The ability of the lead acid battery to provide a voltage and current is due to the chemical structure of the positive and negative electrodes. It is the difference in potential between the positive plate (PbO2) and the negative plate (Pb) in dilute sulphuric acid which creates the lead acid battery. It is the formation process which turns these complex lead compounds into electrochemical machines, capable of storing and providing electrical energy. This process is so called, as this is the first time the material has been converted (formed) into the electrodes.
The formation process is the last opportunity in the manufacturing process to ensure that the structures obtained from the previous processes will be converted into suitable and appropriate active material with the best possible properties for the battery application. For this reason, the parameters and conditions necessary for the successful conversion of dry cured paste masses for flooded batteries using pasted plates have to be very carefully controlled
The basis of all batteries is to create a potential difference between plates immersed in electrolyte. This basically means a different state of energy to allow electrons to flow from one plate to another until both plates have similar chemical energies, much like two containers with different amounts of water and each joined by a valve. When the valve opens the potential energy in the higher level causes the water to flow into the second container until the levels (and therefore the potential energies) are the same. In the case of the lead acid battery we have to create that imbalance by forming two different chemical species for the positive and the negative plate, i.e. PbO2 for the positive and Pb for the negative.
Assuming the active material is correctly processed and the battery properly filled at the point of formation, then the critical aspects affecting the process will become:
- Temperature of the batteries
- Total ampere hours input into the battery
- Total circuit resistance
- Loss of current through tracking
There are consequences to the under formation of batteries through insufficient ampere hours input due to high resistance or tracking. Equally the formation temperature can create conditions which are unsuitable to obtain the desired crystal structure and hence properties for the type of battery being formed, e.g. SLI compared to deep cycle applications. Table 1 gives the conditions necessary for the successful formation of various lead acid battery designs.
TABLE 1 – Comparison of acid filling parameters for different processes and applications
|Process||Acid SG||Soak time||Maximum temperature||Battery types|
|Tank plate formation (all battery types)||1.05||1 – 2 hours||45C||All|
|Container formation single shot (automotive)||1.240||0.5 – 2 hours||55C||Automotive, semi traction, monobloc|
|Container formation two shot (automotive)||1.15||0.5 – 2 hours||55C||Automotive, monobloc|
|2V Container formation (tubular pickled plate)||1.15||1 – 2 hours||65C||2V traction cells|
|2V Container formation (flat plate standby power)||1,240||2-4 hours||55C||Standby power, UPS|
It is quite clear that the effect of temperature increases and losses of coulombic input (ampere hours) through increased resistance and tracking can affect the performance of the battery. The reasons for the loss of coulombic input and higher temperatures are often down to the connectors used to join the batteries in series in the formation bath (or the connection between connector and battery terminal). The integrity and design of the connector and the quality of the connection is vital in assuring that there is no tracking and that the resistance of the series string is uniformly distributed. A high resistance connection will have two effects:
1. To generate heat and to raise the voltage of the battery it is connected to. The higher temperature is easily transmitted to the rest of the battery through the terminal to the grids.
2. The higher resistance will create a higher voltage for a particular battery in a series string which will induce a faster rate of dry-out which increases the battery’s SG and alters the structure of the active material usually reducing the life of the battery.
Typical defects and their causes are listed in Table 2.
