The increased cost of raw materials has had a serious impact on the price of batteries. Now financial considerations join those of sustainability when trying to prolong battery life.
A lead acid battery goes through three life phases: formatting, peak and decline. During the formatting phase the lead plates are being "exercised" to enable the electrolyte to better fill the usable areas, which increases the capacity.
Formatting is most important for deep-cycle batteries and requires 20 to 50 full cycles to reach peak capacity. Field usage does this and there is no need to apply added cycles for the sake of priming, however, manufacturers do advise users not to over stress the battery until broken in.
A deep-cycle battery delivers 100-200 cycles before it starts a gradual decline. Replacement should occur when the capacity drops to 70 or 80%. Applying a fully saturated 14 to 16-hour charge and operating at moderate temperatures assure the longest service times. If possible avoid deep discharges and charge more often.
The primary reason for the relatively short cycle life of a lead acid battery is depletion of active material, but, long before plate and grid-deterioration other problems can develop with use and time. These are corrosion, shorting, sulphation, water loss, acid stratification and surface charge.
Corrosion occurs primarily on the grid and is known as a softening and shedding of lead off the plates, a reaction that cannot be avoided because the electrodes in a lead acid environment are always reactive.
Limiting the depth of discharge, reducing the cycle count, operating at a moderate temperature and controlling overcharge are key in keeping corrosion in check.
To reduce corrosion on long-life batteries, manufacturers keep specific gravity at a moderate 1.200 when fully charged. This, however, reduces the capacity of the battery.
The term "short" is commonly used to describe a general battery fault when no other definition is available. The lead within a battery, especially in deep-cycle units, is mechanically active and when a battery discharges, the lead sulphate causes the plates to expand. This movement reverses during charge and the plates contract.
The cells allow for some expansion but over time the growth of large sulphite crystals can result in a soft short that increases self-discharge. This mechanical action also causes shedding of the lead.
As the battery sheds its lead to the bottom of the container, a conductive layer forms, and once the contaminated material fills the allotted space in the sediment trap, the now conductive liquid reaches the plates and creates a shorting effect.
Another form of "soft" short is mossing. This occurs when the separators and plates are slightly misaligned as a result of poor manufacturing. This causes parts of the plates to become naked.
The exposure promotes the formation of conductive crystal moss around the edges, which leads to elevated self-discharge.
Lead drop also causes of shorting, here large pieces of lead break away from the welded bars connecting the plates. This is mostly a manufacturing defect and cannot be repaired.
The most radical and serious form of short is a mechanical failure in which the suspended plates become loose and touch each other. This results in a sudden high discharge current that can lead to e
xcessive heat buildup and thermal runaway. Sloppy manufacturing as well as excessive shock and vibration are the most common contributors to this failure.
Sulphation occurs when a lead acid battery is deprived of a full charge. Lead acid must periodically be charged for 14-16 hours to attain full saturation.
In use, small sulphate crystals form, but these are normal and harmless. During prolonged charge deprivation, however, the amorphous lead sulphate converts to a stable crystalline form -that deposits on the negative plates.
This leads to the development of large crystals, which reduce the battery's active material exposure that is responsible for high capacity and low resistance. Sulphation also lowers charge acceptance and charging will take longer.
During use, and especially on overcharge, the water in the electrolyte splits into hydrogen and oxygen. The battery begins to gas, which results in water loss.
In flooded batteries, water can be added but in sealed batteries water loss leads to an eventual dry-out and decline in capacity. Water loss from a sealed unit can eventually cause disintegration of the separator.
The initial stages of dry-out can go undetected and the drop in capacity may not immediately be evident. Early detection of this failure is important.
The electrolyte of a stratified battery concentrates on the bottom, starving the upper half of the cell. Acid stratification occurs if the battery dwells at low charge (below 80%), never receives a full charge and has shallow discharges.
Lead acid batteries are sluggish and cannot convert lead sulphate to lead and lead dioxide quickly enough during charge. As a result, most of the charge activities occur on the plate surfaces.
This induces a higher state-of-charge on the outside of the plate than on the inner plate. A battery with surface charge has a slightly elevated voltage.
To normalise the condition, switch on electrical loads to remove about 1% of the battery's capacity, or allow the battery to rest for a few hours. Surface charge is not a battery defect but a reversible condition resulting from charging.
Testing Lead Acid Batteries
According to the Battery University (see end credit) many manufacturers of battery testers claim to measure battery health on the fly.
These instruments work well in finding battery defects that involve voltage anomalies and elevated internal resistance, but other performance criteria remain unknown.
Stating that a battery tester based on internal resistance can also measure capacity is misleading.
The carbon pile, introduced in the 1980s, applies a DC load of short duration to a starter battery, simulating cranking. A major advantage is the ability to detect batteries that have failed due to a shorted cell.
Capacity estimation, however, is not possible, and a battery that has a low state of charge appears as weak. A skilled mechanic can, however, detect a faulty battery based on the voltage signature and loading behavior.
The AC conductance meter appeared in 1992 the non-invasive method injects an AC signal into the battery to measure the internal resistance.
Today, these testers are commonly used to check the CCA (cold cranking amp) of starter batteries and verify resistance change in stationary batteries.
While small and easier to use, AC conductance cannot read capacity. AC conductance testers are common in North America; Europe prefers the DC load method.
Progress has been made towards electrochemical impedance spectroscopy (EIS). Cadex took the EIS technology a step further and developed battery specific models that can estimate the health of a lead acid battery.
Multi-model electrochemical impedance spectroscopy, or Spectro for short, reads battery capacity, CCA and state-of-charge in a single, non-invasive test. (See box Total test in 15 sec).
SOURCE: Information drawn from the on line Battery University sponsored by Cadex Electronics www.Battery University.com.