ignition, lighting, and starting where the current for initiating internal combustion engines are provided by batteries.