Note – The Connector to Battery Terminal must be tight and secure (not loose) with the connector surfaces clean from Oxide residue to allow for the least resistance to prevent the above situations from occurring
TABLE 2– Common formation defects
|Defect||Cause||In service consequences|
|High SG||Incorrect acid filling SG Excessive formation ampere hours Poor final adjustment||Low CCA in automotive Undercharging due to higher on charge voltages, particularly in voltage controlled charging Increased positive corrosion and grid growth in non-voltage regulated charging|
|Low SG||Incorrect acid filling SG insufficient formation ampere hours Poor final adjustment||Low capacity, serious for 2 volt traction and semi traction applications|
|Low CCA (automotive)||High formation temperatures leading to large PbO2 crystals High internal resistance due to low conversion of sulphate to AM||Poor starting characteristics, particularly in cold weather. Early failure under warranty conditions|
|Low charge acceptance||High SG see above||Battery performance reduces with time Early failure under warranty conditions|
|Reduced life||SG imbalance between cells due to poor acid adjustment. Low capacity due to voltage differences between string connected batteries (connectors). Corrosion of positive due to high formation temperature caused by high resistance connections.||For traction series connected cells it is a common cause of poor cycle life Batteries on float charge fail from positive corrosion. High corrosion in the formation stage will reduce the time to positive failure Low capacity in service will reduce the cycle life|
|Low capacity||Batteries under-formed due to insufficient current (tracking problems) or time on charge or too low formation temperature Low SG||Reduced cycle life for traction batteries. Unable to meet performance requirements and fail under warranty terms.|
|Short shelf life||Insufficient formation due to unconverted active material. High residual free lead in the positive.||Poor performance after storage at customers or manufacturers warehouse. Common cause of warranty returns in automotive markets|
** Information courtesy of Dr Michael McDonagh BSc. PhD
What causes tracking and heat generation on the battery/string
To reduce the risk of tracking and heat generation caused from the terminal/connector joint it is important that the connector and terminal connect as efficiently as possible. UK Powertech’s automotive connectors (S & T Type) terminals are manufactured to exactly the same angle (1:9 taper) as the DIN/SAE standards. Before the moulded battery terminal is welded to the internal pillar insert of the battery, the connector is an exact fit. However, following the automatic battery post burning operation this is often not the case. Defective welds can range in severity from slight irregularities to major lead runs, burrs or spatter affecting the contact surface of the battery terminal.
Unfortunately, defects in the battery terminal can prevent the connectors used in the formation process from being correctly fitted. This creates the following problems:
The first and most obvious issue is that any space created in the connector/terminal joint will allow the ingress of acid as vapour or spray during the formation process. Over time, this creates a lead corrosion layer on the connectors contact surface. This layer has a high resistance, which creates heat and a voltage drop. This can induce arcing which damages both the connector and the battery terminal post. The subsequent voltage change in a battery string due to the high resistance can also affect the formation process. This will almost certainly lead to under-formed batteries. If there is acid on the surface of the battery then this creates tracking and heat which combined with arcing can easily result in a fire.
If one connector suffers these issues the voltage drop is carried throughout the complete string.
Cleaning the connectors to remove this oxide residue will allow for better connections and reduce electrical resistance and tracking. A poor connection also creates heat which, in turn is transmitted into the battery which increases the temperature of the acid
Other issues preventing good connections:
The process of welding the pillar inserts to the battery lid terminal creates heat which can distort the surface regularity of the precise conical shape of the terminal lid insert. Spatter from the welding process can attach to the conical contact area of the terminal or a mushroom shape can form over the top of the terminal. The top 2-3 mm of the post can become misshaped or irregular. All of which prevent the connector from efficiently contacting with the battery terminal. ALL will create voltage drop, heat generation and tracking.
Other causes of bad connection can be operator error in fitting the connectors prior to formation. Loose connectors, incorrect angle of fixing of the connectors to the battery terminals will also create high resistance joints. Obviously, this procedure is very intensive for the employee carrying out this operation, but it is important that the connector is fitted tightly, is clean and adequate for the charge currents being applied. The ergonomics and time requirements of the operator to carry out this operation correctly cannot be over-emphasised. High resistance joints caused by this method will be just as severe as defective battery terminals
In conclusion – The Formation Process is a critical procedure. It is important that all aspects of the terminal manufacture and pre formation procedures are controlled very carefully with adequate quality standards. It is also crucial to make sure that your product is finished to the highest possible tolerances. These should be embedded in the quality standards and procedures of those processes which are vital to the assurance of high quality low resistance connector joints during formation.
Technical information taken from Papers written by Dr Michael McDonagh BSc. PhD